qemu/qemu-coroutine.c

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coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
/*
* QEMU coroutines
*
* Copyright IBM, Corp. 2011
*
* Authors:
* Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
* Kevin Wolf <kwolf@redhat.com>
*
* This work is licensed under the terms of the GNU LGPL, version 2 or later.
* See the COPYING.LIB file in the top-level directory.
*
*/
#include "trace.h"
#include "qemu-common.h"
#include "qemu/thread.h"
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
#include "qemu/atomic.h"
#include "block/coroutine.h"
#include "block/coroutine_int.h"
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
enum {
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
POOL_BATCH_SIZE = 64,
};
/** Free list to speed up creation */
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
static QSLIST_HEAD(, Coroutine) release_pool = QSLIST_HEAD_INITIALIZER(pool);
static unsigned int release_pool_size;
static __thread QSLIST_HEAD(, Coroutine) alloc_pool = QSLIST_HEAD_INITIALIZER(pool);
static __thread unsigned int alloc_pool_size;
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
static __thread Notifier coroutine_pool_cleanup_notifier;
static void coroutine_pool_cleanup(Notifier *n, void *value)
{
Coroutine *co;
Coroutine *tmp;
QSLIST_FOREACH_SAFE(co, &alloc_pool, pool_next, tmp) {
QSLIST_REMOVE_HEAD(&alloc_pool, pool_next);
qemu_coroutine_delete(co);
}
}
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
{
Coroutine *co = NULL;
if (CONFIG_COROUTINE_POOL) {
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
co = QSLIST_FIRST(&alloc_pool);
if (!co) {
if (release_pool_size > POOL_BATCH_SIZE) {
/* Slow path; a good place to register the destructor, too. */
if (!coroutine_pool_cleanup_notifier.notify) {
coroutine_pool_cleanup_notifier.notify = coroutine_pool_cleanup;
qemu_thread_atexit_add(&coroutine_pool_cleanup_notifier);
}
/* This is not exact; there could be a little skew between
* release_pool_size and the actual size of release_pool. But
* it is just a heuristic, it does not need to be perfect.
*/
alloc_pool_size = atomic_xchg(&release_pool_size, 0);
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
QSLIST_MOVE_ATOMIC(&alloc_pool, &release_pool);
co = QSLIST_FIRST(&alloc_pool);
}
}
if (co) {
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
QSLIST_REMOVE_HEAD(&alloc_pool, pool_next);
alloc_pool_size--;
}
}
if (!co) {
co = qemu_coroutine_new();
}
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
co->entry = entry;
QTAILQ_INIT(&co->co_queue_wakeup);
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
return co;
}
static void coroutine_delete(Coroutine *co)
{
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
co->caller = NULL;
if (CONFIG_COROUTINE_POOL) {
coroutine: rewrite pool to avoid mutex This patch removes the mutex by using fancy lock-free manipulation of the pool. Lock-free stacks and queues are not hard, but they can suffer from the ABA problem so they are better avoided unless you have some deferred reclamation scheme like RCU. Otherwise you have to stick with adding to a list, and emptying it completely. This is what this patch does, by coupling a lock-free global list of available coroutines with per-CPU lists that are actually used on coroutine creation. Whenever the destruction pool is big enough, the next thread that runs out of coroutines will steal the whole destruction pool. This is positive in two ways: 1) the allocation does not have to do any atomic operation in the fast path, it's entirely using thread-local storage. Once every POOL_BATCH_SIZE allocations it will do a single atomic_xchg. Release does an atomic_cmpxchg loop, that hopefully doesn't cause any starvation, and an atomic_inc. A later patch will also remove atomic operations from the release path, and try to avoid the atomic_xchg altogether---succeeding in doing so if all devices either use ioeventfd or are not submitting requests actively. 2) in theory this should be completely adaptive. The number of coroutines around should be a little more than POOL_BATCH_SIZE * number of allocating threads; so this also empties qemu_coroutine_adjust_pool_size. (The previous pool size was POOL_BATCH_SIZE * number of block backends, so it was a bit more generous. But if you actually have many high-iodepth disks, it's better to put them in different iothreads, which will also use separate thread pools and aio=native file descriptors). This speeds up perf/cost (in tests/test-coroutine) by a factor of ~1.33. No matter if we end with some kind of coroutine bypass scheme or not, it cannot hurt to optimize hot code. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 1417518350-6167-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-12-02 14:05:48 +03:00
if (release_pool_size < POOL_BATCH_SIZE * 2) {
QSLIST_INSERT_HEAD_ATOMIC(&release_pool, co, pool_next);
atomic_inc(&release_pool_size);
return;
}
if (alloc_pool_size < POOL_BATCH_SIZE) {
QSLIST_INSERT_HEAD(&alloc_pool, co, pool_next);
alloc_pool_size++;
return;
}
}
qemu_coroutine_delete(co);
}
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
void qemu_coroutine_enter(Coroutine *co, void *opaque)
{
Coroutine *self = qemu_coroutine_self();
CoroutineAction ret;
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
trace_qemu_coroutine_enter(self, co, opaque);
if (co->caller) {
fprintf(stderr, "Co-routine re-entered recursively\n");
abort();
}
co->caller = self;
co->entry_arg = opaque;
ret = qemu_coroutine_switch(self, co, COROUTINE_ENTER);
qemu_co_queue_run_restart(co);
switch (ret) {
case COROUTINE_YIELD:
return;
case COROUTINE_TERMINATE:
trace_qemu_coroutine_terminate(co);
coroutine_delete(co);
return;
default:
abort();
}
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
}
void coroutine_fn qemu_coroutine_yield(void)
{
Coroutine *self = qemu_coroutine_self();
Coroutine *to = self->caller;
trace_qemu_coroutine_yield(self, to);
if (!to) {
fprintf(stderr, "Co-routine is yielding to no one\n");
abort();
}
self->caller = NULL;
qemu_coroutine_switch(self, to, COROUTINE_YIELD);
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 19:08:14 +03:00
}