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

[Michal Hocko]

On Thu 30-03-23 00:13:16, Yosry Ahmed wrote:
> On Thu, Mar 30, 2023 at 12:06 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
[...]
> > 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.
> 
> There are discussions in [1] about making atomic rstat flush not
> disable irqs throughout the process, so in that case it would only
> result in a similar situation if the caller has irq disabled. The only
> caller that might have irq disabled is the same caller that might be
> in irq context before this series: mem_cgroup_usage().
> 
> On that note, and while I have your attention, I was wondering if we
> can eliminate the flush call completely from mem_cgroup_usage(), and
> read the global stats counters for root memcg instead of the root
> counters. There might be subtle differences, but the root memcg usage
> isn't super accurate now anyway (e.g. it doesn't include kernel
> memory).

root memcg stats are imprecise indeed and I have to admit I do not
really know why we are adding more work for that case. I have tried to
dig into git history for that yesterday but couldn't find a satisfying
answer. It goes all the way down to 2d146aa3aa842 which has done the
switch to rstat. Maybe Johannes remembers.

Anyway, back to this particular patch. I still do not see strong reasons
to be verbose about !in_task case so I would just drop this patch.
-- 
Michal Hocko
SUSE Labs