Re: [PATCH v1 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside task context

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Mar 28, 2023 at 11:59 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Tue, Mar 28, 2023 at 10:49 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Tue, Mar 28, 2023 at 06:16:33AM +0000, Yosry Ahmed wrote:
> > > rstat flushing is too expensive to perform in irq context.
> > > The previous patch removed the only context that may invoke an rstat
> > > flush from irq context, add a WARN_ON_ONCE() to detect future
> > > violations, or those that we are not aware of.
> > >
> > > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > > ---
> > >  kernel/cgroup/rstat.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> > > index d3252b0416b6..c2571939139f 100644
> > > --- a/kernel/cgroup/rstat.c
> > > +++ b/kernel/cgroup/rstat.c
> > > @@ -176,6 +176,8 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
> > >  {
> > >       int cpu;
> > >
> > > +     /* rstat flushing is too expensive for irq context */
> > > +     WARN_ON_ONCE(!in_task());
> > >       lockdep_assert_held(&cgroup_rstat_lock);
> >
> > This seems a bit arbitrary. Why is an irq caller forbidden, but an
> > irq-disabled, non-preemptible section caller is allowed? The latency
> > impact on the system would be the same, right?
>
> Thanks for taking a look.
>
> So in the first patch series the initial purpose was to make sure
> cgroup_rstat_lock was never acquired in an irq context, so that we can
> stop disabling irqs while holding it. Tejun disagreed with this
> approach though.
>
> We currently have one caller that calls flushing with irqs disabled
> (mem_cgroup_usage()) -- so we cannot forbid such callers (yet), but I
> thought we can at least forbid callers from irq context now (or catch
> those that we are not aware of), and then maybe forbid irqs_disabled()
> contexts as well we can get rid of that callsite.
>
> WDYT?

I added more context in the commit log in the v2 respin [1]. Let me
know if you want me to change something else, rephrase the comment, or
drop the patch entirely.

[1]https://lore.kernel.org/linux-mm/20230328221644.803272-5-yosryahmed@google.com/