linux-kernel - Re: [PATCH sched_ext/for-6.12-fixes] Disable SM_IDLE/rq empty path when scx

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Message-ID: <ZvGMjn1Va3aNbieH@slm.duckdns.org>
Date: Mon, 23 Sep 2024 05:43:10 -1000
From: Tejun Heo <tj@...nel.org>
To: Pat Somaru <patso@...ewhatevs.io>
Cc: void@...ifault.com, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	sched-ext@...a.com, peterz@...radead.org
Subject: Re: [PATCH sched_ext/for-6.12-fixes] Disable SM_IDLE/rq empty path
 when scx_enabled

Applied to sched_ext/for-6.12-fixes with minor edits:
------ 8< ------
>From edf1c586e92675c4e0eb27758fcdb55a56838de1 Mon Sep 17 00:00:00 2001
From: Pat Somaru <patso@...ewhatevs.io>
Date: Fri, 20 Sep 2024 15:41:59 -0400
Subject: [PATCH] sched, sched_ext: Disable SM_IDLE/rq empty path when
 scx_enabled()

Disable the rq empty path when scx is enabled. SCX must consult the BPF
scheduler (via the dispatch path in balance) to determine if rq is empty.

This fixes stalls when scx is enabled.

Signed-off-by: Pat Somaru <patso@...ewhatevs.io>
Fixes: 3dcac251b066 ("sched/core: Introduce SM_IDLE and an idle re-entry fast-path in __schedule()")
Signed-off-by: Tejun Heo <tj@...nel.org>
---
 kernel/sched/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b6cc1cf499d6..43e453ab7e20 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6591,7 +6591,8 @@ static void __sched notrace __schedule(int sched_mode)
 	 */
 	prev_state = READ_ONCE(prev->__state);
 	if (sched_mode == SM_IDLE) {
-		if (!rq->nr_running) {
+		/* SCX must consult the BPF scheduler to tell if rq is empty */
+		if (!rq->nr_running && !scx_enabled()) {
 			next = prev;
 			goto picked;
 		}
-- 
2.46.0