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From 4b4fb1cccb8e0307658cee3bc90c77e5f1dde60a Mon Sep 17 00:00:00 2001
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From: Thomas Huth <thuth@redhat.com>
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Date: Fri, 9 Oct 2020 10:08:49 -0400
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Subject: [PATCH 13/14] aio-posix: completely stop polling when disabled
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RH-Author: Thomas Huth <thuth@redhat.com>
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Message-id: <20201009100849.264994-10-thuth@redhat.com>
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Patchwork-id: 98603
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O-Subject: [RHEL-8.4.0 qemu-kvm PATCH 9/9] aio-posix: completely stop polling when disabled
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Bugzilla: 1846975
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RH-Acked-by: Jens Freimann <jfreimann@redhat.com>
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RH-Acked-by: David Hildenbrand <david@redhat.com>
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RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
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From: Stefan Hajnoczi <stefanha@redhat.com>
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One iteration of polling is always performed even when polling is
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disabled. This is done because:
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1. Userspace polling is cheaper than making a syscall. We might get
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lucky.
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2. We must poll once more after polling has stopped in case an event
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occurred while stopping polling.
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However, there are downsides:
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1. Polling becomes a bottleneck when the number of event sources is very
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high. It's more efficient to monitor fds in that case.
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2. A high-frequency polling event source can starve non-polling event
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sources because ppoll(2)/epoll(7) is never invoked.
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This patch removes the forced polling iteration so that poll_ns=0 really
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means no polling.
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IOPS increases from 10k to 60k when the guest has 100
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virtio-blk-pci,num-queues=32 devices and 1 virtio-blk-pci,num-queues=1
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device because the large number of event sources being polled slows down
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the event loop.
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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Link: https://lore.kernel.org/r/20200305170806.1313245-2-stefanha@redhat.com
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Message-Id: <20200305170806.1313245-2-stefanha@redhat.com>
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(cherry picked from commit e4346192f1c2e1683a807b46efac47ef0cf9b545)
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Signed-off-by: Thomas Huth <thuth@redhat.com>
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Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
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---
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util/aio-posix.c | 22 +++++++++++++++-------
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1 file changed, 15 insertions(+), 7 deletions(-)
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diff --git a/util/aio-posix.c b/util/aio-posix.c
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index a4977f538e..abc396d030 100644
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--- a/util/aio-posix.c
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+++ b/util/aio-posix.c
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@@ -340,12 +340,13 @@ void aio_set_event_notifier_poll(AioContext *ctx,
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(IOHandler *)io_poll_end);
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}
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-static void poll_set_started(AioContext *ctx, bool started)
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+static bool poll_set_started(AioContext *ctx, bool started)
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{
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AioHandler *node;
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+ bool progress = false;
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if (started == ctx->poll_started) {
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- return;
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+ return false;
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}
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ctx->poll_started = started;
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@@ -367,8 +368,15 @@ static void poll_set_started(AioContext *ctx, bool started)
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if (fn) {
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fn(node->opaque);
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}
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+
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+ /* Poll one last time in case ->io_poll_end() raced with the event */
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+ if (!started) {
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+ progress = node->io_poll(node->opaque) || progress;
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+ }
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}
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qemu_lockcnt_dec(&ctx->list_lock);
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+
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+ return progress;
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}
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@@ -599,12 +607,12 @@ static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
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}
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}
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- poll_set_started(ctx, false);
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+ if (poll_set_started(ctx, false)) {
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+ *timeout = 0;
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+ return true;
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+ }
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- /* Even if we don't run busy polling, try polling once in case it can make
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- * progress and the caller will be able to avoid ppoll(2)/epoll_wait(2).
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- */
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- return run_poll_handlers_once(ctx, timeout);
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+ return false;
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}
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bool aio_poll(AioContext *ctx, bool blocking)
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--
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2.27.0
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