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[PATCH V7 08/18] blk-throttle: make sure expire time isn't too big

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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>
---
 block/blk-throttle.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 7878ec1..2073b48 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -590,6 +590,17 @@ 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 = 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 = max_expire;
 	mod_timer(&sq->pending_timer, expires);
 	throtl_log(sq, "schedule timer. delay=%lu jiffies=%lu",
 		   expires - jiffies, jiffies);
-- 
2.9.3

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[PATCH V7 00/18] blk-throttle: add .low limit Shaohua Li <shli@fb.com> - 2017-03-27 20:40 +0200
  [PATCH V7 09/18] blk-throttle: make throtl_slice tunable Shaohua Li <shli@fb.com> - 2017-03-27 20:40 +0200
  [PATCH V7 04/18] blk-throttle: add .low interface Shaohua Li <shli@fb.com> - 2017-03-27 20:40 +0200
  [PATCH V7 08/18] blk-throttle: make sure expire time isn't too big Shaohua Li <shli@fb.com> - 2017-03-27 20:40 +0200
  [PATCH V7 07/18] blk-throttle: add downgrade logic Shaohua Li <shli@fb.com> - 2017-03-27 20:40 +0200
  [PATCH V7 03/18] blk-throttle: add configure option for new .low interface Shaohua Li <shli@fb.com> - 2017-03-27 20:50 +0200

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