linux-kernel - Re: [PATCH v4 35/35] mm, slub: convert kmem_cpu_slab protection to local

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Message-ID: <20210817093133.kzwq2vx5gtrwevih@linutronix.de>
Date:   Tue, 17 Aug 2021 11:31:33 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Sven Eckelmann <sven@...fation.org>, akpm@...ux-foundation.org,
        brouer@...hat.com, cl@...ux.com, efault@....de,
        iamjoonsoo.kim@....com, jannh@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mgorman@...hsingularity.net, penberg@...nel.org,
        rientjes@...gle.com, tglx@...utronix.de,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 35/35] mm, slub: convert kmem_cpu_slab protection to
 local_lock

On 2021-08-17 11:17:18 [+0200], Vlastimil Babka wrote:
> On 8/17/21 11:12 AM, Sebastian Andrzej Siewior wrote:
> > On 2021-08-17 10:37:48 [+0200], Vlastimil Babka wrote:
> >> OK reproduced. Thanks, will investigate.
> > 
> > With the local_lock at the top, the needed alignment gets broken for dbl
> > cmpxchg. On RT it was working ;)
> 
> I'd rather have page and partial in the same cacheline as well, is it ok
> to just move the lock as last and not care about whether it straddles
> cachelines or not? (with CONFIG_SLUB_CPU_PARTIAL it will naturally start
> with the next cacheline).

Moving like you suggested appears to be more efficient and saves a bit
of memory:

RT+ debug:
struct kmem_cache_cpu {
        void * *                   freelist;             /*     0     8 */
        long unsigned int          tid;                  /*     8     8 */
        struct page *              page;                 /*    16     8 */
        struct page *              partial;              /*    24     8 */
        local_lock_t               lock;                 /*    32   144 */

        /* size: 176, cachelines: 3, members: 5 */
        /* last cacheline: 48 bytes */
};

RT no debug:
struct kmem_cache_cpu {
        void * *                   freelist;             /*     0     8 */
        long unsigned int          tid;                  /*     8     8 */
        struct page *              page;                 /*    16     8 */
        struct page *              partial;              /*    24     8 */
        local_lock_t               lock;                 /*    32    32 */

        /* size: 64, cachelines: 1, members: 5 */
};

no-RT, no-debug:
struct kmem_cache_cpu {
        void * *                   freelist;             /*     0     8 */
        long unsigned int          tid;                  /*     8     8 */
        struct page *              page;                 /*    16     8 */
        struct page *              partial;              /*    24     8 */
        local_lock_t               lock;                 /*    32     0 */

        /* size: 32, cachelines: 1, members: 5 */
        /* last cacheline: 32 bytes */
};

no-RT, debug:
struct kmem_cache_cpu {
        void * *                   freelist;             /*     0     8 */
        long unsigned int          tid;                  /*     8     8 */
        struct page *              page;                 /*    16     8 */
        struct page *              partial;              /*    24     8 */
        local_lock_t               lock;                 /*    32    56 */

        /* size: 88, cachelines: 2, members: 5 */
        /* last cacheline: 24 bytes */
};

Sebastian