linux-kernel - Re: [PATCH RFC 3/3] slub: reparent memcg caches' slabs on memcg offline

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Message-ID: <20140523152642.GD3147@esperanza>
Date:	Fri, 23 May 2014 19:26:43 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...two.org>
CC:	<hannes@...xchg.org>, <mhocko@...e.cz>,
	<akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>
Subject: Re: [PATCH RFC 3/3] slub: reparent memcg caches' slabs on memcg
 offline

On Thu, May 22, 2014 at 02:25:30PM -0500, Christoph Lameter wrote:
> slab_free calls __slab_free which can release slabs via
> put_cpu_partial()/unfreeze_partials()/discard_slab() to the page
> allocator. I'd rather have preemption enabled there.

Hmm, why? IMO, calling __free_pages with preempt disabled won't hurt
latency, because it proceeds really fast. BTW, we already call it for a
bunch of pages from __slab_free() -> put_cpu_partial() ->
unfreeze_partials() with irqs disabled, which is harder. FWIW, SLAB has
the whole obj free path executed under local_irq_save/restore, and it
doesn't bother enabling irqs for freeing pages.

IMO, the latency improvement we can achieve by enabling preemption while
calling __free_pages is rather minor, and it isn't worth complicating
the code.

Thanks.
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