Re: [PATCH] mm: memcg: provide accurate stats for userspace reads

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Aug 11, 2023 at 7:29 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Fri, Aug 11, 2023 at 7:12 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> [...]
> >
> > I am worried that writing to a stat for flushing then reading will
> > increase the staleness window which we are trying to reduce here.
> > Would it be acceptable to add a separate interface to explicitly read
> > flushed stats without having to write first? If the distinction
> > disappears in the future we can just short-circuit both interfaces.
>
> What is the acceptable staleness time window for your case? It is hard
> to imagine that a write+read will always be worse than just a read.
> Even the proposed patch can have an unintended and larger than
> expected staleness window due to some processing on
> return-to-userspace or some scheduling delay.

Maybe I am worrying too much, we can just go for writing to
memory.stat for explicit stats refresh.

Do we still want to go with the mutex approach Michal suggested for
do_flush_stats() to support either waiting for ongoing flushes
(mutex_lock) or skipping (mutex_trylock)?