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Re: [Qemu-devel] [PATCH] linux-aio: keep processing events if MAX_EVENTS
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] linux-aio: keep processing events if MAX_EVENTS reached |
Date: |
Mon, 27 Jun 2016 18:42:33 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 |
On 27/06/2016 18:37, Stefan Hajnoczi wrote:
> Commit ccb9dc10129954d0bcd7814298ed445e684d5a2a ("linux-aio: Cancel BH
> if not needed") exposed a side-effect of scheduling the BH for nested
> event loops. When io_getevents(2) fetches MAX_EVENTS events then it's
> necessary to call it again. Failure to do so can lead to hung I/O
> because the eventfd has already been cleared and the completion BH will
> not run again.
>
> This patch fixes the hang by calling io_getevents(2) again if the events
> array was totally full.
>
> Reported-by: Roman Penyaev <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> block/linux-aio.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/block/linux-aio.c b/block/linux-aio.c
> index e468960..af15f85 100644
> --- a/block/linux-aio.c
> +++ b/block/linux-aio.c
> @@ -117,6 +117,7 @@ static void qemu_laio_completion_bh(void *opaque)
> LinuxAioState *s = opaque;
>
> /* Fetch more completion events when empty */
> +more_events:
> if (s->event_idx == s->event_max) {
> do {
> struct timespec ts = { 0 };
> @@ -146,6 +147,10 @@ static void qemu_laio_completion_bh(void *opaque)
> qemu_laio_process_completion(laiocb);
> }
>
> + if (s->event_idx == MAX_EVENTS) {
> + goto more_events; /* there might still be events waiting for us */
> + }
> +
> if (!s->io_q.plugged && !QSIMPLEQ_EMPTY(&s->io_q.pending)) {
> ioq_submit(s);
> }
>
Do we want to do it after submitting, in order to reduce latency and
perhaps even get some results immediately?
Paolo