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

[Vincent Guittot]

On 13 October 2016 at 20:49, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> On 13/10/16 17:48, Vincent Guittot wrote:
>> On 13 October 2016 at 17:52, Joseph Salisbury
>> <joseph.salisbury@xxxxxxxxxxxxx> wrote:
>>> On 10/13/2016 06:58 AM, Vincent Guittot wrote:
>>>> Hi,
>>>>
>>>> On 12 October 2016 at 18:21, Joseph Salisbury
>>>> <joseph.salisbury@xxxxxxxxxxxxx> wrote:
>>>>> On 10/12/2016 08:20 AM, Vincent Guittot wrote:
>>>>>> On 8 October 2016 at 13:49, Mike Galbraith <efault@xxxxxx> wrote:
>>>>>>> On Sat, 2016-10-08 at 13:37 +0200, Vincent Guittot wrote:
>>>>>>>> On 8 October 2016 at 10:39, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>>>>>>>> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>>>>>>>>>> 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.

This is normal as se->avg.load_avg is initialized to
scale_load_down(se->load.weight) and this se->avg.load_avg will be
added to tg_x[cpu]->cfs_rq->avg.load_avg when attached to the cfs_rq

>
> 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
>

This looks coherent as tg_x->load_avg = 5120 == sum of
cfs_rq[cpu]->load_avg and cfs_rq->tg_load_avg_contrib will be set to
cfs_rq[cpu]->load_avg at the end of the function call
In the buggy case, tg->load_avg = 381697 but sum of
cfs_rq[cpu]->tg_load_avg_contrib = 1013 = cfs_rq[cpu]->avg.load_avg so
there is no way to remove the 381697

>  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.

IMO, this comes from the decay of tg_y_2->se[cpu]->avg.load_avg before
being attached to tg_x

But I don't think that it's the same issue as the tg_x->load_avg is
still coherent with the sum of cfs_rq->avg.load_avg and the sume of
cfs_rq->tg_load_avg_contrib

>
> 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

There is several dozen of childs in system.slice but all

> related cfs_rq[x]:/autogroups-xxx on my Ubuntu-16.04 Desktop system.
>
> [...]