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aio-posix: move RCU_READ_LOCK() into run_poll_handlers()

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Now that run_poll_handlers_once() is only called by run_poll_handlers()
we can improve the CPU time profile by moving the expensive
RCU_READ_LOCK() out of the polling loop.

This reduces the run_poll_handlers() from 40% CPU to 10% CPU in perf's
sampling profiler output.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-3-stefanha@redhat.com
Message-Id: <20200305170806.1313245-3-stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-03-05 17:08:01 +00:00
parent e4346192f1
commit 3aa221b382
1 changed files with 10 additions and 10 deletions
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20
util/aio-posix.c
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@ -583,16 +583,6 @@ static bool run_poll_handlers_once(AioContext *ctx, int64_t *timeout)
bool progress = false;
AioHandler *node;
/*
* Optimization: ->io_poll() handlers often contain RCU read critical
* sections and we therefore see many rcu_read_lock() -> rcu_read_unlock()
* -> rcu_read_lock() -> ... sequences with expensive memory
* synchronization primitives. Make the entire polling loop an RCU
* critical section because nested rcu_read_lock()/rcu_read_unlock() calls
* are cheap.
*/
RCU_READ_LOCK_GUARD();
QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
if (!QLIST_IS_INSERTED(node, node_deleted) && node->io_poll &&
aio_node_check(ctx, node->is_external) &&
@ -636,6 +626,16 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
trace_run_poll_handlers_begin(ctx, max_ns, *timeout);
/*
* Optimization: ->io_poll() handlers often contain RCU read critical
* sections and we therefore see many rcu_read_lock() -> rcu_read_unlock()
* -> rcu_read_lock() -> ... sequences with expensive memory
* synchronization primitives. Make the entire polling loop an RCU
* critical section because nested rcu_read_lock()/rcu_read_unlock() calls
* are cheap.
*/
RCU_READ_LOCK_GUARD();
start_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
do {
progress = run_poll_handlers_once(ctx, timeout);