On Tue, 27 Aug 2019 at 17:06, Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx> wrote: > > On Tue, Aug 27, 2019 at 03:52:17PM +0100, Russell King - ARM Linux admin wrote: > > On Tue, Aug 27, 2019 at 03:36:34PM +0100, Russell King - ARM Linux admin wrote: > > > On Tue, Aug 27, 2019 at 03:55:23PM +0200, Ulf Hansson wrote: > > > > On Tue, 27 Aug 2019 at 15:43, Russell King - ARM Linux admin > > > > <linux@xxxxxxxxxxxxxxx> wrote: > > > > > > > > > > Hi, > > > > > > > > > > While dd'ing the contents of a SD card, I get hung task timeout > > > > > messages as per below. However, the dd is making progress. Any > > > > > ideas? > > > > > > > > > > Presumably, mmc_rescan doesn't get a look-in while IO is progressing > > > > > for the card? > > > > > > > > Is it a regression? > > > > > > > > There not much of recent mmc core and mmc block changes, that I can > > > > think of at this point. > > > > > > No idea - I just repaired the SD socket after the D2 line became > > > disconnected, and decided to run that command as a test. > > > > > > > > ARM64 host, Macchiatobin, uSD card. > > > > > > > > What mmc host driver is it? mmci? > > > > > > sdhci-xenon. > > > > > > I'm just trying with one CPU online, then I'll try with two. My > > > suspicion is that there's a problem in the ARM64 arch code where > > > unlocking a mutex doesn't get noticed on other CPUs. > > > > > > Hmm, I thought I'd try bringing another CPU online, but it seems > > > like the ARM64 CPU hotplug code is broken: > > > > > > [ 3552.029689] CPU1: shutdown > > > [ 3552.031099] psci: CPU1 killed. > > > [ 3949.835212] CPU1: failed to come online > > > [ 3949.837753] CPU1: failed in unknown state : 0x0 > > > > > > which means I can only take CPUs down, I can't bring them back > > > online without rebooting. > > > > Okay, running on a single CPU shows no problems. > > > > Running on four CPUs (as originally) shows that the kworker thread > > _never_ gets scheduled, so the warning is not false. > > > > With three CPUs, same problem. > > > > root@arm-d06300000000:~# ps aux | grep ' D ' > > root 34 0.0 0.0 0 0 ? D 15:38 0:00 [kworker/1:1+events_freezable] > > root@arm-d06300000000:~# cat /proc/34/sched > > kworker/1:1 (34, #threads: 1) > > ------------------------------------------------------------------- > > se.exec_start : 318689.992440 > > se.vruntime : 37750.882357 > > se.sum_exec_runtime : 9.421240 > > se.nr_migrations : 0 > > nr_switches : 1174 > > nr_voluntary_switches : 1171 > > nr_involuntary_switches : 3 > > se.load.weight : 1048576 > > se.runnable_weight : 1048576 > > se.avg.load_sum : 6 > > se.avg.runnable_load_sum : 6 > > se.avg.util_sum : 5170 > > se.avg.load_avg : 0 > > se.avg.runnable_load_avg : 0 > > se.avg.util_avg : 0 > > se.avg.last_update_time : 318689991680 > > se.avg.util_est.ewma : 10 > > se.avg.util_est.enqueued : 0 > > policy : 0 > > prio : 120 > > clock-delta : 0 > > > > The only thing that changes there is "clock-delta". When I kill the > > dd, I get: > > > > root@arm-d06300000000:~# cat /proc/34/sched > > kworker/1:1 (34, #threads: 1) > > ------------------------------------------------------------------- > > se.exec_start : 574025.791680 > > se.vruntime : 79996.657300 > > se.sum_exec_runtime : 10.916400 > > se.nr_migrations : 0 > > nr_switches : 1403 > > nr_voluntary_switches : 1400 > > nr_involuntary_switches : 3 > > se.load.weight : 1048576 > > se.runnable_weight : 1048576 > > se.avg.load_sum : 15 > > se.avg.runnable_load_sum : 15 > > se.avg.util_sum : 15007 > > se.avg.load_avg : 0 > > se.avg.runnable_load_avg : 0 > > se.avg.util_avg : 0 > > se.avg.last_update_time : 574025791488 > > se.avg.util_est.ewma : 10 > > se.avg.util_est.enqueued : 0 > > policy : 0 > > prio : 120 > > clock-delta : 40 > > > > so the thread makes forward progress. > > > > Down to two CPUs: > > > > root@arm-d06300000000:~# ps aux | grep ' D ' > > root 34 0.0 0.0 0 0 ? D 15:38 0:00 [kworker/1:1+events_freezable] > > > > Same symptoms. dd and md5sum switch between CPU 0 and CPU1. > > Hmm. > > static blk_status_t mmc_mq_queue_rq(struct blk_mq_hw_ctx *hctx, > const struct blk_mq_queue_data *bd) > > mq->in_flight[issue_type] += 1; > get_card = (mmc_tot_in_flight(mq) == 1); > > if (get_card) > mmc_get_card(card, &mq->ctx); > > mmc_get_card() gets the host lock according to the card. > > So, if we always have requests in flight (which is probably the case > here) we never drop the host lock, and mmc_rescan() never gets a look > in - hence blocking the kworker. Ahh, you are right. However, this isn't a new problem I believe. Even if we did some re-work of the locking mechanism while converting to blk-mq, I still think the worker could starve the mmc_rescan work before. In practice this shouldn't be a problem though, unless I am overlooking something. This is because it's not until there is an I/O error, that causes the block worker to release the host, to it makes sense to let mmc_rescan to claim the host to check for card removal. > > So this is a real issue with MMC, and not down to something in the > arch. Yep, thanks for running the test and providing more details! > > I suspect the reason that single-CPU doesn't show it is because it is > unable to keep multiple requests in flight. Yes, most likely. Now, how to solve this problem I need to think more about.... FYI: The long term goal has been to try to remove the big fat host lock altogether and slowly we have moved more an more things to be executed as a part of the block worker, which is one of the needed steps. Like the mmc ioctls for example... Kind regards Uffe
