Re: [PATCH v16 5/7] sched: Add an initial sketch of the find_proxy_task() function

[Juri Lelli]

Hi John,

On 11/04/25 23:02, John Stultz wrote:
> Add a find_proxy_task() function which doesn't do much.
> 
> When we select a blocked task to run, we will just deactivate it
> and pick again. The exception being if it has become unblocked
> after find_proxy_task() was called.
> 
> Greatly simplified from patch by:
>   Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>   Juri Lelli <juri.lelli@xxxxxxxxxx>
>   Valentin Schneider <valentin.schneider@xxxxxxx>
>   Connor O'Brien <connoro@xxxxxxxxxx>
> 
> Cc: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> Cc: Qais Yousef <qyousef@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Metin Kaya <Metin.Kaya@xxxxxxx>
> Cc: Xuewen Yan <xuewen.yan94@xxxxxxxxx>
> Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Cc: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> Cc: kernel-team@xxxxxxxxxxx
> [jstultz: Split out from larger proxy patch and simplified
>  for review and testing.]
> Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
> ---

...

> +static struct task_struct *
> +find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
> +{
> +	struct task_struct *p = donor;
> +	struct mutex *mutex;
> +
> +	mutex = p->blocked_on;
> +	/* Something changed in the chain, so pick again */
> +	if (!mutex)
> +		return NULL;
> +	/*
> +	 * By taking mutex->wait_lock we hold off concurrent mutex_unlock()
> +	 * and ensure @owner sticks around.
> +	 */
> +	guard(raw_spinlock)(&mutex->wait_lock);
> +
> +	/* Check again that p is blocked with blocked_lock held */
> +	if (!task_is_blocked(p) || mutex != __get_task_blocked_on(p)) {
> +		/*
> +		 * Something changed in the blocked_on chain and
> +		 * we don't know if only at this level. So, let's
> +		 * just bail out completely and let __schedule
> +		 * figure things out (pick_again loop).
> +		 */
> +		return NULL; /* do pick_next_task again */
> +	}
> +	return proxy_deactivate(rq, donor);

Nitpick. We might either want to use donor (and remove p) or use p
everywhere in the function; think makes it more clear.

Best,
Juri