On 2021-08-17 11:17:18 [+0200], Vlastimil Babka wrote:

On 8/17/21 11:12 AM, Sebastian Andrzej Siewior wrote:

quoted

On 2021-08-17 10:37:48 [+0200], Vlastimil Babka wrote:

quoted

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