linux-kernel - Re: [PATCH] Make SPLIT_RSS

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Date:   Fri, 4 Oct 2019 16:26:24 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Daniel Colascione <dancol@...gle.com>, Qian Cai <cai@....pw>,
        Tim Murray <timmurray@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        linux-mm@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] Make SPLIT_RSS_COUNTING configurable

On Fri, Oct 04, 2019 at 02:33:49PM +0200, Michal Hocko wrote:
> On Wed 02-10-19 19:08:16, Daniel Colascione wrote:
> > On Wed, Oct 2, 2019 at 6:56 PM Qian Cai <cai@....pw> wrote:
> > > > On Oct 2, 2019, at 4:29 PM, Daniel Colascione <dancol@...gle.com> wrote:
> > > >
> > > > Adding the correct linux-mm address.
> > > >
> > > >
> > > >> +config SPLIT_RSS_COUNTING
> > > >> +       bool "Per-thread mm counter caching"
> > > >> +       depends on MMU
> > > >> +       default y if NR_CPUS >= SPLIT_PTLOCK_CPUS
> > > >> +       help
> > > >> +         Cache mm counter updates in thread structures and
> > > >> +         flush them to visible per-process statistics in batches.
> > > >> +         Say Y here to slightly reduce cache contention in processes
> > > >> +         with many threads at the expense of decreasing the accuracy
> > > >> +         of memory statistics in /proc.
> > > >> +
> > > >> endmenu
> > >
> > > All those vague words are going to make developers almost impossible to decide the right selection here. It sounds like we should kill SPLIT_RSS_COUNTING at all to simplify the code as the benefit is so small vs the side-effect?
> > 
> > Killing SPLIT_RSS_COUNTING would be my first choice; IME, on mobile
> > and a basic desktop, it doesn't make a difference. I figured making it
> > a knob would help allay concerns about the performance impact in more
> > extreme configurations.
> 
> I do agree with Qian. Either it is really helpful (is it? probably on
> the number of cpus) and it should be auto-enabled or it should be
> dropped altogether. You cannot really expect people know how to enable
> this without a deep understanding of the MM internals. Not to mention
> all those users using distro kernels/configs.
> 
> A config option sounds like a bad way forward.

And I don't see much point anyway. Reading RSS counters from proc is
inherently racy. It can just either way after the read due to process
behaviour.

-- 
 Kirill A. Shutemov