linux-kernel - [PATCH 3/3] sched_ext, scx_qmap: Add and use SCX_ENQ_CPU

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Message-ID: <20240927234838.152112-4-tj@kernel.org>
Date: Fri, 27 Sep 2024 13:46:13 -1000
From: Tejun Heo <tj@...nel.org>
To: void@...ifault.com,
	peterz@...radead.org,
	mingo@...hat.com
Cc: kernel-team@...a.com,
	linux-kernel@...r.kernel.org,
	sched-ext@...a.com,
	Tejun Heo <tj@...nel.org>,
	Daniel Hodges <hodges.daniel.scott@...il.com>,
	Changwoo Min <multics69@...il.com>,
	Andrea Righi <andrea.righi@...ux.dev>,
	Dan Schatzberg <schatzberg.dan@...il.com>
Subject: [PATCH 3/3] sched_ext, scx_qmap: Add and use SCX_ENQ_CPU_SELECTED

scx_qmap and other schedulers in the SCX repo are using SCX_ENQ_WAKEUP to
tell whether ops.select_cpu() was called. This is incorrect as
ops.select_cpu() can be skipped in the wakeup path and leads to e.g.
incorrectly skipping direct dispatch for tasks that are bound to a single
CPU.

sched core has been udpated to specify ENQUEUE_RQ_SELECTED if
->select_task_rq() was called. Map it to SCX_ENQ_CPU_SELECTED and update
scx_qmap to test it instead of SCX_ENQ_WAKEUP.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>
Cc: Daniel Hodges <hodges.daniel.scott@...il.com>
Cc: Changwoo Min <multics69@...il.com>
Cc: Andrea Righi <andrea.righi@...ux.dev>
Cc: Dan Schatzberg <schatzberg.dan@...il.com>
---
 kernel/sched/ext.c             | 1 +
 tools/sched_ext/scx_qmap.bpf.c | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index c09e3dc38c34..9f00c8b629f1 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -691,6 +691,7 @@ enum scx_enq_flags {
 	/* expose select ENQUEUE_* flags as enums */
 	SCX_ENQ_WAKEUP		= ENQUEUE_WAKEUP,
 	SCX_ENQ_HEAD		= ENQUEUE_HEAD,
+	SCX_ENQ_CPU_SELECTED	= ENQUEUE_RQ_SELECTED,
 
 	/* high 32bits are SCX specific */
 
diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index 83c8f54c1e31..588b7dce44fa 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -230,8 +230,8 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
 		return;
 	}
 
-	/* if !WAKEUP, select_cpu() wasn't called, try direct dispatch */
-	if (!(enq_flags & SCX_ENQ_WAKEUP) &&
+	/* if select_cpu() wasn't called, try direct dispatch */
+	if (!(enq_flags & SCX_ENQ_CPU_SELECTED) &&
 	    (cpu = pick_direct_dispatch_cpu(p, scx_bpf_task_cpu(p))) >= 0) {
 		__sync_fetch_and_add(&nr_ddsp_from_enq, 1);
 		scx_bpf_dispatch(p, SCX_DSQ_LOCAL_ON | cpu, slice_ns, enq_flags);
-- 
2.46.2