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aio-posix: ensure poll mode is left when aio_notify is called 

With aio=thread, adaptive polling makes latency worse rather than
better, because it delays the execution of the ThreadPool's
completion bottom half.

event_notifier_poll() does run while polling, detecting that
a bottom half was scheduled by a worker thread, but because
ctx->notifier is explicitly ignored in run_poll_handlers_once(),
scheduling the BH does not count as making progress and
run_poll_handlers() keeps running.  Fix this by recomputing
the deadline after *timeout could have changed.

With this change, ThreadPool still cannot participate in polling
but at least it does not suffer from extra latency.

Reported-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20190409122823.12416-1-pbonzini@redhat.com
Cc: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1553692145-86728-1-git-send-email-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190409122823.12416-1-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Paolo Bonzini 2019-04-09 14:28:23 +02:00 committed by Stefan Hajnoczi
parent 118f99442d
commit 993ed89f35
1 changed files with 8 additions and 4 deletions
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12
util/aio-posix.c
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@ -519,6 +519,10 @@ static bool run_poll_handlers_once(AioContext *ctx, int64_t *timeout)
if (!node->deleted && node->io_poll && if (!node->deleted && node->io_poll &&
aio_node_check(ctx, node->is_external) && aio_node_check(ctx, node->is_external) &&
node->io_poll(node->opaque)) { node->io_poll(node->opaque)) {
/*
* Polling was successful, exit try_poll_mode immediately
* to adjust the next polling time.
*/
*timeout = 0; *timeout = 0;
if (node->opaque != &ctx->notifier) { if (node->opaque != &ctx->notifier) {
progress = true; progress = true;
@ -558,8 +562,9 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
do { do {
progress = run_poll_handlers_once(ctx, timeout); progress = run_poll_handlers_once(ctx, timeout);
elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time; elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
} while (!progress && elapsed_time < max_ns max_ns = qemu_soonest_timeout(*timeout, max_ns);
&& !atomic_read(&ctx->poll_disable_cnt)); assert(!(max_ns && progress));
} while (elapsed_time < max_ns && !atomic_read(&ctx->poll_disable_cnt));
/* If time has passed with no successful polling, adjust *timeout to /* If time has passed with no successful polling, adjust *timeout to
* keep the same ending time. * keep the same ending time.
@ -585,8 +590,7 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
*/ */
static bool try_poll_mode(AioContext *ctx, int64_t *timeout) static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
{ {
/* See qemu_soonest_timeout() uint64_t hack */ int64_t max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
int64_t max_ns = MIN((uint64_t)*timeout, (uint64_t)ctx->poll_ns);
if (max_ns && !atomic_read(&ctx->poll_disable_cnt)) { if (max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
poll_set_started(ctx, true); poll_set_started(ctx, true);