linux-kernel - Re: [tip: sched/core] sched/rt: Make rt_rq->pushable_tasks updates drive rto

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Message-ID: <20230925101127.GB31921@noisy.programming.kicks-ass.net>
Date:   Mon, 25 Sep 2023 12:11:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org
Subject: Re: [tip: sched/core] sched/rt: Make rt_rq->pushable_tasks updates
 drive rto_mask

On Mon, Sep 25, 2023 at 08:55:10AM -0000, tip-bot2 for Valentin Schneider wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     612f769edd06a6e42f7cd72425488e68ddaeef0a
> Gitweb:        https://git.kernel.org/tip/612f769edd06a6e42f7cd72425488e68ddaeef0a
> Author:        Valentin Schneider <vschneid@...hat.com>
> AuthorDate:    Fri, 11 Aug 2023 12:20:44 +01:00
> Committer:     Ingo Molnar <mingo@...nel.org>
> CommitterDate: Mon, 25 Sep 2023 10:25:29 +02:00
> 
> sched/rt: Make rt_rq->pushable_tasks updates drive rto_mask
> 
> Sebastian noted that the rto_push_work IRQ work can be queued for a CPU
> that has an empty pushable_tasks list, which means nothing useful will be
> done in the IPI other than queue the work for the next CPU on the rto_mask.
> 
> rto_push_irq_work_func() only operates on tasks in the pushable_tasks list,
> but the conditions for that irq_work to be queued (and for a CPU to be
> added to the rto_mask) rely on rq_rt->nr_migratory instead.
> 
> nr_migratory is increased whenever an RT task entity is enqueued and it has
> nr_cpus_allowed > 1. Unlike the pushable_tasks list, nr_migratory includes a
> rt_rq's current task. This means a rt_rq can have a migratible current, N
> non-migratible queued tasks, and be flagged as overloaded / have its CPU
> set in the rto_mask, despite having an empty pushable_tasks list.
> 
> Make an rt_rq's overload logic be driven by {enqueue,dequeue}_pushable_task().
> Since rt_rq->{rt_nr_migratory,rt_nr_total} become unused, remove them.
> 
> Note that the case where the current task is pushed away to make way for a
> migration-disabled task remains unchanged: the migration-disabled task has
> to be in the pushable_tasks list in the first place, which means it has
> nr_cpus_allowed > 1.
> 
> Reported-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Tested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Link: https://lore.kernel.org/r/20230811112044.3302588-1-vschneid@redhat.com
> ---
>  kernel/sched/debug.c |  3 +--
>  kernel/sched/rt.c    | 70 ++++++-------------------------------------
>  kernel/sched/sched.h |  2 +-
>  3 files changed, 10 insertions(+), 65 deletions(-)
> 

> @@ -358,53 +357,6 @@ static inline void rt_clear_overload(struct rq *rq)
>  	cpumask_clear_cpu(rq->cpu, rq->rd->rto_mask);
>  }
>  
> -static void update_rt_migration(struct rt_rq *rt_rq)
> -{
> -	if (rt_rq->rt_nr_migratory && rt_rq->rt_nr_total > 1) {
> -		if (!rt_rq->overloaded) {
> -			rt_set_overload(rq_of_rt_rq(rt_rq));
> -			rt_rq->overloaded = 1;
> -		}
> -	} else if (rt_rq->overloaded) {
> -		rt_clear_overload(rq_of_rt_rq(rt_rq));
> -		rt_rq->overloaded = 0;
> -	}
> -}
> -
> -static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> -{
> -	struct task_struct *p;
> -
> -	if (!rt_entity_is_task(rt_se))
> -		return;
> -
> -	p = rt_task_of(rt_se);
> -	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
> -
> -	rt_rq->rt_nr_total++;
> -	if (p->nr_cpus_allowed > 1)
> -		rt_rq->rt_nr_migratory++;
> -
> -	update_rt_migration(rt_rq);
> -}
> -
> -static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> -{
> -	struct task_struct *p;
> -
> -	if (!rt_entity_is_task(rt_se))
> -		return;
> -
> -	p = rt_task_of(rt_se);
> -	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
> -
> -	rt_rq->rt_nr_total--;
> -	if (p->nr_cpus_allowed > 1)
> -		rt_rq->rt_nr_migratory--;
> -
> -	update_rt_migration(rt_rq);
> -}

sched/deadline.c has something very similar, does that need updating
too?