linux-kernel - [PATCH 14/17] sched/eevdf: Better handle mixed slice length

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Message-ID: <20230328110354.562078801@infradead.org>
Date:   Tue, 28 Mar 2023 11:26:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, vincent.guittot@...aro.org
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, corbet@....net, qyousef@...alina.io,
        chris.hyser@...cle.com, patrick.bellasi@...bug.net, pjt@...gle.com,
        pavel@....cz, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
        yu.c.chen@...el.com, youssefesmat@...omium.org,
        joel@...lfernandes.org, efault@....de
Subject: [PATCH 14/17] sched/eevdf: Better handle mixed slice length

In the case where (due to latency-nice) there are different request
sizes in the tree, the smaller requests tend to be dominated by the
larger. Also note how the EEVDF lag limits are based on r_max.

Therefore; add a heuristic that for the mixed request size case, moves
smaller requests to placement strategy #2 which ensures they're
immidiately eligible and and due to their smaller (virtual) deadline
will cause preemption.

NOTE: this relies on update_entity_lag() to impose lag limits above
a single slice.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/fair.c     |   14 ++++++++++++++
 kernel/sched/features.h |    1 +
 kernel/sched/sched.h    |    1 +
 3 files changed, 16 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -616,6 +616,7 @@ avg_vruntime_add(struct cfs_rq *cfs_rq,
 	s64 key = entity_key(cfs_rq, se);
 
 	cfs_rq->avg_vruntime += key * weight;
+	cfs_rq->avg_slice += se->slice * weight;
 	cfs_rq->avg_load += weight;
 }
 
@@ -626,6 +627,7 @@ avg_vruntime_sub(struct cfs_rq *cfs_rq,
 	s64 key = entity_key(cfs_rq, se);
 
 	cfs_rq->avg_vruntime -= key * weight;
+	cfs_rq->avg_slice -= se->slice * weight;
 	cfs_rq->avg_load -= weight;
 }
 
@@ -4832,6 +4834,18 @@ place_entity(struct cfs_rq *cfs_rq, stru
 		lag = se->vlag;
 
 		/*
+		 * For latency sensitive tasks; those that have a shorter than
+		 * average slice and do not fully consume the slice, transition
+		 * to EEVDF placement strategy #2.
+		 */
+		if (sched_feat(PLACE_FUDGE) &&
+		    cfs_rq->avg_slice > se->slice * cfs_rq->avg_load) {
+			lag += vslice;
+			if (lag > 0)
+				lag = 0;
+		}
+
+		/*
 		 * If we want to place a task and preserve lag, we have to
 		 * consider the effect of the new entity on the weighted
 		 * average and compensate for this, otherwise lag can quickly
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -5,6 +5,7 @@
  * sleep+wake cycles. EEVDF placement strategy #1, #2 if disabled.
  */
 SCHED_FEAT(PLACE_LAG, true)
+SCHED_FEAT(PLACE_FUDGE, true)
 SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)
 
 /*
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -559,6 +559,7 @@ struct cfs_rq {
 	unsigned int		idle_h_nr_running; /* SCHED_IDLE */
 
 	s64			avg_vruntime;
+	u64			avg_slice;
 	u64			avg_load;
 
 	u64			exec_clock;