qemu_co_queue_next(&queue) arranges that the next queued coroutine is
run at a later point in time. This deferred restart is useful because
the caller may not want to transfer control yet.
This behavior was implemented using QEMUBH in the past, which meant that
CoQueue (and hence CoMutex and CoRwlock) had a dependency on the
AioContext event loop. This hidden dependency causes trouble when we
move to a world with multiple event loops - now qemu_co_queue_next()
needs to know which event loop to schedule the QEMUBH in.
After pondering how to stash AioContext I realized the best solution is
to not use AioContext at all. This patch implements the deferred
restart behavior purely in terms of coroutines and no longer uses
QEMUBH.
Here is how it works:
Each Coroutine has a wakeup queue that starts out empty. When
qemu_co_queue_next() is called, the next coroutine is added to our
wakeup queue. The wakeup queue is processed when we yield or terminate.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
CoQueue uses a BH to awake coroutines that were made ready to run again
using qemu_co_queue_next() or qemu_co_queue_restart_all(). The BH
currently runs in the iothread AioContext and would break coroutines
that run in a different AioContext.
This is a slightly tricky problem because the lifetime of the BH exceeds
that of the CoQueue. This means coroutines can be awoken after CoQueue
itself has been freed. Also, there is no qemu_co_queue_destroy()
function which we could use to handle freeing resources.
Introducing qemu_co_queue_destroy() has a ripple effect of requiring us
to also add qemu_co_mutex_destroy() and qemu_co_rwlock_destroy(), as
well as updating all callers. Avoid doing that.
We also cannot switch from BH to GIdle function because aio_poll() does
not dispatch GIdle functions. (GIdle functions make memory management
slightly easier because they free themselves.)
Finally, I don't want to move unlock_queue and unlock_bh into
AioContext. That would break encapsulation - AioContext isn't supposed
to know about CoQueue.
This patch implements a different solution: each qemu_co_queue_next() or
qemu_co_queue_restart_all() call creates a new BH and list of coroutines
to wake up. Callers tend to invoke qemu_co_queue_next() and
qemu_co_queue_restart_all() occasionally after blocking I/O, so creating
a new BH for each call shouldn't be massively inefficient.
Note that this patch does not add an interface for specifying the
AioContext. That is left to future patches which will convert CoQueue,
CoMutex, and CoRwlock to expose AioContext.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Start introducing AioContext, which will let us remove globals from
aio.c/async.c, and introduce multiple I/O threads.
The bottom half functions now take an additional AioContext argument.
A bottom half is created with a specific AioContext that remains the
same throughout the lifetime. qemu_bh_new is just a wrapper that
uses a global context.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
It's common to wake up all waiting coroutines. Introduce the
qemu_co_queue_restart_all() function to do this instead of looping over
qemu_co_queue_next() in every caller.
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Now that AsyncContexts don't exist any more, we can use one global bottom half
for restarting coroutines instead of allocating a new one every time (before
removing AsyncContexts, the problem with having a global BH was that it had to
belong to a single AsyncContexts and wouldn't be executed in a different one -
which leads to deadlocks)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
