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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@efficios.com > Reported-by: kernel test robot > Link: https://lore.kernel.org/oe-lkp/202305151017.27581d75-yujie.liu@intel.com > Signed-off-by: Mathieu Desnoyers Reviewed-by: Aaron Lu