linux-kernel - Re: [RFC 04/26] mm, slub: simplify kmem_cache

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Message-ID: <20210525114715.GN30378@techsingularity.net>
Date:   Tue, 25 May 2021 12:47:15 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [RFC 04/26] mm, slub: simplify kmem_cache_cpu and tid setup

On Tue, May 25, 2021 at 01:39:24AM +0200, Vlastimil Babka wrote:
> In slab_alloc_node() and do_slab_free() fastpaths we need to guarantee that
> our kmem_cache_cpu pointer is from the same cpu as the tid value. Currently
> that's done by reading the tid first using this_cpu_read(), then the
> kmem_cache_cpu pointer and verifying we read the same tid using the pointer and
> plain READ_ONCE().
> 
> This can be simplified to just fetching kmem_cache_cpu pointer and then reading
> tid using the pointer. That guarantees they are from the same cpu. We don't
> need to read the tid using this_cpu_read() because the value will be validated
> by this_cpu_cmpxchg_double(), making sure we are on the correct cpu and the
> freelist didn't change by anyone preempting us since reading the tid.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Wow, that's a fun approach to avoiding disabling preemption but the
validation check against preemption remains the same so;

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs