Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads

[Michal Hocko]

On Tue 22-08-23 08:30:05, Yosry Ahmed wrote:
> On Tue, Aug 22, 2023 at 2:06 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Mon 21-08-23 20:54:58, Yosry Ahmed wrote:
[...]
> So to answer your question, I don't think a random user can really
> affect the system in a significant way by constantly flushing. In
> fact, in the test script (which I am now attaching, in case you're
> interested), there are hundreds of threads that are reading stats of
> different cgroups every 1s, and I don't see any negative effects on
> in-kernel flushers in this case (reclaimers).

I suspect you have missed my point. Maybe I am just misunderstanding
the code but it seems to me that the lock dropping inside
cgroup_rstat_flush_locked effectivelly allows unbounded number of
contenders which is really dangerous when it is triggerable from the
userspace. The number of spinners at a moment is always bound by the
number CPUs but depending on timing many potential spinners might be
cond_rescheded and the worst time latency to complete can be really
high. Makes more sense?
-- 
Michal Hocko
SUSE Labs