linux-kernel - [PATCH] sched/core: fix pick_next

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Date:   Tue, 17 Aug 2021 17:56:15 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Vineeth Pillai <vineethrp@...il.com>,
        linux-kernel@...r.kernel.org, Josh Don <joshdon@...gle.com>
Subject: [PATCH] sched/core: fix pick_next_task 'max' tracking

For core-sched, pick_next_task will update the 'max' task if there is a
cookie mismatch (since in this case the new task must be of higher
priority than the current max). However, we fail to update 'max' if
we've found a task with a matching cookie and higher priority than
'max'.

This can result in extra iterations on SMT-X machines, where X > 2.

As an example, on a machine with SMT=3, on core 0, SMT-0 might pick
the following, in order:

- SMT-0: p1, with cookie A, and priority 10 (max = p1)
- SMT-1: p2, with cookie A, and priority 30 (max not updated here)
- SMT-2: p3, with cookie B, and priority 20 (max = p2)
	> invalidate the other picks and retry

Here, we should have instead updated 'max' when picking for SMT-1. Note
that this code would eventually have righted itself, since the retry
loop would re-pick p2, and update 'max' accordingly. However, this patch
avoids the extra round-trip.

Signed-off-by: Josh Don <joshdon@...gle.com>
---
 kernel/sched/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3431939699dc..110ea7582a33 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5623,6 +5623,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 					occ = 1;
 					goto again;
 				}
+			} else if (prio_less(max, p, fi_before)) {
+				max = p;
 			}
 		}
 	}
-- 
2.33.0.rc1.237.g0d66db33f3-goog