Re: [PATCH 2/2 v2] sched: use load_avg for selecting idlest group

[Matt Fleming]

On Fri, 02 Dec, at 07:31:04PM, Brendan Gregg wrote:
> 
> For background, is this from the "A decade of wasted cores" paper's
> patches?

No, this patch fixes an issue I originally reported here,

  https://lkml.kernel.org/r/20160923115808.2330-1-matt@xxxxxxxxxxxxxxxxxxx

Essentially, if you have an idle or partially-idle system and a
workload that consists of fork()'ing a bunch of tasks, where each of
those tasks immediately sleeps waiting for some wakeup, then those
tasks aren't spread across all idle CPUs very well.

We saw this issue when running hackbench with a small loop count, such
that the actual benchmark setup (fork()'ing) is where the majority of
the runtime is spent.

In that scenario, there's a large potential/blocked load, but
essentially no runnable load, and the balance on fork scheduler code
only cares about runnable load without Vincent's patch applied.

The closest thing I can find in the "A decade of wasted cores" paper
is "The Overload-on-Wakeup bug", but I don't think that's the issue
here since,

  a) We're balancing on fork, not wakeup
  b) The fork on balance code balances across nodes OK

> What's the expected typical gain? Thanks,

The results are still coming back from the SUSE performance test grid
but they do show that this patch is mainly a win for multi-socket
machines with more than 8 cores or thereabouts.

 [ Vincent, I'll follow up to your PATCH 1/2 with the results that are
   specifically for that patch ]

Assuming a fork-intensive or fork-dominated workload, and a
multi-socket machine, such as this 2 socket, NUMA, with 12 cores and
HT enabled (48 cpus), we saw a very clear win between +10% and +15%
for processes communicating via pipes,

  (1) tip-sched = tip/sched/core branch
  (2) fix-fig-for-fork = (1) + PATCH 1/2
  (3) fix-sig = (1) + (2) + PATCH 2/2

hackbench-process-pipes
                         4.9.0-rc6             4.9.0-rc6             4.9.0-rc6
                         tip-sched      fix-fig-for-fork               fix-sig
Amean    1        0.0717 (  0.00%)      0.0696 (  2.99%)      0.0730 ( -1.79%)
Amean    4        0.1244 (  0.00%)      0.1200 (  3.56%)      0.1190 (  4.36%)
Amean    7        0.1891 (  0.00%)      0.1937 ( -2.42%)      0.1831 (  3.17%)
Amean    12       0.2964 (  0.00%)      0.3116 ( -5.11%)      0.2784 (  6.07%)
Amean    21       0.4011 (  0.00%)      0.4090 ( -1.96%)      0.3574 ( 10.90%)
Amean    30       0.4944 (  0.00%)      0.4654 (  5.87%)      0.4171 ( 15.63%)
Amean    48       0.6113 (  0.00%)      0.6309 ( -3.20%)      0.5331 ( 12.78%)
Amean    79       0.8616 (  0.00%)      0.8706 ( -1.04%)      0.7710 ( 10.51%)
Amean    110      1.1304 (  0.00%)      1.2211 ( -8.02%)      1.0163 ( 10.10%)
Amean    141      1.3754 (  0.00%)      1.4279 ( -3.81%)      1.2803 (  6.92%)
Amean    172      1.6217 (  0.00%)      1.7367 ( -7.09%)      1.5363 (  5.27%)
Amean    192      1.7809 (  0.00%)      2.0199 (-13.42%)      1.7129 (  3.82%)

Things look even better when using threads and pipes, with wins
between 11% and 29% when looking at results outside of the noise,

hackbench-thread-pipes
                         4.9.0-rc6             4.9.0-rc6             4.9.0-rc6
                         tip-sched      fix-fig-for-fork               fix-sig
Amean    1        0.0736 (  0.00%)      0.0794 ( -7.96%)      0.0779 ( -5.83%)
Amean    4        0.1709 (  0.00%)      0.1690 (  1.09%)      0.1663 (  2.68%)
Amean    7        0.2836 (  0.00%)      0.3080 ( -8.61%)      0.2640 (  6.90%)
Amean    12       0.4393 (  0.00%)      0.4843 (-10.24%)      0.4090 (  6.89%)
Amean    21       0.5821 (  0.00%)      0.6369 ( -9.40%)      0.5126 ( 11.95%)
Amean    30       0.6557 (  0.00%)      0.6459 (  1.50%)      0.5711 ( 12.90%)
Amean    48       0.7924 (  0.00%)      0.7760 (  2.07%)      0.6286 ( 20.68%)
Amean    79       1.0534 (  0.00%)      1.0551 ( -0.16%)      0.8481 ( 19.49%)
Amean    110      1.5286 (  0.00%)      1.4504 (  5.11%)      1.1121 ( 27.24%)
Amean    141      1.9507 (  0.00%)      1.7790 (  8.80%)      1.3804 ( 29.23%)
Amean    172      2.2261 (  0.00%)      2.3330 ( -4.80%)      1.6336 ( 26.62%)
Amean    192      2.3753 (  0.00%)      2.3307 (  1.88%)      1.8246 ( 23.19%)

Somewhat surprisingly, I can see improvements for UMA machines with
fewer cores when the workload heavily saturates the machine and the
workload isn't dominated by fork. Such heavy saturation isn't super
realistic, but still interesting. I haven't dug into why these results
occurred, but I am happy things didn't instead fall off a cliff.

Here's a 4-cpu UMA box showing some improvement at the higher end,

hackbench-process-pipes
                        4.9.0-rc6             4.9.0-rc6             4.9.0-rc6
                        tip-sched      fix-fig-for-fork               fix-sig
Amean    1       3.5060 (  0.00%)      3.5747 ( -1.96%)      3.5117 ( -0.16%)
Amean    3       7.7113 (  0.00%)      7.8160 ( -1.36%)      7.7747 ( -0.82%)
Amean    5      11.4453 (  0.00%)     11.5710 ( -1.10%)     11.3870 (  0.51%)
Amean    7      15.3147 (  0.00%)     15.9420 ( -4.10%)     15.8450 ( -3.46%)
Amean    12     25.5110 (  0.00%)     24.3410 (  4.59%)     22.6717 ( 11.13%)
Amean    16     32.3010 (  0.00%)     28.5897 ( 11.49%)     25.7473 ( 20.29%)