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Date:   Fri, 8 Jan 2021 14:40:58 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Jiang Biao <benbjiang@...il.com>
Subject: Re: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost
 accounting

On Fri, Jan 08, 2021 at 02:41:19PM +0100, Vincent Guittot wrote:
> > 1. avg_scan_cost is now based on the average scan cost of a rq but
> >    avg_idle is still scaled to the domain size. This is a bit problematic
> >    because it's comparing scan cost of a single rq with the estimated
> >    average idle time of a domain. As a result, the scan depth can be much
> >    larger than it was before the patch and led to some regressions.
> 
> Point 1 makes sense to me too
> 
> >
> > 2. Accounting for the scan cost of success makes sense but there is a
> >    big difference between a scan that finds an idle CPU and one that fails.
> >    For failures, the scan cost is wasted CPU time where as a success
> >    means that an uncontested CPU is used. This can cause a search to be
> >    truncated earlier than it should be when the domain is lightly loaded.
> 
> But I'm not sure to catch your problem with point 2.
> track the average cost to scan one rq so looping all rqs are only few
> should not impact (much) the avg_scan_cost
> 
> Trying to bias the avg_scan_cost with:  loops <<= 2;
> will just make avg_scan_cost lost any kind of meaning because it
> doesn't reflect the avg cost of scanning a rq anymore
> 

Before the series, the avg_scan_cost also did not represent the cost of
scanning a RQ before either. Treating scan failures and successes equally
can problems with the depth of the scan conducted. Previously the "cost"
of a successful scan was 0 so successful scans allowed deeper scans in
the near future. This partially illustrates the problem.

                          5.11.0-rc2             5.11.0-rc2             5.11.0-rc2
                       baseline-v2r1          acctscan-v2r1           altscan-v2r8
Hmean     1        429.47 (   0.00%)      420.90 *  -2.00%*      414.27 *  -3.54%*
Hmean     2        709.39 (   0.00%)      796.05 *  12.22%*      791.98 *  11.64%*
Hmean     4       1449.19 (   0.00%)     1445.14 (  -0.28%)     1319.09 *  -8.98%*
Hmean     8       2765.65 (   0.00%)     2750.07 *  -0.56%*     2756.17 *  -0.34%*
Hmean     16      5158.47 (   0.00%)     5056.59 *  -1.97%*     5030.67 *  -2.48%*
Hmean     32      8969.96 (   0.00%)     8796.96 *  -1.93%*     8768.34 *  -2.25%*
Hmean     64     11210.05 (   0.00%)     9910.39 * -11.59%*    11073.42 *  -1.22%*
Hmean     128    17978.21 (   0.00%)    17031.41 *  -5.27%*    17037.76 *  -5.23%*
Hmean     256    16143.32 (   0.00%)    15636.59 *  -3.14%*    15761.12 *  -2.37%*
Hmean     320    16388.59 (   0.00%)    15591.78 *  -4.86%*    15588.85 *  -4.88%*

Note the impact of Peters patch (accescan-v2r1) for 64 threads. The
machine is 2-socket (40 cores, 80 threads) so 64 is the load is
balancing between two domains (load balancing vs wakeup migrations).
altscan is my suggested patch on top and with Peter's patch, there is a
11.59% regression that is negligible with my patch on top.

The impact is machine-specific or specific to the CPU generation. Here
is just comparing just the suggested alteration on a slightly older
generation.

                          5.11.0-rc2             5.11.0-rc2
                       acctscan-v2r1           altscan-v2r8
Hmean     1        155.44 (   0.00%)      183.32 *  17.94%*
Hmean     2        445.46 (   0.00%)      548.51 *  23.13%*
Hmean     4       1080.25 (   0.00%)     1112.49 *   2.98%*
Hmean     8       2253.48 (   0.00%)     2457.46 *   9.05%*
Hmean     16      3996.73 (   0.00%)     4244.59 *   6.20%*
Hmean     32      5318.93 (   0.00%)     5798.17 *   9.01%*
Hmean     64      9301.55 (   0.00%)     9563.24 *   2.81%*
Hmean     128     8560.89 (   0.00%)     8873.72 *   3.65%*
Hmean     192     8526.92 (   0.00%)     8843.43 *   3.71%*

And another 2-socket machine on a newer generation.

Hmean     1        551.16 (   0.00%)      503.75 *  -8.60%*
Hmean     2       1074.19 (   0.00%)     1078.08 *   0.36%*
Hmean     4       2024.72 (   0.00%)     2049.29 *   1.21%*
Hmean     8       3762.49 (   0.00%)     4002.24 *   6.37%*
Hmean     16      6589.98 (   0.00%)     6688.21 *   1.49%*
Hmean     32     10080.23 (   0.00%)    10270.34 *   1.89%*
Hmean     64     11349.16 (   0.00%)    12452.68 *   9.72%*
Hmean     128    21670.93 (   0.00%)    21823.70 *   0.70%*
Hmean     256    20605.62 (   0.00%)    20615.01 *   0.05%*
Hmean     320    20974.29 (   0.00%)    20565.11 *  -1.95%*

For hackbench with processes communicating via pipes on the first
machine

                          5.11.0-rc2             5.11.0-rc2
                       acctscan-v2r1           altscan-v2r8
Amean     1        0.3927 (   0.00%)      0.3943 (  -0.42%)
Amean     4        0.9247 (   0.00%)      0.9267 (  -0.22%)
Amean     7        1.4587 (   0.00%)      1.5147 *  -3.84%*
Amean     12       2.3637 (   0.00%)      2.4507 *  -3.68%*
Amean     21       4.0700 (   0.00%)      4.1757 *  -2.60%*
Amean     30       5.6573 (   0.00%)      5.7390 *  -1.44%*
Amean     48       8.9037 (   0.00%)      8.8053 *   1.10%*
Amean     79      14.9190 (   0.00%)     14.4360 *   3.24%*
Amean     110     22.5703 (   0.00%)     21.9210 (   2.88%)
Amean     141     29.2400 (   0.00%)     28.0110 *   4.20%*
Amean     172     36.3720 (   0.00%)     34.7963 (   4.33%)
Amean     203     43.5783 (   0.00%)     42.5537 *   2.35%*
Amean     234     50.3653 (   0.00%)     47.3463 *   5.99%*
Amean     265     57.6153 (   0.00%)     55.6247 *   3.46%*
Amean     296     62.7370 (   0.00%)     62.0720 (   1.06%)

Adjusting the scan cost for successes is neither a universal win or
failure but it's closer to historical behaviour and the strict
accounting does hit corner cases.  If a deep scan is finding an idle CPU,
it makes some sense to continue scanning deeply by adjusting the weight
instead of prematurely failing.

The testing of the full series previously showed that some loads never
recovered from side-effects of the first patch and the last patch in the
series introduced new problems of its own. Hence, I would like to limit
the negative impact of the first patch and, if necessary, cut the last
patch altogether.

-- 
Mel Gorman
SUSE Labs

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