Re: [PATCH] mm: Move mm_count into its own cache line

[Aaron Lu]

On Mon, May 15, 2023 at 10:35:36AM -0400, Mathieu Desnoyers wrote:
> The mm_struct mm_count field is frequently updated by mmgrab/mmdrop
> performed by context switch. This causes false-sharing for surrounding
> mm_struct fields which are read-mostly.
> 
> This has been observed on a 2sockets/112core/224cpu Intel Sapphire
> Rapids server running hackbench, and by the kernel test robot
> will-it-scale testcase.
> 
> Move the mm_count field into its own cache line to prevent false-sharing
> with other mm_struct fields.
> 
> Move mm_count to the first field of mm_struct to minimize the amount of
> padding required: rather than adding padding before and after the
> mm_count field, padding is only added after mm_count.
> 
> Note that I noticed this odd comment in mm_struct:
> 
> commit 2e3025434a6b ("mm: relocate 'write_protect_seq' in struct mm_struct")
> 
>                 /*
>                  * With some kernel config, the current mmap_lock's offset
>                  * inside 'mm_struct' is at 0x120, which is very optimal, as
>                  * its two hot fields 'count' and 'owner' sit in 2 different
>                  * cachelines,  and when mmap_lock is highly contended, both
>                  * of the 2 fields will be accessed frequently, current layout
>                  * will help to reduce cache bouncing.
>                  *
>                  * So please be careful with adding new fields before
>                  * mmap_lock, which can easily push the 2 fields into one
>                  * cacheline.
>                  */
>                 struct rw_semaphore mmap_lock;
> 
> This comment is rather odd for a few reasons:
> 
> - It requires addition/removal of mm_struct fields to carefully consider
>   field alignment of _other_ fields,
> - It expresses the wish to keep an "optimal" alignment for a specific
>   kernel config.
> 
> I suspect that the author of this comment may want to revisit this topic
> and perhaps introduce a split-struct approach for struct rw_semaphore,
> if the need is to place various fields of this structure in different
> cache lines.
> 
> Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
> Fixes: af7f588d8f73 ("sched: Introduce per-memory-map concurrency ID")
> Link: https://lore.kernel.org/lkml/7a0c1db1-103d-d518-ed96-1584a28fbf32@xxxxxxxxxxxx
> Reported-by: kernel test robot <yujie.liu@xxxxxxxxx>
> Link: https://lore.kernel.org/oe-lkp/202305151017.27581d75-yujie.liu@xxxxxxxxx
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

Reviewed-by: Aaron Lu <aaron.lu@xxxxxxxxx>