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

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Message-ID: <94cc6deb-f93e-60ec-5834-e84a8b98e73c@arm.com>
Date:   Mon, 17 Oct 2016 23:52:39 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Joseph Salisbury <joseph.salisbury@...onical.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Mike Galbraith <efault@....de>, omer.akram@...onical.com
Subject: Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

On 10/17/2016 02:54 PM, Vincent Guittot wrote:
> On 17 October 2016 at 15:19, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Mon, Oct 17, 2016 at 12:49:55PM +0100, Dietmar Eggemann wrote:

[...]

>>> BTW, I guess we can reach .tg_load_avg up to ~300000-400000 on such a system
>>> initially because systemd will create all ~100 services (and therefore the
>>> corresponding 2. level tg's) at once. In my previous example, there was 500ms
>>> between the creation of 2 tg's so there was a lot of decaying going on in between.
>>
>> Cute... on current kernels that translates to simply removing the call
>> to update_tg_load_avg(), lets see if we can figure out what goes
>> sideways first though, because it _should_ decay back out. And if that
> 
> yes, Reaching ~300000-400000 is not an issue in itself, the problem is
> that load_avg has decayed but it has not been reflected in
> tg->load_avg in the buggy case
> 
>> can fail here, I'm not seeing why that wouldn't fail elsewhere either.
>>
>> I'll see if I can reproduce this with a script creating heaps of cgroups
>> in a hurry, I have a total lack of system-disease on all my machines.


Something looks weird related to the use of for_each_possible_cpu(i) in
online_fair_sched_group() on my i5-3320M CPU (4 logical cpus).

In case I print out cpu id and the cpu masks inside the for_each_possible_cpu(i)
I get:

[ 5.462368]  cpu=0  cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
[ 5.462370]  cpu=1  cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
[ 5.462370]  cpu=2  cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
[ 5.462371]  cpu=3  cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
[ 5.462372] *cpu=4* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
[ 5.462373] *cpu=5* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
[ 5.462374] *cpu=6* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3
[ 5.462375] *cpu=7* cpu_possible_mask=0-7 cpu_online_mask=0-3 cpu_present_mask=0-3 cpu_active_mask=0-3

T430:/sys/fs/cgroup/cpu,cpuacct/system.slice# ls -l | grep '^d' | wc -l
80

/proc/sched_debug:

cfs_rq[0]:/system.slice
  ...
  .tg_load_avg                   : 323584
  ...

80 * 1024 * 4 (not existent cpu4-cpu7) = 327680 (with a little bit of decay,
this could be this extra load on the systen.slice tg)

Using for_each_online_cpu(i) instead of for_each_possible_cpu(i) in
online_fair_sched_group() works on this machine, i.e. the .tg_load_avg
of system.slice tg is 0 after startup.