Re: single aio thread is migrated crazily by scheduler

Vincent Guittot
2019-11-18T14:54:48Z

On Mon, 18 Nov 2019 at 10:21, Peter Zijlstra <peterz@infradead.org> wrote:
>
>
> +Vincent
>
> On Sat, Nov 16, 2019 at 10:40:05AM +1100, Dave Chinner wrote:
> > On Fri, Nov 15, 2019 at 03:08:43PM +0800, Ming Lei wrote:
> > > On Fri, Nov 15, 2019 at 03:56:34PM +1100, Dave Chinner wrote:
> > > > On Fri, Nov 15, 2019 at 09:08:24AM +0800, Ming Lei wrote:
> > > I can reproduce the issue with 4k block size on another RH system, and
> > > the login info of that system has been shared to you in RH BZ.
> > >
> > > 1)
> > > sysctl kernel.sched_min_granularity_ns=10000000
> > > sysctl kernel.sched_wakeup_granularity_ns=15000000
> >
> > So, these settings definitely influence behaviour.
> >
> > If these are set to kernel defaults (4ms and 3ms each):
> >
> > sysctl kernel.sched_min_granularity_ns=4000000
> > sysctl kernel.sched_wakeup_granularity_ns=3000000
> >
> > The migration problem largely goes away - the fio task migration
> > event count goes from ~2,000 a run down to 200/run.
> >
> > That indicates that the migration trigger is likely load/timing
> > based. The analysis below is based on the 10/15ms numbers above,
> > because it makes it so much easier to reproduce.
> >
> > > 2)
> > > ./xfs_complete 4k
> > >
> > > Then you should see 1k~1.5k fio io thread migration in above test,
> > > either v5.4-rc7(build with rhel8 config) or RHEL 4.18 kernel.
> >
> > Almost all the fio task migrations are coming from migration/X
> > kernel threads. i.e it's the scheduler active balancing that is
> > causing the fio thread to bounce around.
> >
> > This is typical a typical trace, trimmed to remove extraneous noise.
> > The fio process is running on CPU 10:
> >
> > fio-3185 [010] 50419.285954: sched_stat_runtime: comm=fio pid=3185 runtime=1004014 [ns] vruntime=27067882290 [ns]
> > fio-3185 [010] 50419.286953: sched_stat_runtime: comm=fio pid=3185 runtime=979458 [ns] vruntime=27068861748 [ns]
> > fio-3185 [010] 50419.287998: sched_stat_runtime: comm=fio pid=3185 runtime=1028471 [ns] vruntime=27069890219 [ns]
> > fio-3185 [010] 50419.289973: sched_stat_runtime: comm=fio pid=3185 runtime=989989 [ns] vruntime=27071836208 [ns]
> > fio-3185 [010] 50419.290958: sched_stat_runtime: comm=fio pid=3185 runtime=963914 [ns] vruntime=27072800122 [ns]
> > fio-3185 [010] 50419.291952: sched_stat_runtime: comm=fio pid=3185 runtime=972532 [ns] vruntime=27073772654 [ns]
> >
> > fio consumes CPU for several milliseconds, then:
> >
> > fio-3185 [010] 50419.292935: sched_stat_runtime: comm=fio pid=3185 runtime=966032 [ns] vruntime=27074738686 [ns]
> > fio-3185 [010] 50419.292941: sched_switch: fio:3185 [120] S ==> kworker/10:0:2763 [120]
> > kworker/10:0-2763 [010] 50419.292954: sched_stat_runtime: comm=kworker/10:0 pid=2763 runtime=13423 [ns] vruntime=27052479694 [ns]
> > kworker/10:0-2763 [010] 50419.292956: sched_switch: kworker/10:0:2763 [120] R ==> fio:3185 [120]
> > fio-3185 [010] 50419.293115: sched_waking: comm=kworker/10:0 pid=2763 prio=120 target_cpu=010
> > fio-3185 [010] 50419.293116: sched_stat_runtime: comm=fio pid=3185 runtime=160370 [ns] vruntime=27074899056 [ns]
> > fio-3185 [010] 50419.293118: sched_wakeup: kworker/10:0:2763 [120] success=1 CPU:010
> >
> > A context switch out to a kworker, then 13us later we immediately
> > switch back to the fio process, and go on running. No doubt
> > somewhere in what the fio process is doing, we queue up more work to
> > be run on the cpu, but the fio task keeps running
> > (due to CONFIG_PREEMPT=n).
> >
