Paolo Bonzini <pbonz...@redhat.com> writes:
> 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 <s...@redhat.com>
> Cc: Stefan Hajnoczi <stefa...@gmail.com>
> Cc: Kevin Wolf <kw...@redhat.com>
> Cc: qemu-bl...@nongnu.org
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> Message-Id: <1553692145-86728-1-git-send-email-pbonz...@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
> v1->v2: use qemu_soonest_timeout to handle timeout == -1
> util/aio-posix.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/util/aio-posix.c b/util/aio-posix.c
> index 6fbfa7924f..db11021287 100644
> --- a/util/aio-posix.c
> +++ b/util/aio-posix.c
> @@ -519,6 +519,10 @@ static bool run_poll_handlers_once(AioContext *ctx,
> int64_t *timeout)
> if (!node->deleted && node->io_poll &&
> aio_node_check(ctx, node->is_external) &&
> node->io_poll(node->opaque)) {
> + /*
> + * Polling was successful, exit try_poll_mode immediately
> + * to adjust the next polling time.
> + */
> *timeout = 0;
> if (node->opaque != &ctx->notifier) {
> progress = true;
> @@ -558,8 +562,9 @@ static bool run_poll_handlers(AioContext *ctx, int64_t
> max_ns, int64_t *timeout)
> do {
> progress = run_poll_handlers_once(ctx, timeout);
> elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
> - } while (!progress && elapsed_time < max_ns
> - && !atomic_read(&ctx->poll_disable_cnt));
> + max_ns = qemu_soonest_timeout(*timeout, max_ns);
> + 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
> * 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)
> {
> - /* See qemu_soonest_timeout() uint64_t hack */
> - int64_t max_ns = MIN((uint64_t)*timeout, (uint64_t)ctx->poll_ns);
> + int64_t max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
>
> if (max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
> poll_set_started(ctx, true);Thanks, this one does the trick. Reviewed-by: Sergio Lopez <s...@redhat.com>
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