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From:   Xi Wang <xii@google.com>
Date:   Thu, 5 Mar 2020 13:41:36 -0800
Message-ID: <CAOBoifgTZ_h4MJv7-yJwrQ4jAYQtHfajxBCyZNJkerKko+e9nQ@mail.gmail.com>
Subject: Re: [PATCH] sched: watchdog: Touch kernel watchdog in sched code
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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Measuring jitter from a userspace busy loop showed a 4us peak was
flattened in the histogram (cascadelake). So the effect is likely
reducing overhead/jitter by 4us.

Code in resched_for_watchdog should be ok since it is always called
from the watchdog hrtimer function?

Why supporting the option to alternate between thread context and
touch in sched: Might be a little risky to completely switch to the
touch in sched method. Touch in sched for 9 out of 10 times still
captures most of the latency benefit. I can remove it or change it to
on/off if desired.

Advertising the knob on random blogs: Maybe I should create a blog :)

-Xi


On Thu, Mar 5, 2020 at 10:07 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Peter Zijlstra <peterz@infradead.org> writes:
>
> > On Wed, Mar 04, 2020 at 01:39:41PM -0800, Xi Wang wrote:
> >> The main purpose of kernel watchdog is to test whether scheduler can
> >> still schedule tasks on a cpu. In order to reduce latency from
> >> periodically invoking watchdog reset in thread context, we can simply
> >> touch watchdog from pick_next_task in scheduler. Compared to actually
> >> resetting watchdog from cpu stop / migration threads, we lose coverage
> >> on: a migration thread actually get picked and we actually context
> >> switch to the migration thread. Both steps are heavily protected by
> >> kernel locks and unlikely to silently fail. Thus the change would
> >> provide the same level of protection with less overhead.
> >>
> >> The new way vs the old way to touch the watchdogs is configurable
> >> from:
> >>
> >> /proc/sys/kernel/watchdog_touch_in_thread_interval
> >>
> >> The value means:
> >> 0: Always touch watchdog from pick_next_task
> >> 1: Always touch watchdog from migration thread
> >> N (N>0): Touch watchdog from migration thread once in every N
> >>          invocations, and touch watchdog from pick_next_task for
> >>          other invocations.
> >>
> >
> > This is configurable madness. What are we really trying to do here?
>
> Create yet another knob which will be advertised in random web blogs to
> solve all problems of the world and some more. Like the one which got
> silently turned into a NOOP ~10 years ago :)
>
>