linux-kernel - Re: [PATCH 01/12] sched: Employ sched

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Message-ID: <20251008065103.GQ4067720@noisy.programming.kicks-ass.net>
Date: Wed, 8 Oct 2025 08:51:03 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Andrea Righi <arighi@...dia.com>
Cc: tj@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, longman@...hat.com,
	hannes@...xchg.org, mkoutny@...e.com, void@...ifault.com,
	changwoo@...lia.com, cgroups@...r.kernel.org,
	sched-ext@...ts.linux.dev, liuwenfang@...or.com, tglx@...utronix.de
Subject: Re: [PATCH 01/12] sched: Employ sched_change guards

On Tue, Oct 07, 2025 at 10:20:44AM +0200, Andrea Righi wrote:
> Hi Peter,
> 
> On Mon, Oct 06, 2025 at 12:44:03PM +0200, Peter Zijlstra wrote:
> > As proposed a long while ago -- and half done by scx -- wrap the
> > scheduler's 'change' pattern in a guard helper.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Acked-by: Tejun Heo <tj@...nel.org>
> > ---
> ...
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -3885,23 +3885,22 @@ extern void check_class_changed(struct r
> >  extern struct balance_callback *splice_balance_callbacks(struct rq *rq);
> >  extern void balance_callbacks(struct rq *rq, struct balance_callback *head);
> >  
> > -#ifdef CONFIG_SCHED_CLASS_EXT
> > -/*
> > - * Used by SCX in the enable/disable paths to move tasks between sched_classes
> > - * and establish invariants.
> > - */
> > -struct sched_enq_and_set_ctx {
> 
> Not necessarily for this patch, we can add it later, but I kinda liked the
> comment that briefly explained how the context is used. Maybe having
> something along these lines could be helpful?

I have changed it thus:


--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3885,6 +3885,22 @@ extern void check_class_changed(struct r
 extern struct balance_callback *splice_balance_callbacks(struct rq *rq);
 extern void balance_callbacks(struct rq *rq, struct balance_callback *head);
 
+/*
+ * The 'sched_change' pattern is the safe, easy and slow way of changing a
+ * task's scheduling properties. It dequeues a task, such that the scheduler
+ * is fully unaware of it; at which point its properties can be modified;
+ * after which it is enqueued again.
+ *
+ * Typically this must be called while holding task_rq_lock, since most/all
+ * properties are serialized under those locks. There is currently one
+ * exception to this rule in sched/ext which only holds rq->lock.
+ */
+
+/*
+ * This structure is a temporary, used to preserve/convey the queueing state
+ * of the task between sched_change_begin() and sched_change_end(). Ensuring
+ * the task's queueing state is idempotent across the operation.
+ */
 struct sched_change_ctx {
 	struct task_struct	*p;
 	int			flags;