linux-kernel - RE: [PATCH 0/2] execve scalability issues, part 1

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Date:   Wed, 23 Aug 2023 10:49:41 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jan Kara' <jack@...e.cz>, Mateusz Guzik <mjguzik@...il.com>
CC:     Dennis Zhou <dennis@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tj@...nel.org" <tj@...nel.org>, "cl@...ux.com" <cl@...ux.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "shakeelb@...gle.com" <shakeelb@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: RE: [PATCH 0/2] execve scalability issues, part 1

From: Jan Kara
> Sent: Wednesday, August 23, 2023 10:49 AM
....
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -737,7 +737,11 @@ struct mm_struct {
> >
> >                 unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for
> > /proc/PID/auxv */
> >
> > -               struct percpu_counter rss_stat[NR_MM_COUNTERS];
> > +               union {
> > +                       struct percpu_counter rss_stat[NR_MM_COUNTERS];
> > +                       u64 *rss_stat_single;
> > +               };
> > +               bool    magic_flag_stuffed_elsewhere;

I wouldn't use a union to save a pointer - it is asking for trouble.

> >
> >                 struct linux_binfmt *binfmt;
> >
> >
> > Then for single-threaded case an area is allocated for NR_MM_COUNTERS
> > countes * 2 -- first set updated without any synchro by current
> > thread. Second set only to be modified by others and protected with
> > mm->arg_lock. The lock protects remote access to the union to begin
> > with.
> 
> arg_lock seems a bit like a hack. How is it related to rss_stat? The scheme
> with two counters is clever but I'm not 100% convinced the complexity is
> really worth it. I'm not sure the overhead of always using an atomic
> counter would really be measurable as atomic counter ops in local CPU cache
> tend to be cheap. Did you try to measure the difference?

A separate lock is worse than atomics.
(Although some 32bit arch may have issues with 64bit atomics.)

I think you'll be surprised just how slow atomic ops are.
Even when present in the local cache.
(Probably because any other copies have to be invalidated.)

	David

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