blk-throttle: make sure expire time isn't too big

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>

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 7878ec1ed5e8976362160ebeb36261c1e992d7f5..2073b4839ba53a7d26f72ff4e33ed5c1a193ab07 100644 (file)
--- 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);