linux-kernel - [PATCH 2/4] sched_ext: Keep bypass on between enable failure and scx_disable

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Message-ID: <20250902234817.279206-3-tj@kernel.org>
Date: Tue,  2 Sep 2025 13:48:04 -1000
From: Tejun Heo <tj@...nel.org>
To: void@...ifault.com,
	arighi@...dia.com,
	multics69@...il.com
Cc: linux-kernel@...r.kernel.org,
	sched-ext@...a.com,
	Tejun Heo <tj@...nel.org>
Subject: [PATCH 2/4] sched_ext: Keep bypass on between enable failure and scx_disable_workfn()

scx_enable() turns on the bypass mode while enable is in progress. If
enabling fails, it turns off the bypass mode and then triggers scx_error().
scx_error() will trigger scx_disable_workfn() which will turn on the bypass
mode again and unload the failed scheduler.

This moves the system out of bypass mode between the enable error path and
the disable path, which is unnecessary and can be brittle - e.g. the thread
running scx_enable() may already be on the failed scheduler and can be
switched out before it triggers scx_error() leading to a stall. The watchdog
would eventually kick in, so the situation isn't critical but is still
suboptimal.

There is nothing to be gained by turning off the bypass mode between
scx_enable() failure and scx_disable_workfn(). Keep bypass on.

Signed-off-by: Tejun Heo <tj@...nel.org>
---
 kernel/sched/ext.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7f799345c899..fda2b4e85ee3 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5794,7 +5794,7 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 err_disable_unlock_all:
 	scx_cgroup_unlock();
 	percpu_up_write(&scx_fork_rwsem);
-	scx_bypass(false);
+	/* we'll soon enter disable path, keep bypass on */
 err_disable:
 	mutex_unlock(&scx_enable_mutex);
 	/*
-- 
2.51.0