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

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Date:	Mon, 14 Nov 2011 21:19:11 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Gilad Ben-Yossef <gilad@...yossef.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 10:57 PM, Gilad Ben-Yossef <gilad@...yossef.com> wrote:
> 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.
>
Even with the introduced overhead of allocation, IPIs could not go down
as much as we wish, right?
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