[PATCH] sched/fair: Consider RT/IRQ pressure in capacity_spare_wake

[Joel Fernandes]

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)
------------------------------------------------------------
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 |
+--------+-----+-------+---------+---------+-----------------+---------+

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@xxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Morten Ramussen <morten.rasmussen@xxxxxxx>
Cc: Brendan Jackman <brendan.jackman@xxxxxxx>
Tested-by: Rohit Jain <rohit.k.jain@xxxxxxxxxx>
Tested-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
---
 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);
 }
 
 /*
-- 
2.15.0.448.gf294e3d99a-goog