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

[Yosry Ahmed]

On Mon, Aug 28, 2023 at 10:00 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> On Mon, Aug 28, 2023 at 9:15 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> [...]
> > >
> > > Well, I really have to say that I do not like the notion that reading
> > > stats is unpredictable. This just makes it really hard to use. If
> > > the precision is to be sarificed then this should be preferable over
> > > potentially high global lock contention. We already have that model in
> > > place of /proc/vmstat (configurable timeout for flusher and a way to
> > > flush explicitly). I appreciate you would like to have a better
> > > precision but as you have explored the locking is really hard to get rid
> > > of here.
> >
> > Reading the stats *is* unpredictable today. In terms of
> > accuracy/staleness and cost. Avoiding the flush entirely on the read
> > path will surely make the cost very stable and cheap, but will make
> > accuracy even less predictable.
> >
>
> On average you would get the stats at most 2 second old, so I would
> not say it is less predictable.
>
> > >
> > > So from my POV I would prefer to avoid flushing from the stats reading
> > > path and implement force flushing by writing to stat file. If the 2s
> > > flushing interval is considered to coarse I would be OK to allow setting
> > > it from userspace. This way this would be more in line with /proc/vmstat
> > > which seems to be working quite well.
> > >
> > > If this is not accaptable or deemed a wrong approach long term then it
> > > would be good to reonsider the current cgroup_rstat_lock at least.
> > > Either by turning it into mutex or by dropping the yielding code which
> > > can severly affect the worst case latency AFAIU.
> >
> > Honestly I think it's better if we do it the other way around. We make
> > flushing on the stats reading path non-unified and deterministic. That
> > model also exists and is used for cpu.stat. If we find a problem with
> > the locking being held from userspace, we can then remove flushing
> > from the read path and add interface(s) to configure the periodic
> > flusher and do a force flush.
> >
>
> Here I agree with you. Let's go with the approach which is easy to
> undo for now. Though I prefer the new explicit interface for flushing,
> that step would be very hard to undo. Let's reevaluate if the proposed
> approach shows negative impact on production traffic and I think
> Cloudflare folks can give us the results soon.

Do you prefer we also switch to using a mutex (with preemption
disabled) to avoid the scenario Michal described where flushers give
up the lock and sleep resulting in an unbounded wait time in the worst
case?