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Rik van Riel c722f35b51 sched/fair: Bring back select_idle_smt(), but differently 
Mel Gorman did some nice work in 9fe1f127b9 ("sched/fair: Merge
select_idle_core/cpu()"), resulting in the kernel being more efficient
at finding an idle CPU, and in tasks spending less time waiting to be
run, both according to the schedstats run_delay numbers, and according
to measured application latencies. Yay.

The flip side of this is that we see more task migrations (about 30%
more), higher cache misses, higher memory bandwidth utilization, and
higher CPU use, for the same number of requests/second.

This is most pronounced on a memcache type workload, which saw a
consistent 1-3% increase in total CPU use on the system, due to those
increased task migrations leading to higher L2 cache miss numbers, and
higher memory utilization. The exclusive L3 cache on Skylake does us
no favors there.

On our web serving workload, that effect is usually negligible.

It appears that the increased number of CPU migrations is generally a
good thing, since it leads to lower cpu_delay numbers, reflecting the
fact that tasks get to run faster. However, the reduced locality and
the corresponding increase in L2 cache misses hurts a little.

The patch below appears to fix the regression, while keeping the
benefit of the lower cpu_delay numbers, by reintroducing
select_idle_smt with a twist: when a socket has no idle cores, check
to see if the sibling of "prev" is idle, before searching all the
other CPUs.

This fixes both the occasional 9% regression on the web serving
workload, and the continuous 2% CPU use regression on the memcache
type workload.

With Mel's patches and this patch together, task migrations are still
high, but L2 cache misses, memory bandwidth, and CPU time used are
back down to what they were before. The p95 and p99 response times for
the memcache type application improve by about 10% over what they were
before Mel's patches got merged.

Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210326151932.2c187840@imladris.surriel.com
2021-04-09 18:01:39 +02:00
arch * Doc fixes 2021-03-04 11:26:17 -08:00
block block/bfq: update comments and default value in docs for fifo_expire 2021-03-02 11:25:38 -07:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto Keyrings miscellany 2021-02-23 16:09:23 -08:00
Documentation block-5.12-2021-03-05 2021-03-05 12:59:37 -08:00
drivers kcov: Remove kcov include from sched.h and move it to its users. 2021-03-06 12:40:21 +01:00
fs pstore fixes for v5.12-rc2 2021-03-05 17:21:25 -08:00
include sched: Fix various typos 2021-03-22 00:11:52 +01:00
init kbuild: fix UNUSED_KSYMS_WHITELIST for Clang LTO 2021-02-28 15:19:21 +09:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel sched/fair: Bring back select_idle_smt(), but differently 2021-04-09 18:01:39 +02:00
lib Merge branch 'kmap-conversion-for-5.12' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux 2021-03-01 11:24:18 -08:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm swap: fix swapfile read/write offset 2021-03-02 17:25:46 -07:00
net kcov: Remove kcov include from sched.h and move it to its users. 2021-03-06 12:40:21 +01:00
samples treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
scripts gcc-plugins fixes for v5.12-rc2 2021-03-05 17:23:03 -08:00
security Keyrings miscellany 2021-02-23 16:09:23 -08:00
sound ALSA: hda/realtek: Apply dual codec quirks for MSI Godlike X570 board 2021-03-03 15:25:03 +01:00
tools * Doc fixes 2021-03-04 11:26:17 -08:00
usr Kbuild updates for v5.12 2021-02-25 10:17:31 -08:00
virt KVM: x86/mmu: Consider the hva in mmu_notifier retry 2021-02-22 13:16:53 -05:00
.clang-format cxl for 5.12 2021-02-24 09:38:36 -08:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS RISC-V Patches for the 5.12 Merge Window 2021-02-26 10:28:35 -08:00
Makefile Linux 5.12-rc2 2021-03-05 17:33:41 -08:00
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.