On 09/30/2017 12:38 AM, Joseph Qi wrote: > 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. Applied for 4.14, thanks. -- Jens Axboe
