linux-kernel - Re: [PATCH v3 4/5] slub: Only IPI CPUs that have per cpu obj to flush

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Message-ID: <CAOtvUMe+Um-t3k=VC2Kz4hnOdKYszn9_OG8fa2tp8qK=FLpz0Q@mail.gmail.com>
Date:	Sun, 13 Nov 2011 16:57:36 +0200
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Hillf Danton <dhillf@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Russell King <linux@....linux.org.uk>, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH v3 4/5] slub: Only IPI CPUs that have per cpu obj to flush

On Sun, Nov 13, 2011 at 2:20 PM, Hillf Danton <dhillf@...il.com> wrote:
>
> On Sun, Nov 13, 2011 at 6:17 PM, Gilad Ben-Yossef <gilad@...yossef.com> wrote:
>
...
>
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 7d2a996..caf4b3a 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2006,7 +2006,20 @@ static void flush_cpu_slab(void *d)
> >
> >  static void flush_all(struct kmem_cache *s)
> >  {
> > -       on_each_cpu(flush_cpu_slab, s, 1);
> > +       cpumask_var_t cpus;
> > +       struct kmem_cache_cpu *c;
> > +       int cpu;
> > +
> > +       if (likely(zalloc_cpumask_var(&cpus, GFP_ATOMIC))) {
>
> Perhaps, the technique of local_cpu_mask defined in kernel/sched_rt.c
> could be used to replace the above atomic allocation.
>

Thank you for taking the time to review my patch :-)

That is indeed the direction I went with inthe previous iteration of
this patch, with the small change that because of observing that the
allocation will only actually occurs for CPUMASK_OFFSTACK=y which by
definition are systems with lots and lots of CPUs and, it is actually
better to allocate the cpumask per kmem_cache rather then per CPU,
since on system where it matters we are bound to have more CPUs (e.g.
4096) then kmem_caches (~160). See
https://lkml.org/lkml/2011/10/23/151.

I then went a head and further optimized the code to only incur the
memory overhead of allocating those cpumasks for CPUMASK_OFFSTACK=y
systems. See https://lkml.org/lkml/2011/10/23/152.

As you can see from the discussion that evolved, there seems to be an
agreement that the code complexity overhead involved is simply not
worth it for what is, unlike sched_rt, a rather esoteric case and one
where allocation failure is easily dealt with.

Thanks!
Gilad
--
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"Unfortunately, cache misses are an equal opportunity pain provider."
-- Mike Galbraith, LKML
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