Re: single aio thread is migrated crazily by scheduler

[Peter Zijlstra]

+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'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@xxxxxxxxxxxxx