Subject: Re: Continuous SD IO causes hung task messages

From: Russell King - ARM Linux admin <[email-masked]>
Date: Tue, 27 Aug 2019 15:52:17 +0100

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
> > <[email-masked]> 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.

-- 
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