[PATCH v6 2/4] sched/fair: Ignore SIS_UTIL when has_idle_core

From: Abel Wu
Date: Wed Oct 19 2022 - 08:48:21 EST

When SIS_UTIL is enabled, SIS domain scan will be skipped if the
LLC is overloaded even the has_idle_core hint is true. Since idle
load balancing is triggered at tick boundary, the idle cores can
stay cold for the whole tick period wasting time meanwhile some
of other cpus might be overloaded.

Give it a chance to scan for idle cores if the hint implies a
worthy effort.

Benchmark
=========

All of the benchmarks are done inside a normal cpu cgroup in a clean
environment with cpu turbo disabled, and test machines are:

A) A dual socket machine modeled Intel Xeon(R) Platinum 8260 with SNC
disabled, so there are 2 NUMA nodes each of which has 24C/48T. Each
NUMA shares an LLC.

B) A dual socket machine modeled AMD EPYC 7Y83 64-Core Processor with
NPS1 enabled, so there are 2 NUMA nodes each of which has 64C/128T.
Each NUMA node contains several LLCs sized of 16 cpus.

Based on tip sched/core fb04563d1cae (v5.19.0).

Results
=======

hackbench-process-pipes
(A) vanilla patched
Amean 1 0.2767 ( 0.00%) 0.2540 ( 8.19%)
Amean 4 0.6080 ( 0.00%) 0.6220 ( -2.30%)
Amean 7 0.7923 ( 0.00%) 0.8020 ( -1.22%)
Amean 12 1.3917 ( 0.00%) 1.1823 ( 15.04%)
Amean 21 3.6747 ( 0.00%) 2.7717 ( 24.57%)
Amean 30 6.7070 ( 0.00%) 5.1200 * 23.66%*
Amean 48 9.3537 ( 0.00%) 8.5890 * 8.18%*
Amean 79 11.6627 ( 0.00%) 11.2580 ( 3.47%)
Amean 110 13.4473 ( 0.00%) 13.1283 ( 2.37%)
Amean 141 16.4747 ( 0.00%) 15.5967 * 5.33%*
Amean 172 19.0000 ( 0.00%) 18.1153 * 4.66%*
Amean 203 21.4200 ( 0.00%) 21.1340 ( 1.34%)
Amean 234 24.2250 ( 0.00%) 23.8227 ( 1.66%)
Amean 265 27.2400 ( 0.00%) 26.8293 ( 1.51%)
Amean 296 30.6937 ( 0.00%) 29.5800 * 3.63%*
(B)
Amean 1 0.3543 ( 0.00%) 0.3650 ( -3.01%)
Amean 4 0.4623 ( 0.00%) 0.4837 ( -4.61%)
Amean 7 0.5117 ( 0.00%) 0.4997 ( 2.35%)
Amean 12 0.5707 ( 0.00%) 0.5863 ( -2.75%)
Amean 21 0.9717 ( 0.00%) 0.8930 * 8.10%*
Amean 30 1.4423 ( 0.00%) 1.2530 ( 13.13%)
Amean 48 2.3520 ( 0.00%) 1.9743 * 16.06%*
Amean 79 5.7193 ( 0.00%) 3.4933 * 38.92%*
Amean 110 6.9893 ( 0.00%) 5.5963 * 19.93%*
Amean 141 9.1103 ( 0.00%) 7.6550 ( 15.97%)
Amean 172 10.2490 ( 0.00%) 8.8323 * 13.82%*
Amean 203 11.3727 ( 0.00%) 10.8683 ( 4.43%)
Amean 234 12.7627 ( 0.00%) 11.8683 ( 7.01%)
Amean 265 13.8947 ( 0.00%) 13.4717 ( 3.04%)
Amean 296 14.1093 ( 0.00%) 13.8130 ( 2.10%)

The results can approximately divided into 3 sections:
- busy, e.g. <12 groups on A and <21 groups on B
- overloaded, e.g. 12~48 groups on A and 21~172 groups on B
- saturated, the rest part

For the busy part the result is neutral with slight wins or loss.
It is probably because there are still idle cpus not hard to be find
so the effort we paid for locating an idle core will bring limited
benefit which can be negated by the cost of full scan easily.

While for the overloaded but not saturated part, great improvement
can be seen due to exploiting the cpu resources by more actively
kicking idle cores working. But once all cpus are totally saturated,
scanning for idle cores doesn't help much.

One concern of the full scan is that the cost gets bigger in larger
LLCs, but the test result seems still positive. One possible reason
is due to the low SIS success rate (<2%), so the paid effort will
indeed trade for efficiency.

tbench4 Throughput
(A) vanilla patched
Hmean 1 275.61 ( 0.00%) 280.53 * 1.78%*
Hmean 2 541.28 ( 0.00%) 561.94 * 3.82%*
Hmean 4 1102.62 ( 0.00%) 1109.14 * 0.59%*
Hmean 8 2149.58 ( 0.00%) 2229.39 * 3.71%*
Hmean 16 4305.40 ( 0.00%) 4383.06 * 1.80%*
Hmean 32 7088.36 ( 0.00%) 7124.14 * 0.50%*
Hmean 64 8609.16 ( 0.00%) 8815.41 * 2.40%*
Hmean 128 19304.92 ( 0.00%) 19519.35 * 1.11%*
Hmean 256 19147.04 ( 0.00%) 19392.24 * 1.28%*
Hmean 384 18970.86 ( 0.00%) 19201.07 * 1.21%*
(B)
Hmean 1 519.62 ( 0.00%) 515.98 * -0.70%*
Hmean 2 1042.92 ( 0.00%) 1031.54 * -1.09%*
Hmean 4 1959.10 ( 0.00%) 1953.44 * -0.29%*
Hmean 8 3842.82 ( 0.00%) 3622.52 * -5.73%*
Hmean 16 6768.50 ( 0.00%) 6545.82 * -3.29%*
Hmean 32 12589.50 ( 0.00%) 13697.73 * 8.80%*
Hmean 64 24797.23 ( 0.00%) 25589.59 * 3.20%*
Hmean 128 38036.66 ( 0.00%) 35667.64 * -6.23%*
Hmean 256 65069.93 ( 0.00%) 65215.85 * 0.22%*
Hmean 512 61147.99 ( 0.00%) 66035.57 * 7.99%*
Hmean 1024 48542.73 ( 0.00%) 53391.64 * 9.99%*

The tbench4 test has a ~40% success rate on used target, prev or
recent cpus, and ~45% of total success rate. And the core scan is
also not very frequent, so the benefit this patch brings is limited
while still some gains can be seen.

netperf

The netperf has an almost 100% success rate on used target, prev or
recent cpus, so the domain scan is generally not performed and not
affected by this patch.

Conclusion
==========

Taking full scan for idle cores is generally good for making better
use of the cpu resources.

Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Tested-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
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 e7f82fa92c5b..7b668e16812e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6436,7 +6436,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
time = cpu_clock(this);
}

- if (sched_feat(SIS_UTIL)) {
+ if (sched_feat(SIS_UTIL) && !has_idle_core) {
sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
if (sd_share) {
/* because !--nr is the condition to stop scan */
--
2.37.3