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Mathieu Desnoyers c1753fd02a mm: move mm_count into its own cache line 
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 2e3025434a ("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>
History 2023-06-19 16:19:35 -07:00
arch sh: move the ARCH_DMA_MINALIGN definition to asm/cache.h 2023-06-19 16:19:24 -07:00
block filemap: add a kiocb_write_and_wait helper 2023-06-09 16:25:52 -07:00
certs KEYS: Add missing function documentation 2023-04-24 16:15:52 +03:00
crypto This push fixes the following problems: 2023-05-07 10:57:14 -07:00
Documentation kasan: add support for kasan.fault=panic_on_write 2023-06-19 16:19:33 -07:00
drivers zram: further limit recompression threshold 2023-06-19 16:19:33 -07:00
fs buffer: convert block_truncate_page() to use a folio 2023-06-19 16:19:32 -07:00
include mm: move mm_count into its own cache line 2023-06-19 16:19:35 -07:00
init cachestat: implement cachestat syscall 2023-06-09 16:25:16 -07:00
io_uring mm/gup: remove vmas parameter from pin_user_pages() 2023-06-09 16:25:26 -07:00
ipc Merge branch 'work.namespace' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2023-02-24 19:20:07 -08:00
kernel kernel: pid_namespace: remove unused set_memfd_noexec_scope() 2023-06-19 16:19:28 -07:00
lib maple_tree: simplify and clean up mas_wr_node_store() 2023-06-09 16:25:45 -07:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm mm/memcg: remove return value of mem_cgroup_scan_tasks() 2023-06-19 16:19:35 -07:00
net mm/gup: remove vmas parameter from pin_user_pages() 2023-06-09 16:25:26 -07:00
rust Rust changes for v6.4 2023-04-30 11:20:22 -07:00
samples kmemleak-test: drop __init to get better backtrace 2023-06-09 16:25:40 -07:00
scripts Locking changes in v6.4: 2023-05-05 12:56:55 -07:00
security mm/gup: remove vmas parameter from get_user_pages_remote() 2023-06-09 16:25:26 -07:00
sound ALSA: hda/realtek: Enable headset onLenovo M70/M90 2023-05-24 14:18:59 +02:00
tools selftests: mm: remove duplicate unneeded defines 2023-06-19 16:19:25 -07:00
usr initramfs: Check negative timestamp to prevent broken cpio archive 2023-04-16 17:37:01 +09:00
virt mm: ptep_get() conversion 2023-06-19 16:19:25 -07:00
.clang-format cxl for v6.4 2023-04-30 11:51:51 -07:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for *.dtso files 2023-02-26 15:28:23 +09:00
.gitignore linux-kselftest-kunit-6.4-rc1 2023-04-24 12:31:32 -07:00
.mailmap mailmap: add entries for Nikolay Aleksandrov 2023-05-17 09:35:05 +01:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: sctp: move Neil to CREDITS 2023-05-12 08:51:32 +01:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS selftests: add selftests for cachestat 2023-06-09 16:25:17 -07:00
Makefile Linux 6.4-rc4 2023-05-28 07:49:00 -04:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

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.