linux-kernel - Re: [BUG] schedutil governor produces regular max freq spikes because of lockup detector watchdog threads

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Message-ID: <CAJZ5v0gytg+ESkcH0uW+AYKaeiT_cb8vdUnhPzds13CaDR521g@mail.gmail.com>
Date:   Wed, 10 Jan 2018 13:35:46 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Leonard Crestez <leonard.crestez@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Anson Huang <anson.huang@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [BUG] schedutil governor produces regular max freq spikes because
 of lockup detector watchdog threads

On Wed, Jan 10, 2018 at 11:54 AM, Juri Lelli <juri.lelli@...hat.com> wrote:
> On 09/01/18 16:50, Rafael J. Wysocki wrote:
>> On Tue, Jan 9, 2018 at 3:43 PM, Leonard Crestez <leonard.crestez@....com> wrote:
>
> [...]
>
>> > Every 4 seconds (really it's /proc/sys/kernel/watchdog_thresh * 2 / 5
>> > and watchdog_thresh defaults to 10). There is a per-cpu hrtimer which
>> > wakes the per-cpu thread in order to check that tasks can still
>> > execute, this works very well against bugs like infinite loops in
>> > softirq mode. The timers are synchronized initially but can get
>> > staggered (for example by hotplug).
>> >
>> > My guess is that it's only marked RT so that it executes ahead of other
>> > threads and the watchdog doesn't trigger simply when there are lots of
>> > userspace tasks.
>>
>> I think so too.
>>
>> I see a couple of more-or-less hackish ways to avoid the issue, but
>> nothing particularly attractive ATM.
>>
>> I wouldn't change the general behavior with respect to RT tasks
>> because of this, though, as we would quickly find a case in which that
>> would turn out to be not desirable.
>
> I agree we cannot generalize to all RT tasks, but what Patrick proposed
> (clamping utilization of certain known tasks) might help here:
>
> lkml.kernel.org/r/20170824180857.32103-1-patrick.bellasi@....com
>
> Maybe with a per-task interface instead of using cgroups?

The problem here is that this is a kernel thing and user space should
not be expected to have to do anything about fixing this IMO.

> The other option would be to relax DL tasks affinity constraints, so
> that a case like this might be handled. Daniel and Tommaso proposed
> possible approaches, this might be a driving use case. Not sure how we
> would come up with a proper runtime for the watchdog, though.

That is a problem.

Basically, it needs to run as soon as possible, but it will be running
for a very short time, every time.  Overall, using a thread for that
seems wasteful ...

Thanks,
Rafael