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Peter Zijlstra 94aafc3ee3 sched/fair: Age the average idle time
This is a partial forward-port of Peter Ziljstra's work first posted
at:

   https://lore.kernel.org/lkml/20180530142236.667774973@infradead.org/

Currently select_idle_cpu()'s proportional scheme uses the average idle
time *for when we are idle*, that is temporally challenged.  When a CPU
is not at all idle, we'll happily continue using whatever value we did
see when the CPU goes idle. To fix this, introduce a separate average
idle and age it (the existing value still makes sense for things like
new-idle balancing, which happens when we do go idle).

The overall goal is to not spend more time scanning for idle CPUs than
we're idle for. Otherwise we're inhibiting work. This means that we need to
consider the cost over all the wake-ups between consecutive idle periods.
To track this, the scan cost is subtracted from the estimated average
idle time.

The impact of this patch is related to workloads that have domains that
are fully busy or overloaded. Without the patch, the scan depth may be
too high because a CPU is not reaching idle.

Due to the nature of the patch, this is a regression magnet. It
potentially wins when domains are almost fully busy or overloaded --
at that point searches are likely to fail but idle is not being aged
as CPUs are active so search depth is too large and useless. It will
potentially show regressions when there are idle CPUs and a deep search is
beneficial. This tbench result on a 2-socket broadwell machine partially
illustates the problem

                          5.13.0-rc2             5.13.0-rc2
                             vanilla     sched-avgidle-v1r5
Hmean     1        445.02 (   0.00%)      451.36 *   1.42%*
Hmean     2        830.69 (   0.00%)      846.03 *   1.85%*
Hmean     4       1350.80 (   0.00%)     1505.56 *  11.46%*
Hmean     8       2888.88 (   0.00%)     2586.40 * -10.47%*
Hmean     16      5248.18 (   0.00%)     5305.26 *   1.09%*
Hmean     32      8914.03 (   0.00%)     9191.35 *   3.11%*
Hmean     64     10663.10 (   0.00%)    10192.65 *  -4.41%*
Hmean     128    18043.89 (   0.00%)    18478.92 *   2.41%*
Hmean     256    16530.89 (   0.00%)    17637.16 *   6.69%*
Hmean     320    16451.13 (   0.00%)    17270.97 *   4.98%*

Note that 8 was a regression point where a deeper search would have helped
but it gains for high thread counts when searches are useless. Hackbench
is a more extreme example although not perfect as the tasks idle rapidly

hackbench-process-pipes
                          5.13.0-rc2             5.13.0-rc2
                             vanilla     sched-avgidle-v1r5
Amean     1        0.3950 (   0.00%)      0.3887 (   1.60%)
Amean     4        0.9450 (   0.00%)      0.9677 (  -2.40%)
Amean     7        1.4737 (   0.00%)      1.4890 (  -1.04%)
Amean     12       2.3507 (   0.00%)      2.3360 *   0.62%*
Amean     21       4.0807 (   0.00%)      4.0993 *  -0.46%*
Amean     30       5.6820 (   0.00%)      5.7510 *  -1.21%*
Amean     48       8.7913 (   0.00%)      8.7383 (   0.60%)
Amean     79      14.3880 (   0.00%)     13.9343 *   3.15%*
Amean     110     21.2233 (   0.00%)     19.4263 *   8.47%*
Amean     141     28.2930 (   0.00%)     25.1003 *  11.28%*
Amean     172     34.7570 (   0.00%)     30.7527 *  11.52%*
Amean     203     41.0083 (   0.00%)     36.4267 *  11.17%*
Amean     234     47.7133 (   0.00%)     42.0623 *  11.84%*
Amean     265     53.0353 (   0.00%)     47.7720 *   9.92%*
Amean     296     60.0170 (   0.00%)     53.4273 *  10.98%*
Stddev    1        0.0052 (   0.00%)      0.0025 (  51.57%)
Stddev    4        0.0357 (   0.00%)      0.0370 (  -3.75%)
Stddev    7        0.0190 (   0.00%)      0.0298 ( -56.64%)
Stddev    12       0.0064 (   0.00%)      0.0095 ( -48.38%)
Stddev    21       0.0065 (   0.00%)      0.0097 ( -49.28%)
Stddev    30       0.0185 (   0.00%)      0.0295 ( -59.54%)
Stddev    48       0.0559 (   0.00%)      0.0168 (  69.92%)
Stddev    79       0.1559 (   0.00%)      0.0278 (  82.17%)
Stddev    110      1.1728 (   0.00%)      0.0532 (  95.47%)
Stddev    141      0.7867 (   0.00%)      0.0968 (  87.69%)
Stddev    172      1.0255 (   0.00%)      0.0420 (  95.91%)
Stddev    203      0.8106 (   0.00%)      0.1384 (  82.92%)
Stddev    234      1.1949 (   0.00%)      0.1328 (  88.89%)
Stddev    265      0.9231 (   0.00%)      0.0820 (  91.11%)
Stddev    296      1.0456 (   0.00%)      0.1327 (  87.31%)

