linux-kernel - [PATCH 4.14 172/185] md: only call set_in_sync() when it is expected to succeed.

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Message-Id: <20191003154519.575402106@linuxfoundation.org>
Date:   Thu,  3 Oct 2019 17:54:10 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, NeilBrown <neilb@...e.com>,
        Song Liu <songliubraving@...com>,
        Jack Wang <jinpu.wang@...ud.ionos.com>
Subject: [PATCH 4.14 172/185] md: only call set_in_sync() when it is expected to succeed.

From: NeilBrown <neilb@...e.com>

commit 480523feae581ab714ba6610388a3b4619a2f695 upstream.

Since commit 4ad23a976413 ("MD: use per-cpu counter for
writes_pending"), set_in_sync() is substantially more expensive: it
can wait for a full RCU grace period which can be 10s of milliseconds.

So we should only call it when the cost is justified.

md_check_recovery() currently calls set_in_sync() every time it finds
anything to do (on non-external active arrays).  For an array
performing resync or recovery, this will be quite often.
Each call will introduce a delay to the md thread, which can noticeable
affect IO submission latency.

In md_check_recovery() we only need to call set_in_sync() if
'safemode' was non-zero at entry, meaning that there has been not
recent IO.  So we save this "safemode was nonzero" state, and only
call set_in_sync() if it was non-zero.

This measurably reduces mean and maximum IO submission latency during
resync/recovery.

Reported-and-tested-by: Jack Wang <jinpu.wang@...ud.ionos.com>
Fixes: 4ad23a976413 ("MD: use per-cpu counter for writes_pending")
Cc: stable@...r.kernel.org (v4.12+)
Signed-off-by: NeilBrown <neilb@...e.com>
Signed-off-by: Song Liu <songliubraving@...com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/md/md.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -8765,6 +8765,7 @@ void md_check_recovery(struct mddev *mdd

 	if (mddev_trylock(mddev)) {
 		int spares = 0;
+		bool try_set_sync = mddev->safemode != 0;

 		if (!mddev->external && mddev->safemode == 1)
 			mddev->safemode = 0;
@@ -8810,7 +8811,7 @@ void md_check_recovery(struct mddev *mdd
 			}
 		}

-		if (!mddev->external && !mddev->in_sync) {
+		if (try_set_sync && !mddev->external && !mddev->in_sync) {
 			spin_lock(&mddev->lock);
 			set_in_sync(mddev);
 			spin_unlock(&mddev->lock);