Messages in this thread | | | Date | Tue, 27 Aug 2019 16:06:14 +0100 | From | Russell King - ARM Linux admin <> | Subject | Re: Continuous SD IO causes hung task messages |
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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@armlinux.org.uk> 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.
So this is a real issue with MMC, and not down to something in the arch.
I suspect the reason that single-CPU doesn't show it is because it is unable to keep multiple requests in flight.
-- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
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