Re: [PATCH] sched: watchdog: Touch kernel watchdog in sched code

[Peter Zijlstra]

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?