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* [PATCH] sched/core: fix pick_next_task 'max' tracking
@ 2021-08-18  0:56 Josh Don
  2021-08-18  4:35 ` Tao Zhou
  2021-08-23 11:16 ` Peter Zijlstra
  0 siblings, 2 replies; 17+ messages in thread
From: Josh Don @ 2021-08-18  0:56 UTC (permalink / raw)
  To: Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot
  Cc: Dietmar Eggemann, Steven Rostedt, Ben Segall, Mel Gorman,
	Daniel Bristot de Oliveira, Joel Fernandes, Vineeth Pillai,
	linux-kernel, Josh Don

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@google.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


^ permalink raw reply	[flat|nested] 17+ messages in thread

end of thread, other threads:[~2021-10-05 14:13 UTC | newest]

Thread overview: 17+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-08-18  0:56 [PATCH] sched/core: fix pick_next_task 'max' tracking Josh Don
2021-08-18  4:35 ` Tao Zhou
2021-08-18 15:18   ` Tao Zhou
2021-08-23 11:16 ` Peter Zijlstra
2021-08-23 15:38   ` Tao Zhou
2021-08-23 20:25   ` Vineeth Pillai
2021-08-23 22:57     ` Tao Zhou
2021-08-24  9:03     ` Peter Zijlstra
2021-08-24  9:38       ` [PATCH] sched/core: Simplify core-wide task selection Peter Zijlstra
2021-08-24 12:15         ` Tao Zhou
2021-08-24 17:40         ` Josh Don
2021-08-24 18:28         ` Vineeth Pillai
2021-09-09 11:18         ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-10-05 14:12         ` tip-bot2 for Peter Zijlstra
2021-08-23 23:24   ` [PATCH] sched/core: fix pick_next_task 'max' tracking Josh Don
2021-08-24  3:01     ` Tao Zhou
2021-08-24  8:55     ` Peter Zijlstra

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