Re: [PATCH v2 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside task context

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Mar 30, 2023 at 1:39 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 30-03-23 01:19:29, Yosry Ahmed wrote:
> > On Thu, Mar 30, 2023 at 1:15 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Thu 30-03-23 01:06:26, Yosry Ahmed wrote:
> > > [...]
> > > > If we achieve that, do you think it makes sense to add
> > > > WARN_ON_ONCE(irqs_disabled()) instead to prevent future users from
> > > > flushing while disabling irqs or in irq context?
> > >
> > > WARN_ON (similar to BUG_ON) will not prevent anybody from doing bad
> > > things. We already have means to shout about sleepable code being
> > > invoked from an atomic context and there is no reason to duplicate that.
> > > As I've said earlier WARN_ON might panic the system in some
> > > configurations (and yes they are used also in production systems - do
> > > not ask me why...). So please be careful about that and use that only
> > > when something really bad (yet recoverable) is going on.
> >
> > Thanks for the information (I was about to ask why about production
> > systems, but okay..). I will avoid WARN_ON completely. For the
> > purposes of this series I will drop this patch anyway.
>
> Thanks! People do strange things sometimes...
>
> > Any idea how to shout about "hey this may take too long, why are you
> > doing it with irqs disabled?!"?
>
> Well we have a hard lockup detector. It hits at a much higher stall by
> default but if you care about IRQ latencies in general then you likely
> want to lower. Another thing would be IRQ tracing. In any case this code
> path shouldn't be any special. Sure it can take long on large systems
> but I bet there are more of those.

We did see hard lockups in extreme cases, and we do have setups with
"nmi_watchdog=panic" that will panic when this happens, so we would
rather catch the code paths that can lead to that before it actually
happens.

Maybe we can add a primitive like might_sleep() for this, just food for thought.

> --
> Michal Hocko
> SUSE Labs