AioContext: optimize clearing the EventNotifier
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
Stefan Hajnoczi
parent
21a03d17f2
commit
05e514b1d4
|
|
@ -276,7 +276,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
|
|||
aio_context_acquire(ctx);
|
||||
}
|
||||
|
||||
event_notifier_test_and_clear(&ctx->notifier);
|
||||
aio_notify_accept(ctx);
|
||||
|
||||
/* if we have any readable fds, dispatch event */
|
||||
if (ret > 0) {
|
||||
|
|
|
|||
|
|
@ -338,7 +338,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
|
|||
}
|
||||
|
||||
if (first) {
|
||||
event_notifier_test_and_clear(&ctx->notifier);
|
||||
aio_notify_accept(ctx);
|
||||
progress |= aio_bh_poll(ctx);
|
||||
first = false;
|
||||
}
|
||||
|
|
|
|||
10
async.c
10
async.c
|
|
@ -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);
|
||||
}
|
||||
}
|
||||
|
||||
|
|
|
|||
|
|
@ -0,0 +1,152 @@
|
|||
/*
|
||||
* This model describes the interaction between ctx->notified
|
||||
* and ctx->notifier.
|
||||
*
|
||||
* Author: Paolo Bonzini <pbonzini@redhat.com>
|
||||
*
|
||||
* This file is in the public domain. If you really want a license,
|
||||
* the WTFPL will do.
|
||||
*
|
||||
* To verify the buggy version:
|
||||
* spin -a -DBUG1 docs/aio_notify_bug.promela
|
||||
* gcc -O2 pan.c
|
||||
* ./a.out -a -f
|
||||
* (or -DBUG2)
|
||||
*
|
||||
* To verify the fixed version:
|
||||
* spin -a docs/aio_notify_bug.promela
|
||||
* gcc -O2 pan.c
|
||||
* ./a.out -a -f
|
||||
*
|
||||
* Add -DCHECK_REQ to test an alternative invariant and the
|
||||
* "notify_me" optimization.
|
||||
*/
|
||||
|
||||
int notify_me;
|
||||
bool notified;
|
||||
bool event;
|
||||
bool req;
|
||||
bool notifier_done;
|
||||
|
||||
#ifdef CHECK_REQ
|
||||
#define USE_NOTIFY_ME 1
|
||||
#else
|
||||
#define USE_NOTIFY_ME 0
|
||||
#endif
|
||||
|
||||
#ifdef BUG
|
||||
#error Please define BUG1 or BUG2 instead.
|
||||
#endif
|
||||
|
||||
active proctype notifier()
|
||||
{
|
||||
do
|
||||
:: true -> {
|
||||
req = 1;
|
||||
if
|
||||
:: !USE_NOTIFY_ME || notify_me ->
|
||||
#if defined BUG1
|
||||
/* CHECK_REQ does not detect this bug! */
|
||||
notified = 1;
|
||||
event = 1;
|
||||
#elif defined BUG2
|
||||
if
|
||||
:: !notified -> event = 1;
|
||||
:: else -> skip;
|
||||
fi;
|
||||
notified = 1;
|
||||
#else
|
||||
event = 1;
|
||||
notified = 1;
|
||||
#endif
|
||||
:: else -> skip;
|
||||
fi
|
||||
}
|
||||
:: true -> break;
|
||||
od;
|
||||
notifier_done = 1;
|
||||
}
|
||||
|
||||
#define AIO_POLL \
|
||||
notify_me++; \
|
||||
if \
|
||||
:: !req -> { \
|
||||
if \
|
||||
:: event -> skip; \
|
||||
fi; \
|
||||
} \
|
||||
:: else -> skip; \
|
||||
fi; \
|
||||
notify_me--; \
|
||||
\
|
||||
atomic { old = notified; notified = 0; } \
|
||||
if \
|
||||
:: old -> event = 0; \
|
||||
:: else -> skip; \
|
||||
fi; \
|
||||
\
|
||||
req = 0;
|
||||
|
||||
active proctype waiter()
|
||||
{
|
||||
bool old;
|
||||
|
||||
do
|
||||
:: true -> AIO_POLL;
|
||||
od;
|
||||
}
|
||||
|
||||
/* Same as waiter(), but disappears after a while. */
|
||||
active proctype temporary_waiter()
|
||||
{
|
||||
bool old;
|
||||
|
||||
do
|
||||
:: true -> AIO_POLL;
|
||||
:: true -> break;
|
||||
od;
|
||||
}
|
||||
|
||||
#ifdef CHECK_REQ
|
||||
never {
|
||||
do
|
||||
:: req -> goto accept_if_req_not_eventually_false;
|
||||
:: true -> skip;
|
||||
od;
|
||||
|
||||
accept_if_req_not_eventually_false:
|
||||
if
|
||||
:: req -> goto accept_if_req_not_eventually_false;
|
||||
fi;
|
||||
assert(0);
|
||||
}
|
||||
|
||||
#else
|
||||
/* There must be infinitely many transitions of event as long
|
||||
* as the notifier does not exit.
|
||||
*
|
||||
* If event stayed always true, the waiters would be busy looping.
|
||||
* If event stayed always false, the waiters would be sleeping
|
||||
* forever.
|
||||
*/
|
||||
never {
|
||||
do
|
||||
:: !event -> goto accept_if_event_not_eventually_true;
|
||||
:: event -> goto accept_if_event_not_eventually_false;
|
||||
:: true -> skip;
|
||||
od;
|
||||
|
||||
accept_if_event_not_eventually_true:
|
||||
if
|
||||
:: !event && notifier_done -> do :: true -> skip; od;
|
||||
:: !event && !notifier_done -> goto accept_if_event_not_eventually_true;
|
||||
fi;
|
||||
assert(0);
|
||||
|
||||
accept_if_event_not_eventually_false:
|
||||
if
|
||||
:: event -> goto accept_if_event_not_eventually_false;
|
||||
fi;
|
||||
assert(0);
|
||||
}
|
||||
#endif
|
||||
|
|
@ -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.
|
||||
*
|
||||
|
|
|
|||
Loading…
Reference in New Issue