linux-kernel - [PATCH 9/9] sched,fair: Alternative sched

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Message-ID: <20210326103935.444833549@infradead.org>
Date:   Fri, 26 Mar 2021 11:34:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, mgorman@...e.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, bristot@...hat.com,
        joshdon@...gle.com, valentin.schneider@....com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org, greg@...ah.com
Subject: [PATCH 9/9] sched,fair: Alternative sched_slice()

The current sched_slice() seems to have issues; there's two possible
things that could be improved:

 - the 'nr_running' used for __sched_period() is daft when cgroups are
   considered. Using the RQ wide h_nr_running seems like a much more
   consistent number.

 - (esp) cgroups can slice it real fine, which makes for easy
   over-scheduling, ensure min_gran is what the name says.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/fair.c     |   15 ++++++++++++++-
 kernel/sched/features.h |    3 +++
 2 files changed, 17 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -680,7 +680,16 @@ static u64 __sched_period(unsigned long
  */
 static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
+	unsigned int nr_running = cfs_rq->nr_running;
+	u64 slice;
+
+	if (sched_feat(ALT_PERIOD))
+		nr_running = rq_of(cfs_rq)->cfs.h_nr_running;
+
+	slice = __sched_period(nr_running + !se->on_rq);
+
+	if (sched_feat(BASE_SLICE))
+		slice -= sysctl_sched_min_granularity;
 
 	for_each_sched_entity(se) {
 		struct load_weight *load;
@@ -697,6 +706,10 @@ static u64 sched_slice(struct cfs_rq *cf
 		}
 		slice = __calc_delta(slice, se->load.weight, load);
 	}
+
+	if (sched_feat(BASE_SLICE))
+		slice += sysctl_sched_min_granularity;
+
 	return slice;
 }
 
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -90,3 +90,6 @@ SCHED_FEAT(WA_BIAS, true)
  */
 SCHED_FEAT(UTIL_EST, true)
 SCHED_FEAT(UTIL_EST_FASTUP, true)
+
+SCHED_FEAT(ALT_PERIOD, true)
+SCHED_FEAT(BASE_SLICE, true)