> > fio-3185 [010] 50419.293934: sched_stat_runtime: comm=fio pid=3185 runtime=803135 [ns] vruntime=27075702191 [ns]
> > fio-3185 [010] 50419.294936: sched_stat_runtime: comm=fio pid=3185 runtime=988478 [ns] vruntime=27076690669 [ns]
> > fio-3185 [010] 50419.295934: sched_stat_runtime: comm=fio pid=3185 runtime=982219 [ns] vruntime=27077672888 [ns]
> > fio-3185 [010] 50419.296935: sched_stat_runtime: comm=fio pid=3185 runtime=984781 [ns] vruntime=27078657669 [ns]
> > fio-3185 [010] 50419.297934: sched_stat_runtime: comm=fio pid=3185 runtime=981703 [ns] vruntime=27079639372 [ns]
> > fio-3185 [010] 50419.298937: sched_stat_runtime: comm=fio pid=3185 runtime=990057 [ns] vruntime=27080629429 [ns]
> > fio-3185 [010] 50419.299935: sched_stat_runtime: comm=fio pid=3185 runtime=977554 [ns] vruntime=27081606983 [ns]
> >
> > About 6ms later, CPU 0 kicks the active load balancer on CPU 10...
> >
> > <idle>-0 [000] 50419.300014: sched_waking: comm=migration/10 pid=70 prio=0 target_cpu=010
> > fio-3185 [010] 50419.300024: sched_wakeup: migration/10:70 [0] success=1 CPU:010
> > fio-3185 [010] 50419.300026: sched_stat_runtime: comm=fio pid=3185 runtime=79291 [ns] vruntime=27081686274 [ns]
> > fio-3185 [010] 50419.300027: sched_switch: fio:3185 [120] S ==> migration/10:70 [0]
> > migration/10-70 [010] 50419.300032: sched_migrate_task: comm=fio pid=3185 prio=120 orig_cpu=10 dest_cpu=12
> > migration/10-70 [010] 50419.300040: sched_switch: migration/10:70 [0] D ==> kworker/10:0:2763 [120]
> >
> > And 10us later the fio process is switched away, the active load
> > balancer work is run and migrates the fio process to CPU 12. Then...
> >
> > kworker/10:0-2763 [010] 50419.300048: sched_stat_runtime: comm=kworker/10:0 pid=2763 runtime=9252 [ns] vruntime=27062908308 [ns]
> > kworker/10:0-2763 [010] 50419.300062: sched_switch: kworker/10:0:2763 [120] R ==> swapper/10:0 [120]
> > <idle>-0 [010] 50419.300067: sched_waking: comm=kworker/10:0 pid=2763 prio=120 target_cpu=010
> > <idle>-0 [010] 50419.300069: sched_wakeup: kworker/10:0:2763 [120] success=1 CPU:010
> > <idle>-0 [010] 50419.300071: sched_switch: swapper/10:0 [120] S ==> kworker/10:0:2763 [120]
> > kworker/10:0-2763 [010] 50419.300073: sched_switch: kworker/10:0:2763 [120] R ==> swapper/10:0 [120]
> >
> > The kworker runs for another 10us and the CPU goes idle. Shortly
> > after this, CPU 12 is woken:
> >
> > <idle>-0 [012] 50419.300113: sched_switch: swapper/12:0 [120] S ==> fio:3185 [120]
> > fio-3185 [012] 50419.300596: sched_waking: comm=kworker/12:1 pid=227 prio=120 target_cpu=012
> > fio-3185 [012] 50419.300598: sched_stat_runtime: comm=fio pid=3185 runtime=561137 [ns] vruntime=20361153275 [ns]
> > fio-3185 [012] 50419.300936: sched_stat_runtime: comm=fio pid=3185 runtime=326187 [ns] vruntime=20361479462 [ns]
> > fio-3185 [012] 50419.301935: sched_stat_runtime: comm=fio pid=3185 runtime=981201 [ns] vruntime=20362460663 [ns]
> > fio-3185 [012] 50419.302935: sched_stat_runtime: comm=fio pid=3185 runtime=983160 [ns] vruntime=20363443823 [ns]
> > fio-3185 [012] 50419.303934: sched_stat_runtime: comm=fio pid=3185 runtime=983855 [ns] vruntime=20364427678 [ns]
> > fio-3185 [012] 50419.304934: sched_stat_runtime: comm=fio pid=3185 runtime=977757 [ns] vruntime=20365405435 [ns]
> > fio-3185 [012] 50419.305948: sched_stat_runtime: comm=fio pid=3185 runtime=999563 [ns] vruntime=20366404998 [ns]
> >
> >
> > and fio goes on running there. The pattern repeats very soon afterwards:
> >
> > <idle>-0 [000] 50419.314982: sched_waking: comm=migration/12 pid=82 prio=0 target_cpu=012
> > fio-3185 [012] 50419.314988: sched_wakeup: migration/12:82 [0] success=1 CPU:012
