Re: [PATCH v8 2/2] sched/fair: Introduce SIS_CURRENT to wake up short task on current CPU

[Mike Galbraith]

On Sat, 2023-04-29 at 07:16 +0800, Chen Yu wrote:
> [Problem Statement]
> For a workload that is doing frequent context switches, the throughput
> scales well until the number of instances reaches a peak point. After
> that peak point, the throughput drops significantly if the number of
> instances continue to increase.
>
> The will-it-scale context_switch1 test case exposes the issue. The
> test platform has 2 x 56C/112T and 224 CPUs in total. will-it-scale
> launches 1, 8, 16 ... instances respectively. Each instance is composed
> of 2 tasks, and each pair of tasks would do ping-pong scheduling via
> pipe_read() and pipe_write(). No task is bound to any CPU. It is found
> that, once the number of instances is higher than 56, the throughput
> drops accordingly:
>
>           ^
> throughput|
>           |                 X
>           |               X   X X
>           |             X         X X
>           |           X               X
>           |         X                   X
>           |       X
>           |     X
>           |   X
>           | X
>           |
>           +-----------------.------------------->
>                             56
>                                  number of instances

Should these buddy pairs not start interfering with one another at 112
instances instead of 56? NR_CPUS/2 buddy pair instances is the point at
which trying to turn waker/wakee overlap into throughput should tend
toward being a loser due to man-in-the-middle wakeup delay pain more
than offsetting overlap recovery gain, rendering sync wakeup thereafter
an ever more likely win.

Anyway..

What I see in my box, and I bet a virtual nickle it's a player in your
box as well, is WA_WEIGHT making a mess of things by stacking tasks,
sometimes very badly.  Below, I start NR_CPUS tbench buddy pairs in
crusty ole i4790 desktop box with WA_WEIGHT turned off, then turn it on
remotely as to not have noisy GUI muck up my demo.

...
   8   3155749  3606.79 MB/sec  warmup  38 sec  latency 3.852 ms
   8   3238485  3608.75 MB/sec  warmup  39 sec  latency 3.839 ms
   8   3321578  3608.59 MB/sec  warmup  40 sec  latency 3.882 ms
   8   3404746  3608.09 MB/sec  warmup  41 sec  latency 2.273 ms
   8   3487885  3607.58 MB/sec  warmup  42 sec  latency 3.869 ms
   8   3571034  3607.12 MB/sec  warmup  43 sec  latency 3.855 ms
   8   3654067  3607.48 MB/sec  warmup  44 sec  latency 3.857 ms
   8   3736973  3608.83 MB/sec  warmup  45 sec  latency 4.008 ms
   8   3820160  3608.33 MB/sec  warmup  46 sec  latency 3.849 ms
   8   3902963  3607.60 MB/sec  warmup  47 sec  latency 14.241 ms
   8   3986117  3607.17 MB/sec  warmup  48 sec  latency 20.290 ms
   8   4069256  3606.70 MB/sec  warmup  49 sec  latency 28.284 ms
   8   4151986  3608.35 MB/sec  warmup  50 sec  latency 17.216 ms
   8   4235070  3608.06 MB/sec  warmup  51 sec  latency 23.221 ms
   8   4318221  3607.81 MB/sec  warmup  52 sec  latency 28.285 ms
   8   4401456  3607.29 MB/sec  warmup  53 sec  latency 20.835 ms
   8   4484606  3607.06 MB/sec  warmup  54 sec  latency 28.943 ms
   8   4567609  3607.32 MB/sec  warmup  55 sec  latency 28.254 ms

Where I turned it on is hard to miss.

Short duration thread pool workers can be stacked all the way to the
ceiling by WA_WEIGHT during burst wakeups, with wake_wide() not being
able to intervene due to lack of cross coupling between waker/wakees
leading to heuristic failure.  A (now long) while ago I caught that
happening with firefox event threads, it launched 32 of 'em in my 8 rq
box (hmm), and them being essentially the scheduler equivalent of
neutrinos (nearly massless), we stuffed 'em all into one rq.. and got
away with it because those particular threads don't seem to do much of
anything.  However, were they to go active, the latency hit that we set
up could have stung mightily. That scenario being highly generic leads
me to suspect that somewhere out there in the big wide world, folks are
eating that burst serialization.

	-Mike