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Date:	Sun, 1 Jan 2012 18:12:10 +0200
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org,
	Chris Metcalf <cmetcalf@...era.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Russell King <linux@....linux.org.uk>, linux-mm@...ck.org,
	Matt Mackall <mpm@...enic.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, apkm@...ux-foundation.org
Subject: Re: [PATCH v4 4/5] slub: Only IPI CPUs that have per cpu obj to flush

On Sun, Jan 1, 2012 at 2:41 PM, Avi Kivity <avi@...hat.com> wrote:
> On 11/23/2011 08:23 AM, Pekka Enberg wrote:
>> On Tue, 22 Nov 2011, Gilad Ben-Yossef wrote:
>>> 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))) {
>>
>> __GFP_NOWARN too maybe?
>>
>>> +        for_each_online_cpu(cpu) {
>>> +            c = per_cpu_ptr(s->cpu_slab, cpu);
>>> +            if (c->page)
>>> +                cpumask_set_cpu(cpu, cpus);
>>> +        }
>>> +        on_each_cpu_mask(cpus, flush_cpu_slab, s, 1);
>>> +        free_cpumask_var(cpus);
>>> +    } else
>>> +        on_each_cpu(flush_cpu_slab, s, 1);
>>> }
>>
>
> Since this seems to be a common pattern, how about:
>
>   zalloc_cpumask_var_or_all_online_cpus(&cpus, GFTP_ATOMIC);
>   ...
>   free_cpumask_var(cpus);
>
> The long-named function at the top of the block either returns a newly
> allocated zeroed cpumask, or a static cpumask with all online cpus set.
> The code in the middle is only allowed to set bits in the cpumask
> (should be the common usage).  free_cpumask_var() needs to check whether
> the freed object is the static variable.

Thanks for the feedback and advice! I totally agree the repeating
pattern needs abstracting.

I ended up chosing to try a different abstraction though - basically a wrapper
on_each_cpu_cond that gets a predicate function to run per CPU to
build the mask
to send the IPI to. It seems cleaner to me not having to mess with
free_cpumask_var
and it abstracts more of the general pattern.

I intend to run the new code through some more testing tomorrow and send out V5
of the patch set. I'd be delighted if you can have a look through it then.

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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