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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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10
async.c
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@@ -203,7 +203,7 @@ aio_ctx_check(GSource *source)
QEMUBH *bh;
atomic_and(&ctx->notify_me, ~1);
event_notifier_test_and_clear(&ctx->notifier);
aio_notify_accept(ctx);
for (bh = ctx->first_bh; bh; bh = bh->next) {
if (!bh->deleted && bh->scheduled) {
@@ -267,6 +267,14 @@ void aio_notify(AioContext *ctx)
smp_mb();
if (ctx->notify_me) {
event_notifier_set(&ctx->notifier);
atomic_mb_set(&ctx->notified, true);
}
}
void aio_notify_accept(AioContext *ctx)
{
if (atomic_xchg(&ctx->notified, false)) {
event_notifier_test_and_clear(&ctx->notifier);
}
}