From: Joseph Qi <qijiang.qj@xxxxxxxxxxxxxxx> There is a case which will lead to io stall. The case is described as follows. /test1 |-subtest1 /test2 |-subtest2 And subtest1 and subtest2 each has 32 queued bios already. Now upgrade to max. In throtl_upgrade_state, it will try to dispatch bios as follows: 1) tg=subtest1, do nothing; 2) tg=test1, transfer 32 queued bios from subtest1 to test1; no pending left, no need to schedule next dispatch; 3) tg=subtest2, do nothing; 4) tg=test2, transfer 32 queued bios from subtest2 to test2; no pending left, no need to schedule next dispatch; 5) tg=/, transfer 8 queued bios from test1 to /, 8 queued bios from test2 to /, 8 queued bios from test1 to /, and 8 queued bios from test2 to /; note that test1 and test2 each still has 16 queued bios left; 6) tg=/, try to schedule next dispatch, but since disptime is now (update in tg_update_disptime, wait=0), pending timer is not scheduled in fact; 7) In throtl_upgrade_state it totally dispatches 32 queued bios and with 32 left. test1 and test2 each has 16 queued bios; 8) throtl_pending_timer_fn sees the left over bios, but could do nothing, because throtl_select_dispatch returns 0, and test1/test2 has no pending tg. The blktrace shows the following: 8,32 0 0 2.539007641 0 m N throtl upgrade to max 8,32 0 0 2.539072267 0 m N throtl /test2 dispatch nr_queued=16 read=0 write=16 8,32 7 0 2.539077142 0 m N throtl /test1 dispatch nr_queued=16 read=0 write=16 So force schedule dispatch if there are pending children. Signed-off-by: Joseph Qi <qijiang.qj@xxxxxxxxxxxxxxx> --- block/blk-throttle.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/block/blk-throttle.c b/block/blk-throttle.c index 0fea76a..17816a0 100644 --- a/block/blk-throttle.c +++ b/block/blk-throttle.c @@ -1911,11 +1911,11 @@ static void throtl_upgrade_state(struct throtl_data *td) tg->disptime = jiffies - 1; throtl_select_dispatch(sq); - throtl_schedule_next_dispatch(sq, false); + throtl_schedule_next_dispatch(sq, true); } rcu_read_unlock(); throtl_select_dispatch(&td->service_queue); - throtl_schedule_next_dispatch(&td->service_queue, false); + throtl_schedule_next_dispatch(&td->service_queue, true); queue_work(kthrotld_workqueue, &td->dispatch_work); } -- 1.9.4