> > fio-3185 [012] 50419.314990: sched_stat_runtime: comm=fio pid=3185 runtime=46342 [ns] vruntime=20375268656 [ns]
> > fio-3185 [012] 50419.314991: sched_switch: fio:3185 [120] S ==> migration/12:82 [0]
> > migration/12-82 [012] 50419.314995: sched_migrate_task: comm=fio pid=3185 prio=120 orig_cpu=12 dest_cpu=5
> > migration/12-82 [012] 50419.315001: sched_switch: migration/12:82 [0] D ==> kworker/12:1:227 [120]
> > kworker/12:1-227 [012] 50419.315022: sched_stat_runtime: comm=kworker/12:1 pid=227 runtime=21453 [ns] vruntime=20359477889 [ns]
> > kworker/12:1-227 [012] 50419.315028: sched_switch: kworker/12:1:227 [120] R ==> swapper/12:0 [120]
> > <idle>-0 [005] 50419.315053: sched_switch: swapper/5:0 [120] S ==> fio:3185 [120]
> > fio-3185 [005] 50419.315286: sched_waking: comm=kworker/5:0 pid=2646 prio=120 target_cpu=005
> > fio-3185 [005] 50419.315288: sched_stat_runtime: comm=fio pid=3185 runtime=287737 [ns] vruntime=33779011507 [ns]
> >
> > And fio is now running on CPU 5 - it only ran on CPU 12 for about
> > 15ms. Hmmm:
> >
> > $ grep fio-3185 ~/tmp/sched.out | awk 'BEGIN {totcpu = 0.0; switches = 0.0; prev_waket = 0.0 }/sched_waking/ { cpu = $2; split($3, t, ":"); waket = t[1]; if (cpu != prev_cpu) { t_on_cpu = waket - prev_waket; if (prev_waket) { print "time on CPU", cpu, "was", t_on_cpu; totcpu += t_on_cpu; switches++ } prev_waket = waket; prev_cpu = cpu; } } END { print "switches", switches, "time on cpu", totcpu, "aver time on cpu", (totcpu / switches) } ' | stats --trim-outliers
> > switches 2211 time on cpu 30.0994 aver time on cpu 0.0136135
> > time on CPU [0-23(8.8823+/-6.2)] was 0.000331-0.330772(0.0134759+/-0.012)
> >
> > Yeah, the fio task averages 13.4ms on any given CPU before being
> > switched to another CPU. Mind you, the stddev is 12ms, so the range
> > of how long it spends on any one CPU is pretty wide (330us to
> > 330ms).
> >
> > IOWs, this doesn't look like a workqueue problem at all - this looks
> > like the scheduler is repeatedly making the wrong load balancing
> > decisions when mixing a very short runtime task (queued work) with a
> > long runtime task on the same CPU....
> >
> > This is not my area of expertise, so I have no idea why this might
> > be happening. Scheduler experts: is this expected behaviour? What
> > tunables directly influence the active load balancer (and/or CONFIG
> > options) to change how aggressive it's behaviour is?
>
> We typically only fall back to the active balancer when there is
> (persistent) imbalance and we fail to migrate anything else (of
> substance).
>
> The tuning mentioned has the effect of less frequent scheduling, IOW,
> leaving (short) tasks on the runqueue longer. This obviously means the
> load-balancer will have a bigger chance of seeing them.
>
> Now; it's been a while since I looked at the workqueue code but one
> possible explanation would be if the kworker that picks up the work item
> is pinned. That would make it runnable but not migratable, the exact
> situation in which we'll end up shooting the current task with active
> balance.

I agree with your explanation above

Although queue_work uses queue_work_on(WORK_CPU_UNBOUND,... At the
end, it uses a bounded workqueue and tries to queue the work on the
local cpu 1st even if the latter is already busy

>
> I'll go see if I can reproduce and stare at the workqueue code a bit.
>
> >
> > > Not reproduced the issue with 512 block size on the RH system yet,
> > > maybe it is related with my kernel config.
> >
> > I doubt it - this looks like a load specific corner case in the
> > scheduling algorithm....
> >
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@fromorbit.com