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

[Michal Hocko <mhocko@suse.com>]

On Wed 29-03-23 19:20:59, Shakeel Butt wrote:
> On Wed, Mar 29, 2023 at 11:41:39AM -0700, Yosry Ahmed wrote:
> > On Wed, Mar 29, 2023 at 4:22 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Tue 28-03-23 22:16:39, 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.
> > > >
> > > > Ideally, we wouldn't flush with irqs disabled either, but we have one
> > > > context today that does so in mem_cgroup_usage(). Forbid callers from
> > > > irq context for now, and hopefully we can also forbid callers with irqs
> > > > disabled in the future when we can get rid of this callsite.
> > >
> > > I am sorry to be late to the discussion. I wanted to follow up on
> > > Johannes reply in the previous version but you are too fast ;)
> > >
> > > I do agree that this looks rather arbitrary. You do not explain how the
> > > warning actually helps. Is the intention to be really verbose to the
> > > kernel log when somebody uses this interface from the IRQ context and
> > > get bug reports? What about configurations with panic on warn? Do we
> > > really want to crash their systems for something like that?
> > 
> > Thanks for taking a look, Michal!
> > 
> > The ultimate goal is not to flush in irq context or with irqs
> > disabled, as in some cases it causes irqs to be disabled for a long
> > time, as flushing is an expensive operation. The previous patch in the
> > series should have removed the only context that flushes in irq
> > context, and the purpose of the WARN_ON_ONCE() is to catch future uses
> > or uses that we might have missed.
> > 
> > There is still one code path that flushes with irqs disabled (also
> > mem_cgroup_usage()), and we cannot remove this just yet; we need to
> > deprecate usage threshold events for root to do that. So we cannot
> > enforce not flushing with irqs disabled yet.
> > 
> > So basically the patch is trying to enforce what we have now, not
> > flushing in irq context, and hopefully at some point we will also be
> > able to enforce not flushing with irqs disabled.
> > 
> > If WARN_ON_ONCE() is the wrong tool for this, please let me know.
> > 
> 
> If I understand Michal's concern, the question is should be start with
> pr_warn_once() instead of WARN_ON_ONCE() and I think yes we should start
> with pr_warn_once().

Yes, I do not really like the WARN_ON here. It is an overkill. pr_warn
would much less intrusive but potentially incomplete because you won't
know who that offender is. So if you really care about those then you
would need to call dump_stack as well.

So the real question is. Do we really care so deeply? After all somebody
might be calling this from within a spin lock or irq disabled section
resulting in a similar situation without noticing.
-- 
Michal Hocko
SUSE Labs