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

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Message-ID: <1317022420.9084.57.camel@twins>
Date:	Mon, 26 Sep 2011 09:33:40 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
Cc:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Chris Metcalf <cmetcalf@...era.com>, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH 5/5] slub: Only IPI CPUs that have per cpu obj to flush

On Sun, 2011-09-25 at 11:54 +0300, Gilad Ben-Yossef wrote:
> +       if (likely(zalloc_cpumask_var(&cpus, GFP_ATOMIC))) {
> +               for_each_online_cpu(cpu) {
> +                       c = per_cpu_ptr(s->cpu_slab, cpu);
> +                       if (c && c->page)
> +                               cpumask_set_cpu(cpu, cpus);
> +               }
> +               on_each_cpu_mask(cpus, flush_cpu_slab, s, 1);
> +               free_cpumask_var(cpus); 

Right, having to do that for_each_oneline_cpu() loop only to then IPI
them can cause a massive cacheline bounce fest.. Ideally you'd want to
keep a cpumask per kmem_cache, although I bet the memory overhead of
that isn't attractive.

Also, what Pekka says, having that alloc here isn't good either.
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