On Mon, 15 May 2023 10:35:36 -0400 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> 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.
>
> ...
>
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -583,6 +583,21 @@ struct mm_cid {
> struct kioctx_table;
> struct mm_struct {
> struct {
> + /*
> + * Fields which are often written to are placed in a separate
> + * cache line.
> + */
> + struct {
> + /**
> + * @mm_count: The number of references to &struct
> + * mm_struct (@mm_users count as 1).
> + *
> + * Use mmgrab()/mmdrop() to modify. When this drops to
> + * 0, the &struct mm_struct is freed.
> + */
> + atomic_t mm_count;
> + } ____cacheline_aligned_in_smp;
> +
Why add the anonymous struct?
atomic_t mm_count ____cacheline_aligned_in_smp;
would suffice?
Secondly, the ____cacheline_aligned_in_smp doesn't actually do
anything? mm_count is at offset 0 which is cacheline aligned anyway.
The next field (mm_mt) will share a cacheline with mm_count.
If the plan is to put mm_count in "its own" cacheline then padding will
be needed?
