Re: single aio thread is migrated crazily by scheduler

[Juri Lelli <juri.lelli@redhat.com>]

Hi,

On 02/12/19 10:46, Ming Lei wrote:
> On Thu, Nov 28, 2019 at 10:53:33AM +0100, Vincent Guittot wrote:
> > On Thu, 28 Nov 2019 at 10:40, Hillf Danton <hdanton@sina.com> wrote:
> > >
> > >
> > > On Sat, 16 Nov 2019 10:40:05 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).
> > > >
> > > Hey Dave
> > >
> > > > IOWs, this doesn't look like a workqueue problem at all - this looks
> > >
> > > Surprised to see you're so sure it has little to do with wq,
> > >
> > > > 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....
> > > >
> > > and it helps more to know what is driving lb to make decisions like
> > > this. Because for 70+ per cent of communters in cities like London it
> > > is supposed tube is better than cab on work days, the end_io cb is
> > > tweaked to be a lookalike of execute_in_process_context() in the diff
> > > with the devoted s_dio_done_wq taken out of account. It's interesting
> > > to see what difference lb will make in the tube environment.
> > >
> > > Hillf
> > >
> > > > 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?
> > > >
> > > > > 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
> > >
> > > --- a/fs/iomap/direct-io.c
> > > +++ b/fs/iomap/direct-io.c
> > > @@ -157,10 +157,8 @@ static void iomap_dio_bio_end_io(struct
> > >                         WRITE_ONCE(dio->submit.waiter, NULL);
> > >                         blk_wake_io_task(waiter);
> > >                 } else if (dio->flags & IOMAP_DIO_WRITE) {
> > > -                       struct inode *inode = file_inode(dio->iocb->ki_filp);
> > > -
> > >                         INIT_WORK(&dio->aio.work, iomap_dio_complete_work);
> > > -                       queue_work(inode->i_sb->s_dio_done_wq, &dio->aio.work);
> > > +                       schedule_work(&dio->aio.work);
> > 
> > I'm not sure that this will make a real difference because it ends up
> > to call queue_work(system_wq, ...) and system_wq is bounded as well so
> > the work will still be pinned to a CPU
> > Using system_unbound_wq should make a difference because it doesn't
> > pin the work on a CPU
> >  +                       queue_work(system_unbound_wq, &dio->aio.work);
> 
> Indeed, just run a quick test on my KVM guest, looks the following patch
> makes a difference:
> 
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 9329ced91f1d..2f4488b0ecec 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -613,7 +613,8 @@ int sb_init_dio_done_wq(struct super_block *sb)
>  {
>         struct workqueue_struct *old;
>         struct workqueue_struct *wq = alloc_workqueue("dio/%s",
> -                                                     WQ_MEM_RECLAIM, 0,
> +                                                     WQ_MEM_RECLAIM |
> +                                                     WQ_UNBOUND, 0,
>                                                       sb->s_id);

Chiming in to report that, after Vincent suggested to try the very same
change out on a private IRC conversation, I saw the very same positive
effects. E.g.,

Baseline is using 'slow' sched parameters (10ms/15ms), baseline 3/4 is
same kernel just with default 'fast' sched params and dio unbound 10/15
is a kernel patched with the above patch and using 'slow' sched params.

fsperf-4k-5G                            baseline 3/4    dio unbound 10/15	
                                        improv.	        improv.
sequential initial write throughput	-0.12	        -0.87
                         latency	1.15	        2.87
           overwrite     throughput	1186.32	        1232.74
                         latency	11.35           11.26
           read          throughput     0.80            -2.38
                         latency        2.79            3.03
random     initial write throughput     84.73           105.34
                         latency        0.00            1.14
           overwrite     throughput     254.27          273.17
                         latency        1.23            7.37
           read	         throughput	40.05           3.24
                         latency        8.03            7.36

Best,

Juri