Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 13/10/16 17:48, Vincent Guittot wrote:
> On 13 October 2016 at 17:52, Joseph Salisbury
> <joseph.salisbury@canonical.com> wrote:
>> On 10/13/2016 06:58 AM, Vincent Guittot wrote:
>>> Hi,
>>>
>>> On 12 October 2016 at 18:21, Joseph Salisbury
>>> <joseph.salisbury@canonical.com> wrote:
>>>> On 10/12/2016 08:20 AM, Vincent Guittot wrote:
>>>>> On 8 October 2016 at 13:49, Mike Galbraith <efault@gmx.de> wrote:
>>>>>> On Sat, 2016-10-08 at 13:37 +0200, Vincent Guittot wrote:
>>>>>>> On 8 October 2016 at 10:39, Ingo Molnar <mingo@kernel.org> wrote:
>>>>>>>> * Peter Zijlstra <peterz@infradead.org> wrote:
>>>>>>>>
>>>>>>>>> On Fri, Oct 07, 2016 at 03:38:23PM -0400, Joseph Salisbury wrote:
>>>>>>>>>> Hello Peter,
>>>>>>>>>>
>>>>>>>>>> A kernel bug report was opened against Ubuntu [0].  After a
>>>>>>>>>> kernel
>>>>>>>>>> bisect, it was found that reverting the following commit
>>>>>>>>>> resolved this bug:
>>>>>>>>>>
>>>>>>>>>> commit 3d30544f02120b884bba2a9466c87dba980e3be5
>>>>>>>>>> Author: Peter Zijlstra <peterz@infradead.org>
>>>>>>>>>> Date:   Tue Jun 21 14:27:50 2016 +0200
>>>>>>>>>>
>>>>>>>>>>     sched/fair: Apply more PELT fixes
>>>>>>> This patch only speeds up the update of task group load in order to
>>>>>>> reflect the new load balance but It should not change the final value
>>>>>>> and as a result the final behavior. I will try to reproduce it in my
>>>>>>> target later today
>>>>>> FWIW, I tried and failed w/wo autogroup on 4.8 and master.
>>>>> Me too
>>>>>
>>>>> Is it possible to get some dump of  /proc/sched_debug while the problem occurs ?
>>>>>
>>>>> Vincent
>>>>>
>>>>>>         -Mike
>>>> The output from /proc/shed_debug can be seen here:
>>>> http://paste.ubuntu.com/23312351/
>>> I have looked at the dump and there is something very odd for
>>> system.slice task group where the display manager is running.
>>> system.slice->tg_load_avg is around 381697 but  tg_load_avg is
>>> normally equal to Sum of system.slice[cpu]->tg_load_avg_contrib
>>> whereas Sum of system.slice[cpu]->tg_load_avg_contrib = 1013 in our
>>> case. We can have some differences because the dump of
>>> /proc/shed_debug is not atomic and some changes can happen but nothing
>>> like this difference.
>>>
>>> The main effect of this quite high value is that the weight/prio of
>>> the sched_entity that represents system.slice in root cfs_rq is very
>>> low (lower than task with the smallest nice prio) so the system.slice
>>> task group will not get the CPU quite often compared to the user.slice
>>> task group: less than 1% for the system.slice where lightDM and xorg
>>> are running compared 99% for the user.slice where the stress tasks are
>>> running. This is confirmed by the se->avg.util_avg value of the task
>>> groups which reflect how much time each task group is effectively
>>> running on a CPU:
>>> system.slice[CPU3].se->avg.util_avg = 8 whereas
>>> user.slice[CPU3].se->avg.util_avg = 991
>>>
>>> This difference of weight/priority explains why the system becomes
>>> unresponsive. For now, I can't explain is why
>>> system.slice->tg_load_avg = 381697 whereas is should be around 1013
>>> and how the patch can generate this situation.
>>>
>>> Is it possible to have a dump of /proc/sched_debug before starting
>>> stress command ? to check if the problem is there from the beginning
>>> but not seen because not overloaded. Or if it the problem comes when
>>> user starts to load the system
>> Here is the dump before stress is started:
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1627108/+attachment/4760437/+files/dump_nonbuggy
> 
> This one is ok.
> The dump indicates Sched Debug Version: v0.11, 4.8.0-11-generic
> #12~lp1627108Commit3d30544Reverted
> so this is without the culprit commit
> 
>>
>> Here it is after:
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1627108/+attachment/4760436/+files/dump_buggy
>>
> 
> This one has the exact same odds values for system.slice->tg_load_avg
> than the 1st dump that you sent yesterday
> The dump indicates Sched Debug Version: v0.11, 4.8.0-22-generic #24-Ubuntu
> So this dump has been done with a different kernel than for the dump above.
> As I can't find any stress task in the dump, i tend to believe that
> the dump has been done before starting the stress tasks and not after
> starting them. Can you confirm ?
> 
> If i'm right, it mean that the problem was already there before
> starting stress tasks.

