Re: [PATCH] watchdog: softdog: fire watchdog even if softirqs do not get to run

From: Peter Zijlstra
Date: Mon Feb 20 2017 - 03:37:13 EST

On Sun, Feb 19, 2017 at 08:46:28AM -0800, Guenter Roeck wrote:
> Cc: Wolfram for input.
> On 02/17/2017 10:25 AM, Niklas Cassel wrote:
> >From: Niklas Cassel <niklas.cassel@xxxxxxxx>
> >
> >Checking for timer expiration is done from the softirq TIMER_SOFTIRQ.
> >
> >Since commit 4cd13c21b207 ("softirq: Let ksoftirqd do its job"),
> >pending softirqs are no longer always handled immediately, instead,
> >if there are pending softirqs, and ksoftirqd is in state TASK_RUNNING,
> >the handling of the softirqs are deferred, and are instead supposed
> >to be handled by ksoftirqd, when ksoftirqd gets scheduled.
> >
> >If a user space process with a real-time policy starts to misbehave
> >by never relinquishing the CPU while ksoftirqd is in state TASK_RUNNING,
> >what will happen is that all softirqs will get deferred, while ksoftirqd,
> >which is supposed to handle the deferred softirqs, will never get to run.
> >
> >To make sure that the watchdog is able to fire even when we do not get
> >to run softirqs, replace the timers with hrtimers.
> >
> This makes the driver dependent on HIGH_RES_TIMERS, which is not available
> on all architectures. Before adding that restriction, I would like to see
> some discussion if this is the only feasible solution.

It does no such thing; the hrtimer interface is always available.