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AioContext: optimize clearing the EventNotifier

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It is pretty rare for aio_notify to actually set the EventNotifier.  It
can happen with worker threads such as thread-pool.c's, but otherwise it
should never be set thanks to the ctx->notify_me optimization.  The
previous patch, unfortunately, added an unconditional call to
event_notifier_test_and_clear; now add a userspace fast path that
avoids the call.

Note that it is not possible to do the same with event_notifier_set;
it would break, as proved (again) by the included formal model.

This patch survived over 3000 reboots on aarch64 KVM.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-7-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini 2015-07-21 16:07:53 +02:00 • committed by Stefan Hajnoczi
parent 21a03d17f2
commit 05e514b1d4
5 changed files with 194 additions and 4 deletions
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32
include/block/aio.h
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@ -99,7 +99,19 @@ struct AioContext {
*/
int walking_bh;
/* Used for aio_notify. */
/* Used by aio_notify.
*
* "notified" is used to avoid expensive event_notifier_test_and_clear
* calls. When it is clear, the EventNotifier is clear, or one thread
* is going to clear "notified" before processing more events. False
* positives are possible, i.e. "notified" could be set even though the
* EventNotifier is clear.
*
* Note that event_notifier_set *cannot* be optimized the same way. For
* more information on the problem that would result, see "#ifdef BUG2"
* in the docs/aio_notify_accept.promela formal model.
*/
bool notified;
EventNotifier notifier;
/* Thread pool for performing work and receiving completion callbacks */
@ -173,6 +185,24 @@ QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
*/
void aio_notify(AioContext *ctx);
/**
* aio_notify_accept: Acknowledge receiving an aio_notify.
*
* aio_notify() uses an EventNotifier in order to wake up a sleeping
* aio_poll() or g_main_context_iteration(). Calls to aio_notify() are
* usually rare, but the AioContext has to clear the EventNotifier on
* every aio_poll() or g_main_context_iteration() in order to avoid
* busy waiting. This event_notifier_test_and_clear() cannot be done
* using the usual aio_context_set_event_notifier(), because it must
* be done before processing all events (file descriptors, bottom halves,
* timers).
*
* aio_notify_accept() is an optimized event_notifier_test_and_clear()
* that is specific to an AioContext's notifier; it is used internally
* to clear the EventNotifier only if aio_notify() had been called.
*/
void aio_notify_accept(AioContext *ctx);
/**
* aio_bh_poll: Poll bottom halves for an AioContext.
*