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From:   brookxu <brookxu.cn@gmail.com>
To:     axboe@kernel.dk
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH v3] block: fix the problem of io_ticks becoming smaller
Date:   Tue,  6 Jul 2021 05:47:26 +0800
Message-Id: <1625521646-1069-1-git-send-email-brookxu.cn@gmail.com>
Precedence: bulk

From: Chunguang Xu <brookxu@tencent.com>

On the IO submission path, blk_account_io_start() may interrupt
the system interruption. When the interruption returns, the value
of part->stamp may have been updated by other cores, so the time
value collected before the interruption may be less than part->
stamp. So when this happens, we should do nothing to make io_ticks
more accurate? For kernels less than 5.0, this may cause io_ticks
to become smaller, which in turn may cause abnormal ioutil values.

v3: update the commit log
v2: sorry, fix compile error due to the missed ')'

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
---
 block/blk-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 514838c..bbf56ae 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1243,7 +1243,7 @@ static void update_io_ticks(struct block_device *part, unsigned long now,
 	unsigned long stamp;
 again:
 	stamp = READ_ONCE(part->bd_stamp);
-	if (unlikely(stamp != now)) {
+	if (unlikely(time_after(now, stamp))) {
 		if (likely(cmpxchg(&part->bd_stamp, stamp, now) == stamp))
 			__part_stat_add(part, io_ticks, end ? now - stamp : 1);
 	}
-- 
1.8.3.1