linux-kernel - Re: [PATCH 1/3] sched/core: Fix the bug that task won't enqueue into core tree when update cookie

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Message-ID: <Ys//3pspEdzEGg2G@worktop.programming.kicks-ass.net>
Date:   Thu, 14 Jul 2022 13:37:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Cruz Zhao <CruzZhao@...ux.alibaba.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched/core: Fix the bug that task won't enqueue into
 core tree when update cookie

On Mon, Jul 04, 2022 at 10:53:20AM +0200, Peter Zijlstra wrote:
> On Tue, Jun 28, 2022 at 03:57:23PM +0800, Cruz Zhao wrote:
> > In function sched_core_update_cookie(), a task will enqueue into the
> > core tree only when it enqueued before, that is, if an uncookied task
> > is cookied, it will not enqueue into the core tree until it enqueue
> > again, which will result in unnecessary force idle.
> > 
> > Here follows the scenario:
> >   CPU x and CPU y are a pair of SMT siblings.
> >   1. Start task a running on CPU x without sleeping, and task b and
> >      task c running on CPU y without sleeping.
> >   2. We create a cookie and share it to task a and task b, and then
> >      we create another cookie and share it to task c.
> >   3. Simpling core_forceidle_sum of task a and b from /proc/PID/sched
> > 
> > And we will find out that core_forceidle_sum of task a takes 30%
> > time of the sampling period, which shouldn't happen as task a and b
> > have the same cookie.
> > 
> > Then we migrate task a to CPU x', migrate task b and c to CPU y', where
> > CPU x' and CPU y' are a pair of SMT siblings, and sampling again, we
> > will found out that core_forceidle_sum of task a and b are almost zero.
> > 
> > To solve this problem, we enqueue the task into the core tree if it's
> > on rq.
> > 
> > Fixes: 6e33cad0af49("sched: Trivial core scheduling cookie management")
> > Signed-off-by: Cruz Zhao <CruzZhao@...ux.alibaba.com>
> > ---
> >  kernel/sched/core_sched.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
> > index 38a2cec..ba2466c 100644
> > --- a/kernel/sched/core_sched.c
> > +++ b/kernel/sched/core_sched.c
> > @@ -75,7 +75,7 @@ static unsigned long sched_core_update_cookie(struct task_struct *p,
> >  	old_cookie = p->core_cookie;
> >  	p->core_cookie = cookie;
> >  
> > -	if (enqueued)
> > +	if (task_on_rq_queued(p))
> >  		sched_core_enqueue(rq, p);
> >  
> >  	/*
> 
> Yeah; I suppose that's true. However if we want to consider the
> asymmetric case, we should be complete and also consider the case where
> we clear the cookie.
> 
> And if you remove the second use of @enqueued, having that variable is
> rather redudant, which then leaves me with something like this.

Can you please confirm the below works for you so I can queue it?

> ---
> Subject: sched/core: Fix the bug that task won't enqueue into core tree when update cookie
> From: Cruz Zhao <CruzZhao@...ux.alibaba.com>
> Date: Tue, 28 Jun 2022 15:57:23 +0800
> 
> From: Cruz Zhao <CruzZhao@...ux.alibaba.com>
> 
> In function sched_core_update_cookie(), a task will enqueue into the
> core tree only when it enqueued before, that is, if an uncookied task
> is cookied, it will not enqueue into the core tree until it enqueue
> again, which will result in unnecessary force idle.
> 
> Here follows the scenario:
>   CPU x and CPU y are a pair of SMT siblings.
>   1. Start task a running on CPU x without sleeping, and task b and
>      task c running on CPU y without sleeping.
>   2. We create a cookie and share it to task a and task b, and then
>      we create another cookie and share it to task c.
>   3. Simpling core_forceidle_sum of task a and b from /proc/PID/sched
> 
> And we will find out that core_forceidle_sum of task a takes 30%
> time of the sampling period, which shouldn't happen as task a and b
> have the same cookie.
> 
> Then we migrate task a to CPU x', migrate task b and c to CPU y', where
> CPU x' and CPU y' are a pair of SMT siblings, and sampling again, we
> will found out that core_forceidle_sum of task a and b are almost zero.
> 
> To solve this problem, we enqueue the task into the core tree if it's
> on rq.
> 
> Fixes: 6e33cad0af49("sched: Trivial core scheduling cookie management")
> Signed-off-by: Cruz Zhao <CruzZhao@...ux.alibaba.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/1656403045-100840-2-git-send-email-CruzZhao@linux.alibaba.com
> ---
>  kernel/sched/core_sched.c |    9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> --- a/kernel/sched/core_sched.c
> +++ b/kernel/sched/core_sched.c
> @@ -56,7 +56,6 @@ static unsigned long sched_core_update_c
>  	unsigned long old_cookie;
>  	struct rq_flags rf;
>  	struct rq *rq;
> -	bool enqueued;
>  
>  	rq = task_rq_lock(p, &rf);
>  
> @@ -68,14 +67,16 @@ static unsigned long sched_core_update_c
>  	 */
>  	SCHED_WARN_ON((p->core_cookie || cookie) && !sched_core_enabled(rq));
>  
> -	enqueued = sched_core_enqueued(p);
> -	if (enqueued)
> +	if (sched_core_enqueued(p))
>  		sched_core_dequeue(rq, p, DEQUEUE_SAVE);
>  
>  	old_cookie = p->core_cookie;
>  	p->core_cookie = cookie;
>  
> -	if (enqueued)
> +	/*
> +	 * Consider the cases: !prev_cookie and !cookie.
> +	 */
> +	if (cookie && task_on_rq_queued(p))
>  		sched_core_enqueue(rq, p);
>  
>  	/*