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Tianchen Ding f3dd3f6745 sched: Remove the limitation of WF_ON_CPU on wakelist if wakee cpu is idle 
Wakelist can help avoid cache bouncing and offload the overhead of waker
cpu. So far, using wakelist within the same llc only happens on
WF_ON_CPU, and this limitation could be removed to further improve
wakeup performance.

The commit 518cd62341 ("sched: Only queue remote wakeups when
crossing cache boundaries") disabled queuing tasks on wakelist when
the cpus share llc. This is because, at that time, the scheduler must
send IPIs to do ttwu_queue_wakelist. Nowadays, ttwu_queue_wakelist also
supports TIF_POLLING, so this is not a problem now when the wakee cpu is
in idle polling.

Benefits:
  Queuing the task on idle cpu can help improving performance on waker cpu
  and utilization on wakee cpu, and further improve locality because
  the wakee cpu can handle its own rq. This patch helps improving rt on
  our real java workloads where wakeup happens frequently.

  Consider the normal condition (CPU0 and CPU1 share same llc)
  Before this patch:

         CPU0                                       CPU1

    select_task_rq()                                idle
    rq_lock(CPU1->rq)
    enqueue_task(CPU1->rq)
    notify CPU1 (by sending IPI or CPU1 polling)

                                                    resched()

  After this patch:

         CPU0                                       CPU1

    select_task_rq()                                idle
    add to wakelist of CPU1
    notify CPU1 (by sending IPI or CPU1 polling)

                                                    rq_lock(CPU1->rq)
                                                    enqueue_task(CPU1->rq)
                                                    resched()

  We see CPU0 can finish its work earlier. It only needs to put task to
  wakelist and return.
  While CPU1 is idle, so let itself handle its own runqueue data.

This patch brings no difference about IPI.
  This patch only takes effect when the wakee cpu is:
  1) idle polling
  2) idle not polling

  For 1), there will be no IPI with or without this patch.

  For 2), there will always be an IPI before or after this patch.
  Before this patch: waker cpu will enqueue task and check preempt. Since
  "idle" will be sure to be preempted, waker cpu must send a resched IPI.
  After this patch: waker cpu will put the task to the wakelist of wakee
  cpu, and send an IPI.

Benchmark:
We've tested schbench, unixbench, and hachbench on both x86 and arm64.

On x86 (Intel Xeon Platinum 8269CY):
  schbench -m 2 -t 8

    Latency percentiles (usec)              before        after
        50.0000th:                             8            6
        75.0000th:                            10            7
        90.0000th:                            11            8
        95.0000th:                            12            8
        *99.0000th:                           13           10
        99.5000th:                            15           11
        99.9000th:                            18           14

  Unixbench with full threads (104)
                                            before        after
    Dhrystone 2 using register variables  3011862938    3009935994  -0.06%
    Double-Precision Whetstone              617119.3      617298.5   0.03%
    Execl Throughput                         27667.3       27627.3  -0.14%
    File Copy 1024 bufsize 2000 maxblocks   785871.4      784906.2  -0.12%
    File Copy 256 bufsize 500 maxblocks     210113.6      212635.4   1.20%
    File Copy 4096 bufsize 8000 maxblocks  2328862.2     2320529.1  -0.36%
    Pipe Throughput                      145535622.8   145323033.2  -0.15%
    Pipe-based Context Switching           3221686.4     3583975.4  11.25%
    Process Creation                        101347.1      103345.4   1.97%
    Shell Scripts (1 concurrent)            120193.5      123977.8   3.15%
    Shell Scripts (8 concurrent)             17233.4       17138.4  -0.55%
    System Call Overhead                   5300604.8     5312213.6   0.22%

  hackbench -g 1 -l 100000
                                            before        after
    Time                                     3.246        2.251

On arm64 (Ampere Altra):
  schbench -m 2 -t 8

    Latency percentiles (usec)              before        after
        50.0000th:                            14           10
        75.0000th:                            19           14
        90.0000th:                            22           16
        95.0000th:                            23           16
        *99.0000th:                           24           17
        99.5000th:                            24           17
        99.9000th:                            28           25

