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Mel Gorman 37a4094e82 mremap: remove LATENCY_LIMIT from mremap to reduce the number of TLB shootdowns 
Commit 5d1904204c ("mremap: fix race between mremap() and page
cleanning") fixed races between mremap and other operations for both
file-backed and anonymous mappings.  The file-backed was the most
critical as it allowed the possibility that data could be changed on a
physical page after page_mkclean returned which could trigger data loss
or data integrity issues.

A customer reported that the cost of the TLBs for anonymous regressions
was excessive and resulting in a 30-50% drop in performance overall
since this commit on a microbenchmark.  Unfortunately I neither have
access to the test-case nor can I describe what it does other than
saying that mremap operations dominate heavily.

This patch removes the LATENCY_LIMIT to handle TLB flushes on a PMD
boundary instead of every 64 pages to reduce the number of TLB
shootdowns by a factor of 8 in the ideal case.  LATENCY_LIMIT was almost
certainly used originally to limit the PTL hold times but the latency
savings are likely offset by the cost of IPIs in many cases.  This patch
is not reported to completely restore performance but gets it within an
acceptable percentage.  The given metric here is simply described as
"higher is better".

Baseline that was known good
002:  Metric:       91.05
004:  Metric:      109.45
008:  Metric:       73.08
016:  Metric:       58.14
032:  Metric:       61.09
064:  Metric:       57.76
128:  Metric:       55.43

Current
001:  Metric:       54.98
002:  Metric:       56.56
004:  Metric:       41.22
008:  Metric:       35.96
016:  Metric:       36.45
032:  Metric:       35.71
064:  Metric:       35.73
128:  Metric:       34.96

With patch
001:  Metric:       61.43
002:  Metric:       81.64
004:  Metric:       67.92
008:  Metric:       51.67
016:  Metric:       50.47
032:  Metric:       52.29
064:  Metric:       50.01
128:  Metric:       49.04

So for low threads, it's not restored but for larger number of threads,
it's closer to the "known good" baseline.

Using a different mremap-intensive workload that is not representative
of the real workload there is little difference observed outside of
noise in the headline metrics However, the TLB shootdowns are reduced by
11% on average and at the peak, TLB shootdowns were reduced by 21%.
Interrupts were sampled every second while the workload ran to get those
figures.  It's known that the figures will vary as the
non-representative load is non-deterministic.

An alternative patch was posted that should have significantly reduced
the TLB flushes but unfortunately it does not perform as well as this
version on the customer test case.  If revisited, the two patches can
stack on top of each other.

Link: http://lkml.kernel.org/r/20180606183803.k7qaw2xnbvzshv34@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Aaron Lu <aaron.lu@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2018-06-15 07:55:24 +09:00
arch mm: fix devmem_is_allowed() for sub-page System RAM intersections 2018-06-15 07:55:23 +09:00
block treewide: Use array_size() in vzalloc() 2018-06-12 16:19:22 -07:00
certs certs/blacklist_nohashes.c: fix const confusion in certs blacklist 2018-02-21 15:35:43 -08:00
crypto treewide: Use array_size() in sock_kmalloc() 2018-06-12 16:19:22 -07:00
Documentation platform-drivers-x86 for v4.18-1 2018-06-14 16:30:30 +09:00
drivers SCSI fixes on 20180613 2018-06-14 16:35:32 +09:00
firmware kbuild: remove all dummy assignments to obj- 2017-11-18 11:46:06 +09:00
fs Revert "debugfs: inode: debugfs_create_dir uses mode permission from parent" 2018-06-12 20:52:16 -07:00
include mm: fix race between kmem_cache destroy, create and deactivate 2018-06-15 07:55:23 +09:00
init Kbuild updates for v4.18 (2nd) 2018-06-13 08:40:34 -07:00
ipc treewide: kvmalloc() -> kvmalloc_array() 2018-06-12 16:19:22 -07:00
kernel mm: check for SIGKILL inside dup_mmap() loop 2018-06-15 07:55:24 +09:00
lib Kbuild updates for v4.18 (2nd) 2018-06-13 08:40:34 -07:00
LICENSES LICENSES: Add Linux-OpenIB license text 2018-04-27 16:41:53 -06:00
mm mremap: remove LATENCY_LIMIT from mremap to reduce the number of TLB shootdowns 2018-06-15 07:55:24 +09:00
net - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
samples VFIO updates for v4.18 2018-06-12 13:11:26 -07:00
scripts Kbuild updates for v4.18 (2nd) 2018-06-13 08:40:34 -07:00
security + Features 2018-06-14 16:11:28 +09:00
sound - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
tools Additional power management updates for 4.18-rc1 2018-06-13 07:24:18 -07:00
usr kbuild: rename built-in.o to built-in.a 2018-03-26 02:01:19 +09:00
virt - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
.clang-format clang-format: add configuration file 2018-04-11 10:28:35 -07:00
.cocciconfig
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
.mailmap Merge branch 'asoc-4.17' into asoc-4.18 for compress dependencies 2018-04-26 12:24:28 +01:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS/CREDITS: Drop METAG ARCHITECTURE 2018-03-05 16:34:24 +00:00
Kbuild Kbuild updates for v4.15 2017-11-17 17:45:29 -08:00
Kconfig kconfig: add basic helper macros to scripts/Kconfig.include 2018-05-29 03:31:19 +09:00
MAINTAINERS Merge branch 'i2c/for-4.18' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux 2018-06-14 16:21:46 +09:00
Makefile Kbuild: rename CC_STACKPROTECTOR[_STRONG] config variables 2018-06-14 12:21:18 +09:00
README Docs: Added a pointer to the formatted docs to README 2018-03-21 09:02:53 -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.
See Documentation/00-INDEX for a list of what is contained in each file.

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.