Commit 05e514b1 authored by Paolo Bonzini's avatar Paolo Bonzini Committed by Stefan Hajnoczi

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: default avatarPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: default avatarFam Zheng <famz@redhat.com>
Tested-by: default avatarRichard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-7-git-send-email-pbonzini@redhat.com
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
parent 21a03d17
......@@ -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;
}
......
......@@ -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);
}
}
......
/*
* 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.
*
......
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