linux-kernel - Re: Problem with commit 31e77c93e432 "sched/fair: Update blocked load when newly idle"

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Message-ID: <CAKfTPtAisq0tQ_Kw3OptJYm=u7tSk6vyi3CFZJdhBxD=d=EN1g@mail.gmail.com>
Date:   Thu, 29 Mar 2018 09:41:40 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Problem with commit 31e77c93e432 "sched/fair: Update blocked load
 when newly idle"

On 28 March 2018 at 16:01, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> Hi,
>
> On 28 March 2018 at 12:37, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> Hi,
>>
>> On 03/24/2018 01:47 PM, Heiner Kallweit wrote:
>>>
>>> Am 24.03.2018 um 07:46 schrieb Vincent Guittot:
>>>>
>>>> Hi Heiner,
>>>>
>>>> Le Friday 23 Mar 2018 à 22:28:09 (+0100), Heiner Kallweit a écrit :
>>>>>
>>>>> Recently I started to get the following problems with linux-next:
>>>>>
>>>>> - When working via Putty/SSH on the system the console frequently
>>>>> freezes
>>>>>    for few seconds. Sometimes only opening a second console makes the
>>>>>    first one react again.
>>>>>
>>>>> - I get "INFO: rcu_sched detected stalls on CPUs/tasks:" warnings as
>>>>>    described in [1].
>>>>>
>>
>> I can't catch this issue on my Juno r0 (arm64 big.Little).
>>
>> root@...o:~# uname -r
>> 4.16.0-rc4-00198-g31e77c93e432
>>
>> I'm using openssh-client and openssh-server though.
>
> I think that I have finally been able to reproduce it on my hikey
> (octo cortex-A53) after unplugging 6 cores and waiting for almost 2
> hours
> This seems to happen only on dual core system as I haven't faced that
> before on the hikey which I have used for my tests
>

I'm finally not so sure that i have the right set up to reproduce the
problem as I haven't been able to reproduce it since.

Heiner,

How fast the problem happens on your board ?
Are you doing anything specific on the console that trigger the problem ?

Regards,
Vincent

>>
>>>>> Bisecting the issue resulted in:
>>>>>
>>>>> 31e77c93e432dec79c7d90b888bbfc3652592741 is the first bad commit
>>>>> commit 31e77c93e432dec79c7d90b888bbfc3652592741
>>>>> Author: Vincent Guittot <vincent.guittot@...aro.org>
>>>>> Date:   Wed Feb 14 16:26:46 2018 +0100
>>>>>
>>>>>      sched/fair: Update blocked load when newly idle
>>>>>
>>>>>      When NEWLY_IDLE load balance is not triggered, we might need to
>>>>> update the
>>>>>      blocked load anyway. We can kick an ilb so an idle CPU will take
>>>>> care of
>>>>>      updating blocked load or we can try to update them locally before
>>>>> entering
>>>>>      idle. In the latter case, we reuse part of the nohz_idle_balance.
>>>>>
>>>>> After reversing this commit at least the issue with the freezing console
>>>>> is gone. The second one appeared only sporadically, I still have to see
>>>>> whether it pops up again.
>>
>>
>> [...]
>>