[dpdk-stable] patch 'vhost: avoid concurrency when logging dirty pages' has been queued to stable release 18.02.2

[luca.boccassi at gmail.com]

Hi,

FYI, your patch has been queued to stable release 18.02.2

Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable yet.
It will be pushed if I get no objections before 05/02/18. So please
shout if anyone has objections.

Thanks.

Luca Boccassi

---
>From a5d642c446cc3a3be41694715d99f3e9f9ea44df Mon Sep 17 00:00:00 2001
From: Maxime Coquelin <maxime.coquelin at redhat.com>
Date: Wed, 21 Mar 2018 16:44:13 +0100
Subject: [PATCH] vhost: avoid concurrency when logging dirty pages

[ upstream commit 394313fff39d0f994325c47f7eab39daf5dc9e11 ]

This patch aims at fixing a migration performance regression
faced since atomic operation is used to log pages as dirty when
doing live migration.

Instead of setting a single bit by doing an atomic read-modify-write
operation to log a page as dirty, this patch write 0xFF to the
corresponding byte, and so logs 8 page as dirty.

The advantage is that it avoids concurrent atomic operations by
multiple PMD threads, the drawback is that some clean pages are
marked as dirty and so are transferred twice.

Fixes: 897f13a1f726 ("vhost: make page logging atomic")

Signed-off-by: Maxime Coquelin <maxime.coquelin at redhat.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan at intel.com>
---
 lib/librte_vhost/vhost.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index bab0b456e..a4f23f714 100644
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -247,18 +247,14 @@ struct virtio_net {
 #define VHOST_LOG_PAGE	4096
 
 /*
- * Atomically set a bit in memory.
+ * Mark all pages belonging to the same dirty log bitmap byte
+ * as dirty. The goal is to avoid concurrency between different
+ * threads doing atomic read-modify-writes on the same byte.
  */
-static __rte_always_inline void
-vhost_set_bit(unsigned int nr, volatile uint8_t *addr)
-{
-	__sync_fetch_and_or_8(addr, (1U << nr));
-}
-
 static __rte_always_inline void
 vhost_log_page(uint8_t *log_base, uint64_t page)
 {
-	vhost_set_bit(page % 8, &log_base[page / 8]);
+	log_base[page / 8] = 0xff;
 }
 
 static __rte_always_inline void
-- 
2.14.2