Again, higher thread counts benefit and the standard deviation
shows that results are also a lot more stable when the idle
time is aged.

The patch potentially matters when a socket was multiple LLCs as the
maximum search depth is lower. However, some of the test results were
suspiciously good (e.g. specjbb2005 gaining 50% on a Zen1 machine) and
other results were not dramatically different to other mcahines.

Given the nature of the patch, Peter's full series is not being forward
ported as each part should stand on its own. Preferably they would be
merged at different times to reduce the risk of false bisections.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210615111611.GH30378@techsingularity.net
2021-06-17 14:11:44 +02:00
arch Merge branch 'sched/urgent' into sched/core, to pick up fixes 2021-06-03 19:00:49 +02:00
block block-5.13-2021-05-22 2021-05-22 07:40:34 -10:00
certs Kbuild updates for v5.13 (2nd) 2021-05-08 10:00:11 -07:00
crypto for-5.13/drivers-2021-04-27 2021-04-28 14:39:37 -07:00
Documentation Documentation: Add usecases, design and interface for core scheduling 2021-06-10 09:57:09 +02:00
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fs Merge branch 'sched/urgent' into sched/core, to pick up fixes 2021-06-03 19:00:49 +02:00
include sched/cpufreq: Consider reduced CPU capacity in energy calculation 2021-06-17 14:11:43 +02:00
init Merge branch 'sched/urgent' into sched/core, to pick up fixes 2021-06-03 19:00:49 +02:00
ipc ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry 2021-05-22 15:09:07 -10:00
kernel sched/fair: Age the average idle time 2021-06-17 14:11:44 +02:00
lib Merge branch 'sched/urgent' into sched/core, to pick up fixes 2021-06-03 19:00:49 +02:00
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mm userfaultfd: hugetlbfs: fix new flag usage in error path 2021-05-22 15:09:07 -10:00
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security trusted-keys: match tpm_get_ops on all return paths 2021-05-12 22:36:37 +03:00
sound sound fixes for 5.12-rc4 2021-05-28 08:47:50 -10:00
tools Merge branch 'sched/urgent' into sched/core, to pick up fixes 2021-06-03 19:00:49 +02:00
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virt KVM: VMX: update vcpu posted-interrupt descriptor when assigning device 2021-05-27 07:58:23 -04:00
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.mailmap Merge drm/drm-fixes into drm-misc-fixes 2021-05-11 13:35:52 +02:00
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CREDITS MAINTAINERS: move Murali Karicheri to credits 2021-04-29 15:47:30 -07:00
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MAINTAINERS Merge branch 'i2c/for-current' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux 2021-05-29 18:24:00 -10:00
Makefile Linux 5.13-rc4 2021-05-30 11:58:25 -10:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

Linux kernel
============

There are several guides for kernel developers and users. These guides can
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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.