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From:   Sasha Levin <Alexander.Levin@microsoft.com>
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        "stable@vger.kernel.org" <stable@vger.kernel.org>
CC:     Shaohua Li <shli@fb.com>, Jens Axboe <axboe@fb.com>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.9 081/219] blk-throttle: make sure expire time
 isn't too big
Thread-Topic: [PATCH AUTOSEL for 4.9 081/219] blk-throttle: make sure expire
 time isn't too big
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Date:   Sat, 3 Mar 2018 22:28:42 +0000
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From: Shaohua Li <shli@fb.com>

[ Upstream commit 06cceedcca67a93ac7f7aa93bbd9980c7496d14e ]

cgroup could be throttled to a limit but when all cgroups cross high
limit, queue enters a higher state and so the group should be throttled
to a higher limit. It's possible the cgroup is sleeping because of
throttle and other cgroups don't dispatch IO any more. In this case,
nobody can trigger current downgrade/upgrade logic. To fix this issue,
we could either set up a timer to wakeup the cgroup if other cgroups are
idle or make sure this cgroup doesn't sleep too long. Setting up a timer
means we must change the timer very frequently. This patch chooses the
latter. Making cgroup sleep time not too big wouldn't change cgroup
bps/iops, but could make it wakeup more frequently, which isn't a big
issue because throtl_slice * 8 is already quite big.

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 block/blk-throttle.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index a3ea8260c94c..3a4c9a3c1427 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -499,6 +499,17 @@ static void throtl_dequeue_tg(struct throtl_grp *tg)
 static void throtl_schedule_pending_timer(struct throtl_service_queue *sq,
 					  unsigned long expires)
 {
+	unsigned long max_expire =3D jiffies + 8 * throtl_slice;
+
+	/*
+	 * Since we are adjusting the throttle limit dynamically, the sleep
+	 * time calculated according to previous limit might be invalid. It's
+	 * possible the cgroup sleep time is very long and no other cgroups
+	 * have IO running so notify the limit changes. Make sure the cgroup
+	 * doesn't sleep too long to avoid the missed notification.
+	 */
+	if (time_after(expires, max_expire))
+		expires =3D max_expire;
 	mod_timer(&sq->pending_timer, expires);
 	throtl_log(sq, "schedule timer. delay=3D%lu jiffies=3D%lu",
 		   expires - jiffies, jiffies);
--=20
2.14.1