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md: only call set_in_sync() when it is expected to succeed.

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Since commit 4ad23a9764 ("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 <[email protected]>
Fixes: 4ad23a9764 ("MD: use per-cpu counter for writes_pending")
Cc: [email protected] (v4.12+)
Signed-off-by: NeilBrown <[email protected]>
Signed-off-by: Song Liu <[email protected]>
This commit is contained in:
NeilBrown 2019-08-20 10:21:09 +10:00 • committed by Song Liu
commit 480523feae
1 changed files with 2 additions and 1 deletions
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3
drivers/md/md.c
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@ -8932,6 +8932,7 @@ void md_check_recovery(struct mddev *mddev)
if (mddev_trylock(mddev)) {
int spares = 0;
bool try_set_sync = mddev->safemode != 0;
if (!mddev->external && mddev->safemode == 1)
mddev->safemode = 0;
@ -8977,7 +8978,7 @@ void md_check_recovery(struct mddev *mddev)
}
}
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);