  Unixbench with full threads (80)
                                            before        after
    Dhrystone 2 using register variables  3536194249    3537019613   0.02%
    Double-Precision Whetstone              629383.6      629431.6   0.01%
    Execl Throughput                         65920.5       65846.2  -0.11%
    File Copy 1024 bufsize 2000 maxblocks  1063722.8     1064026.8   0.03%
    File Copy 256 bufsize 500 maxblocks     322684.5      318724.5  -1.23%
    File Copy 4096 bufsize 8000 maxblocks  2348285.3     2328804.8  -0.83%
    Pipe Throughput                      133542875.3   131619389.8  -1.44%
    Pipe-based Context Switching           3215356.1     3576945.1  11.25%
    Process Creation                        108520.5      120184.6  10.75%
    Shell Scripts (1 concurrent)            122636.3        121888  -0.61%
    Shell Scripts (8 concurrent)             17462.1       17381.4  -0.46%
    System Call Overhead                   4429998.9     4435006.7   0.11%

  hackbench -g 1 -l 100000
                                            before        after
    Time                                     4.217        2.916

Our patch has improvement on schbench, hackbench
and Pipe-based Context Switching of unixbench
when there exists idle cpus,
and no obvious regression on other tests of unixbench.
This can help improve rt in scenes where wakeup happens frequently.

Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/20220608233412.327341-3-dtcccc@linux.alibaba.com
2022-06-13 10:30:01 +02:00
arch virtio,vdpa: fixes 2022-06-11 16:32:47 -07:00
block block: remove bioset_init_from_src 2022-06-08 14:04:14 -04:00
certs certs: Convert spaces in certs/Makefile to a tab 2022-06-10 11:42:02 -07:00
crypto This update includes the following changes: 2022-05-27 18:06:49 -07:00
Documentation Kbuild fixes for v5.19 2022-06-12 11:10:07 -07:00
drivers platform-drivers-x86 for v5.19-2 2022-06-12 11:33:42 -07:00
fs 3 smb3 reconnect fixes 2022-06-12 11:05:44 -07:00
include Workqueue fixes for v5.19-rc1 2022-06-12 11:16:00 -07:00
init gcc-12: disable '-Warray-bounds' universally for now 2022-06-09 10:11:12 -07:00
ipc These changes update the ipc sysctls so that they are fundamentally 2022-06-03 15:54:57 -07:00
kernel sched: Remove the limitation of WF_ON_CPU on wakelist if wakee cpu is idle 2022-06-13 10:30:01 +02:00
lib Random number generator fixes for Linux 5.19-rc2. 2022-06-12 10:33:38 -07:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm/huge_memory: Fix xarray node memory leak 2022-06-09 16:24:25 -04:00
net Notable changes: 2022-06-10 17:28:43 -07:00
samples drm for 5.19-rc1 2022-05-25 16:18:27 -07:00
scripts Kbuild fixes for v5.19 2022-06-12 11:10:07 -07:00
security KEYS: trusted: tpm2: Fix migratable logic 2022-06-08 14:12:13 +03:00
sound sound fixes for 5.19-rc2 2022-06-10 10:20:57 -07:00
tools Random number generator fixes for Linux 5.19-rc2. 2022-06-12 10:33:38 -07:00
usr Not a lot of material this cycle. Many singleton patches against various 2022-05-27 11:22:03 -07:00
virt * Fix syzkaller NULL pointer dereference 2022-06-08 09:16:31 -07:00
.clang-format clang-format: Fix space after for_each macros 2022-05-20 19:27:16 +02:00
.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 kbuild: split the second line of *.mod into *.usyms 2022-05-08 03:16:59 +09:00
.mailmap Hot fixes for 5.19-rc1. 2022-06-05 17:05:38 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: replace a Microchip AT91 maintainer 2022-02-09 11:30:01 +01:00
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 Notable changes: 2022-06-10 17:28:43 -07:00
Makefile Linux 5.19-rc2 2022-06-12 16:11:37 -07:00
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.