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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: commit2e3025434a("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. Link: https://lkml.kernel.org/r/20230515143536.114960-1-mathieu.desnoyers@efficios.com Fixes:223baf9d17("sched: Fix performance regression introduced by mm_cid") Fixes:af7f588d8f("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 <yujie.liu@intel.com> Link: https://lore.kernel.org/oe-lkp/202305151017.27581d75-yujie.liu@intel.com Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Reviewed-by: Aaron Lu <aaron.lu@intel.com> Reviewed-by: John Hubbard <jhubbard@nvidia.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Olivier Dion <odion@efficios.com> Cc: <michael.christie@oracle.com> Cc: Feng Tang <feng.tang@intel.com> Cc: Jason Gunthorpe <jgg@nvidia.com> Cc: Peter Xu <peterx@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.