Could it be a problem I'm also seeing on my ARM64 Juno (6 logical cpus) w/o systemd
and w/o autogroup (tip/sched/core 447976ef4fd0):

When I create a tg_root/tg_x/tg_y_1 and a tg_root/tg_x/tg_y_2 group, the tg_x->load_avg
becomes > 6*1024 before any tasks ran in it.

tg_x   : 0xffff800975800d80
tg_y_1 : 0xffff800975800c00
tg_y_2 : 0xffff80097543d200 

 mkdir-2177 [002] 117.235241: bprint: sched_online_group: tg=0xffff800975800d80 tg->parent=0xffff000008fd0300
 mkdir-2177 [002] 117.235244: bprint: online_fair_sched_group: tg=0xffff800975800d80 cpu=0
 mkdir-2177 [002] 117.235247: bprint: online_fair_sched_group: tg=0xffff800975800d80 cpu=1
 mkdir-2177 [002] 117.235249: bprint: online_fair_sched_group: tg=0xffff800975800d80 cpu=2
 mkdir-2177 [002] 117.235251: bprint: online_fair_sched_group: tg=0xffff800975800d80 cpu=3
 mkdir-2177 [002] 117.235253: bprint: online_fair_sched_group: tg=0xffff800975800d80 cpu=4
 mkdir-2177 [002] 117.235255: bprint: online_fair_sched_group: tg=0xffff800975800d80 cpu=5

 mkdir-2181 [002] 117.353155: bprint: sched_online_group: tg=0xffff800975800c00 tg->parent=0xffff800975800d80
 mkdir-2181 [002] 117.353158: bprint: online_fair_sched_group: tg=0xffff800975800c00 cpu=0
 mkdir-2181 [002] 117.353162: bprint: post_init_entity_util_avg: cpu=0 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=0 cfs_rq->avg.load_avg=1024 cfs_rq->tg_load_avg_contrib=0 delta=1024
 mkdir-2181 [002] 117.353164: bprint: online_fair_sched_group: tg=0xffff800975800c00 cpu=1
 mkdir-2181 [002] 117.353167: bprint: post_init_entity_util_avg: cpu=1 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=1024 cfs_rq->avg.load_avg=1024 cfs_rq->tg_load_avg_contrib=0 delta=1024
 mkdir-2181 [002] 117.353168: bprint: online_fair_sched_group: tg=0xffff800975800c00 cpu=2
 mkdir-2181 [002] 117.353171: bprint: post_init_entity_util_avg: cpu=2 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=2048 cfs_rq->avg.load_avg=1024 cfs_rq->tg_load_avg_contrib=0 delta=1024
 mkdir-2181 [002] 117.353173: bprint: online_fair_sched_group: tg=0xffff800975800c00 cpu=3
 mkdir-2181 [002] 117.353175: bprint: post_init_entity_util_avg: cpu=3 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=3072 cfs_rq->avg.load_avg=1024 cfs_rq->tg_load_avg_contrib=0 delta=1024
 mkdir-2181 [002] 117.353177: bprint: online_fair_sched_group: tg=0xffff800975800c00 cpu=4
 mkdir-2181 [002] 117.353179: bprint: post_init_entity_util_avg: cpu=4 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=4096 cfs_rq->avg.load_avg=1024 cfs_rq->tg_load_avg_contrib=0 delta=1024
 mkdir-2181 [002] 117.353180: bprint: online_fair_sched_group: tg=0xffff800975800c00 cpu=5
 mkdir-2181 [002] 117.353183: bprint: post_init_entity_util_avg: cpu=5 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=5120 cfs_rq->avg.load_avg=1024 cfs_rq->tg_load_avg_contrib=0 delta=1024

 mkdir-2185 [001] 117.502980: bprint: sched_online_group: tg=0xffff80097543d200 tg->parent=0xffff800975800d80
 mkdir-2185 [001] 117.502982: bprint: online_fair_sched_group: tg=0xffff80097543d200 cpu=0
 mkdir-2185 [001] 117.502987: bprint: post_init_entity_util_avg: cpu=0 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=6144 cfs_rq->avg.load_avg=1068 cfs_rq->tg_load_avg_contrib=1024 delta=44
 mkdir-2185 [001] 117.502988: bprint: online_fair_sched_group: tg=0xffff80097543d200 cpu=1
 mkdir-2185 [001] 117.502992: bprint: post_init_entity_util_avg: cpu=1 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=6188 cfs_rq->avg.load_avg=1058 cfs_rq->tg_load_avg_contrib=1024 delta=34
 mkdir-2185 [001] 117.502993: bprint: online_fair_sched_group: tg=0xffff80097543d200 cpu=2
 mkdir-2185 [001] 117.502996: bprint: post_init_entity_util_avg: cpu=2 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=6222 cfs_rq->avg.load_avg=1092 cfs_rq->tg_load_avg_contrib=1024 delta=68
 mkdir-2185 [001] 117.502998: bprint: online_fair_sched_group: tg=0xffff80097543d200 cpu=3
 mkdir-2185 [001] 117.503001: bprint: post_init_entity_util_avg: cpu=3 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=6290 cfs_rq->avg.load_avg=1069 cfs_rq->tg_load_avg_contrib=1024 delta=45
 mkdir-2185 [001] 117.503002: bprint: online_fair_sched_group: tg=0xffff80097543d200 cpu=4
 mkdir-2185 [001] 117.503005: bprint: post_init_entity_util_avg: cpu=4 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=6335 cfs_rq->avg.load_avg=1064 cfs_rq->tg_load_avg_contrib=1024 delta=40
 mkdir-2185 [001] 117.503006: bprint: online_fair_sched_group: tg=0xffff80097543d200 cpu=5
 mkdir-2185 [001] 117.503009: bprint: post_init_entity_util_avg: cpu=5 tg=0xffff800975800d80 tg_css_id=2 cfs_rq->tg->load_avg=*6375* cfs_rq->avg.load_avg=1061 cfs_rq->tg_load_avg_contrib=1024 delta=37

We add delta=1024 to tg_x->load_avg during tg_root/tg_x/tg_y_1 initialization but only
delta=~40 for tg_root/tg_x/tg_y_2.   

system.slice->tg_load_avg = 381697 is still pretty much higher and AFAICS, it's an
i5-5300U CPU so only 4 logical cpus. 

How many system.slice/foo's actually exists in this system? I only see user.slice
related cfs_rq[x]:/autogroups-xxx on my Ubuntu-16.04 Desktop system.

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