linux-kernel - [PATCH 3/3] ringtest: poll for new buffers once before updating event index

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Date:   Thu,  6 Oct 2016 11:39:12 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     mst@...hat.com
Subject: [PATCH 3/3] ringtest: poll for new buffers once before updating event index

Updating the event index has a memory barrier and causes more work
on the other side to actually signal the event.  It is unnecessary
if a new buffer has already appeared on the ring, so poll once before
doing the update.

The effect of this on the 0.9 ring implementation is pretty much
invisible, but on the new-style ring it provides a consistent 3%
performance improvement.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 tools/virtio/ringtest/main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/virtio/ringtest/main.c b/tools/virtio/ringtest/main.c
index ac1269f87394..19543799c621 100644
--- a/tools/virtio/ringtest/main.c
+++ b/tools/virtio/ringtest/main.c
@@ -147,7 +147,7 @@ static void __attribute__((__flatten__)) run_guest(void)
 		assert(completed <= bufs);
 		assert(started <= bufs);
 		if (do_sleep) {
-			if (enable_call())
+			if (used_empty() && enable_call())
 				wait_for_call();
 		} else {
 			poll_used();
@@ -172,7 +172,7 @@ static void __attribute__((__flatten__)) run_host(void)
 
 	for (;;) {
 		if (do_sleep) {
-			if (enable_kick())
+			if (avail_empty() && enable_kick())
 				wait_for_kick();
 		} else {
 			poll_avail();
-- 
2.7.4