linux-kernel - [PATCH v2 01/14] sched_ext: Don't set ddsp_dsq_id during select

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Message-ID: <20251110205636.405592-2-tj@kernel.org>
Date: Mon, 10 Nov 2025 10:56:23 -1000
From: Tejun Heo <tj@...nel.org>
To: David Vernet <void@...ifault.com>,
	Andrea Righi <andrea.righi@...ux.dev>,
	Changwoo Min <changwoo@...lia.com>
Cc: Dan Schatzberg <schatzberg.dan@...il.com>,
	Emil Tsalapatis <etsal@...a.com>,
	sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>
Subject: [PATCH v2 01/14] sched_ext: Don't set ddsp_dsq_id during select_cpu in bypass mode

In the default CPU selection path used during bypass mode, select_task_rq_scx()
sets p->scx.ddsp_dsq_id to SCX_DSQ_LOCAL to emulate direct dispatch. However,
do_enqueue_task() ignores ddsp_dsq_id in bypass mode and queues to the global
DSQ instead, rendering the setting unnecessary.

Don't set ddsp_dsq_id from bypass. Just return the selected CPU, which has the
effect of waking up the picked idle CPU. Later patches will implement per-CPU
bypass DSQs to resolve this issue in a more proper way.

v2: Removed incorrect bug fix claim about dangling ddsp_dsq_id triggering
    WARN_ON_ONCE(). dispatch_enqueue() always clears ddsp_dsq_id even in bypass
    mode (Andrea).

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Andrea Righi <andrea.righi@...ux.dev>
---
 kernel/sched/ext.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 652a364e9e4c..cf8d86a2585c 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2521,12 +2521,8 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
 		s32 cpu;
 
 		cpu = scx_select_cpu_dfl(p, prev_cpu, wake_flags, NULL, 0);
-		if (cpu >= 0) {
-			refill_task_slice_dfl(sch, p);
-			p->scx.ddsp_dsq_id = SCX_DSQ_LOCAL;
-		} else {
+		if (cpu < 0)
 			cpu = prev_cpu;
-		}
 		p->scx.selected_cpu = cpu;
 
 		if (rq_bypass)
-- 
2.51.2