linux-kernel - Re: [PATCH v5 16/19] sched: Fix proxy/current (push,pull)ability

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Date:   Tue, 22 Aug 2023 17:20:20 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     John Stultz <jstultz@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Qais Yousef <qyousef@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>,
        Youssef Esmat <youssefesmat@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>, kernel-team@...roid.com,
        Connor O'Brien <connoro@...gle.com>
Subject: Re: [PATCH v5 16/19] sched: Fix proxy/current (push,pull)ability

On 19/08/2023 08:08, John Stultz wrote:
> From: Valentin Schneider <valentin.schneider@....com>

[...]

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bee7082b294f..e8065fc5c894 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6656,6 +6656,21 @@ proxy(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
>  	raw_spin_unlock(&mutex->wait_lock);
>  	return ret;
>  }
> +
> +static inline void proxy_tag_curr(struct rq *rq, struct task_struct *next)
> +{
> +	/*
> +	 * pick_next_task() calls set_next_task() on the selected task
> +	 * at some point, which ensures it is not push/pullable.
> +	 * However, the selected task *and* the ,mutex owner form an
> +	 * atomic pair wrt push/pull.
> +	 *
> +	 * Make sure owner is not pushable. Unfortunately we can only
> +	 * deal with that by means of a dequeue/enqueue cycle. :-/
> +	 */
> +	dequeue_task(rq, next, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
> +	enqueue_task(rq, next, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
> +}
>  #else /* PROXY_EXEC */
>  static struct task_struct *
>  proxy(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
> @@ -6663,6 +6678,8 @@ proxy(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
>  	BUG(); // This should never be called in the !PROXY case
>  	return next;
>  }
> +
> +static inline void proxy_tag_curr(struct rq *rq, struct task_struct *next) { }
>  #endif /* PROXY_EXEC */
>  
>  /*
> @@ -6711,6 +6728,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>  	unsigned long prev_state;
>  	struct rq_flags rf;
>  	struct rq *rq;
> +	bool proxied;
>  	int cpu;
>  
>  	cpu = smp_processor_id();
> @@ -6760,6 +6778,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>  		switch_count = &prev->nvcsw;
>  	}
>  
> +	proxied = (rq_selected(rq) != prev);

Looks like proxied isn't used here. (*)

>  pick_again:
>  	next = pick_next_task(rq, rq_selected(rq), &rf);
>  	rq_set_selected(rq, next);
> @@ -6786,6 +6805,10 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>  		 * changes to task_struct made by pick_next_task().
>  		 */
>  		RCU_INIT_POINTER(rq->curr, next);
> +
> +		if (unlikely(!task_current_selected(rq, next)))
> +			proxy_tag_curr(rq, next);
> +
>  		/*
>  		 * The membarrier system call requires each architecture
>  		 * to have a full memory barrier after updating
> @@ -6810,6 +6833,10 @@ static void __sched notrace __schedule(unsigned int sched_mode)
>  		/* Also unlocks the rq: */
>  		rq = context_switch(rq, prev, next, &rf);
>  	} else {
> +		/* In case next was already curr but just got blocked_donor*/
> +		if (unlikely(!task_current_selected(rq, next)))
> +			proxy_tag_curr(rq, next);

(*) v4 had:

+          /* In case next was already curr but just got blocked_donor*/
+             if (unlikely(!proxied && next->blocked_donor))

> +
>  		rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
>  
>  		rq_unpin_lock(rq, &rf);

I miss changes in enqueue_task_rt() and put_prev_task_rt() related to
'affinity of blocked tasks doesn't matter' from v4.