linux-kernel - [PATCH] sched/fair: ensure tasks spreading in LLC during LB

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Date:   Mon,  2 Nov 2020 11:24:57 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org, riel@...riel.com,
        clm@...com
Cc:     hannes@...xchg.org, Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH] sched/fair: ensure tasks spreading in LLC during LB

schbench shows latency increase for 95 percentile above since:
  commit 0b0695f2b34a ("sched/fair: Rework load_balance()")

Align the behavior of the load balancer with the wake up path, which tries
to select an idle CPU which belongs to the LLC for a waking task.

calculate_imbalance() will use nr_running instead of the spare
capacity when CPUs share resources (ie cache) at the domain level. This
will ensure a better spread of tasks on idle CPUs.

Running schbench on a hikey (8cores arm64) shows the problem:

tip/sched/core :
schbench -m 2 -t 4 -s 10000 -c 1000000 -r 10
Latency percentiles (usec)
	50.0th: 33
	75.0th: 45
	90.0th: 51
	95.0th: 4152
	*99.0th: 14288
	99.5th: 14288
	99.9th: 14288
	min=0, max=14276

tip/sched/core + patch :
schbench -m 2 -t 4 -s 10000 -c 1000000 -r 10
Latency percentiles (usec)
	50.0th: 34
	75.0th: 47
	90.0th: 52
	95.0th: 78
	*99.0th: 94
	99.5th: 94
	99.9th: 94
	min=0, max=94

Fixes: 0b0695f2b34a ("sched/fair: Rework load_balance()")
Reported-by: Chris Mason <clm@...com>
Suggested-by: Rik van Riel <riel@...riel.com>
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index aa4c6227cd6d..210b15f068a6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9031,7 +9031,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 	 * emptying busiest.
 	 */
 	if (local->group_type == group_has_spare) {
-		if (busiest->group_type > group_fully_busy) {
+		if ((busiest->group_type > group_fully_busy) &&
+		    !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
 			/*
 			 * If busiest is overloaded, try to fill spare
 			 * capacity. This might end up creating spare capacity
-- 
2.17.1