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[Qemu-block] [PATCH for-4.0] aio-posix: ensure poll mode is left when ai

From: Paolo Bonzini
Subject: [Qemu-block] [PATCH for-4.0] aio-posix: ensure poll mode is left when aio_notify is called
Date: Wed, 27 Mar 2019 14:09:05 +0100

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 <address@hidden>
Cc: Stefan Hajnoczi <address@hidden>
Cc: Kevin Wolf <address@hidden>
Cc: address@hidden
Signed-off-by: Paolo Bonzini <address@hidden>
 util/aio-posix.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/util/aio-posix.c b/util/aio-posix.c
index 6fbfa79..b166cda 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 = MIN(*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.

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