Re: [Qemu-devel] [PATCH 2/2] AioContext: optimize clearing the EventNotifier

[Fam Zheng <famz@redhat.com>]

On Sat, 07/18 22:21, Paolo Bonzini wrote:
> It is pretty rare for aio_notify to actually set the EventNotifier.  It
> can happen with worker threads such as thread-pool.c's, but otherwise it
> should never be set thanks to the ctx->notify_me optimization.  The
> previous patch, unfortunately, added an unconditional call to
> event_notifier_test_and_clear; now add a userspace fast path that
> avoids the call.
> 
> Note that it is not possible to do the same with event_notifier_set;
> it would break, as proved (again) by the included formal model.
> 
> This patch survived over 800 reboots on aarch64 KVM.

For aio-posix, how about keeping the optimization local which doesn't need
atomic operation? (no idea for win32 :)

diff --git a/aio-posix.c b/aio-posix.c
index 5c8b266..7e98123 100644
--- a/aio-posix.c
+++ b/aio-posix.c
@@ -236,6 +236,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
     int i, ret;
     bool progress;
     int64_t timeout;
+    int aio_notifier_idx = -1;
 
     aio_context_acquire(ctx);
     progress = false;
@@ -256,11 +257,18 @@ bool aio_poll(AioContext *ctx, bool blocking)
     assert(npfd == 0);
 
     /* fill pollfds */
+    i = 0;
     QLIST_FOREACH(node, &ctx->aio_handlers, node) {
         if (!node->deleted && node->pfd.events) {
             add_pollfd(node);
+            if (node->pfd.fd == event_notifier_get_fd(&ctx->notifier)) {
+                assert(aio_notifier_idx == -1);
+                aio_notifier_idx = i;
+            }
+            i++;
         }
     }
+    assert(aio_notifier_idx != -1);
 
     timeout = blocking ? aio_compute_timeout(ctx) : 0;
 
@@ -276,7 +284,9 @@ bool aio_poll(AioContext *ctx, bool blocking)
         aio_context_acquire(ctx);
     }
 
-    event_notifier_test_and_clear(&ctx->notifier);
+    if (pollfds[aio_notifier_idx].revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) {
+        event_notifier_test_and_clear(&ctx->notifier);
+    }
 
     /* if we have any readable fds, dispatch event */
     if (ret > 0) {