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SubjectRe: [PATCH] sched/fair: Consider RT/IRQ pressure in capacity_spare_wake
On 9 November 2017 at 19:52, Joel Fernandes <joelaf@google.com> wrote:
> capacity_spare_wake in the slow path influences choice of idlest groups,
> as we search for groups with maximum spare capacity. In scenarios where
> RT pressure is high, a sub optimal group can be chosen and hurt
> performance of the task being woken up.
>
> Several tests with results are included below to show improvements with
> this change.
>
> 1) Hackbench on Pixel 2 Android device (4x4 ARM64 Octa core)

"4x4 ARM64 Octa core" is confusing . At least for me, 4x4 means 16 cores :-)

> ------------------------------------------------------------
> Here we have RT activity running on big CPU cluster induced with rt-app,
> and running hackbench in parallel. The RT tasks are bound to 4 CPUs on
> the big cluster (cpu 4,5,6,7) and have 100ms periodicity with
> runtime=20ms sleep=80ms.
>
> Hackbench shows big benefit (30%) improvement when number of tasks is 8
> and 32: Note: data is completion time in seconds (lower is better).
> Number of loops for 8 and 16 tasks is 50000, and for 32 tasks its 20000.
> +--------+-----+-------+-------------------+---------------------------+
> | groups | fds | tasks | Without Patch | With Patch |
> +--------+-----+-------+---------+---------+-----------------+---------+
> | | | | Mean | Stdev | Mean | Stdev |
> | | | +-------------------+-----------------+---------+
> | 1 | 8 | 8 | 1.0534 | 0.13722 | 0.7293 (+30.7%) | 0.02653 |
> | 2 | 8 | 16 | 1.6219 | 0.16631 | 1.6391 (-1%) | 0.24001 |
> | 4 | 8 | 32 | 1.2538 | 0.13086 | 1.1080 (+11.6%) | 0.16201 |
> +--------+-----+-------+---------+---------+-----------------+---------+

Out of curiosity, do you know why you don't see any improvement for
16 tasks but only for 8 and 32 tasks ?

>
> 2) Rohit ran barrier.c test (details below) with following improvements:
> ------------------------------------------------------------------------
> This was Rohit's original use case for a patch he posted at [1] however
> from his recent tests he showed my patch can replace his slow path
> changes [1] and there's no need to selectively scan/skip CPUs in
> find_idlest_group_cpu in the slow path to get the improvement he sees.
>
> barrier.c (open_mp code) as a micro-benchmark. It does a number of
> iterations and barrier sync at the end of each for loop.
>
> Here barrier,c is running in along with ping on CPU 0 and 1 as:
> 'ping -l 10000 -q -s 10 -f hostX'
>
> barrier.c can be found at:
> http://www.spinics.net/lists/kernel/msg2506955.html
>
> Following are the results for the iterations per second with this
> micro-benchmark (higher is better), on a 44 core, 2 socket 88 Threads
> Intel x86 machine:
> +--------+------------------+---------------------------+
> |Threads | Without patch | With patch |
> | | | |
> +--------+--------+---------+-----------------+---------+
> | | Mean | Std Dev | Mean | Std Dev |
> +--------+--------+---------+-----------------+---------+
> |1 | 539.36 | 60.16 | 572.54 (+6.15%) | 40.95 |
> |2 | 481.01 | 19.32 | 530.64 (+10.32%)| 56.16 |
> |4 | 474.78 | 22.28 | 479.46 (+0.99%) | 18.89 |
> |8 | 450.06 | 24.91 | 447.82 (-0.50%) | 12.36 |
> |16 | 436.99 | 22.57 | 441.88 (+1.12%) | 7.39 |
> |32 | 388.28 | 55.59 | 429.4 (+10.59%)| 31.14 |
> |64 | 314.62 | 6.33 | 311.81 (-0.89%) | 11.99 |
> +--------+--------+---------+-----------------+---------+
>
> 3) ping+hackbench test on bare-metal sever (Rohit ran this test)
> ----------------------------------------------------------------
> Here hackbench is running in threaded mode along
> with, running ping on CPU 0 and 1 as:
> 'ping -l 10000 -q -s 10 -f hostX'
>
> This test is running on 2 socket, 20 core and 40 threads Intel x86
> machine:
> Number of loops is 10000 and runtime is in seconds (Lower is better).
>
> +--------------+-----------------+--------------------------+
> |Task Groups | Without patch | With patch |
> | +-------+---------+----------------+---------+
> |(Groups of 40)| Mean | Std Dev | Mean | Std Dev |
> +--------------+-------+---------+----------------+---------+
> |1 | 0.851 | 0.007 | 0.828 (+2.77%)| 0.032 |
> |2 | 1.083 | 0.203 | 1.087 (-0.37%)| 0.246 |
> |4 | 1.601 | 0.051 | 1.611 (-0.62%)| 0.055 |
> |8 | 2.837 | 0.060 | 2.827 (+0.35%)| 0.031 |
> |16 | 5.139 | 0.133 | 5.107 (+0.63%)| 0.085 |
> |25 | 7.569 | 0.142 | 7.503 (+0.88%)| 0.143 |
> +--------------+-------+---------+----------------+---------+
>
> [1] https://patchwork.kernel.org/patch/9991635/
>
> Matt Fleming also ran cyclictest and several different hackbench tests
> on his test machines to santiy-check that the patch doesn't harm any
> of his usecases.
>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Morten Ramussen <morten.rasmussen@arm.com>
> Cc: Brendan Jackman <brendan.jackman@arm.com>
> Tested-by: Rohit Jain <rohit.k.jain@oracle.com>
> Tested-by: Matt Fleming <matt@codeblueprint.co.uk>
> Signed-off-by: Joel Fernandes <joelaf@google.com>
> ---
> kernel/sched/fair.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 56f343b8e749..ba9609407cb9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5724,7 +5724,7 @@ static int cpu_util_wake(int cpu, struct task_struct *p);
>
> static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
> {
> - return capacity_orig_of(cpu) - cpu_util_wake(cpu, p);
> + return max_t(long, capacity_of(cpu) - cpu_util_wake(cpu, p), 0);

Make sense

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>


> }
>
> /*
> --
> 2.15.0.448.gf294e3d99a-goog
>

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