linux-kernel - Re: [RFC PATCH] sched/deadline: sched_getattr() returns absolute dl-task information

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Date:   Mon, 23 Jul 2018 11:49:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alessio Balsini <alessio.balsini@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH] sched/deadline: sched_getattr() returns absolute
 dl-task information

On Fri, Jun 29, 2018 at 02:09:47PM +0200, Alessio Balsini wrote:

Joel nailed it wrt the Changelog, that needs improvement.


> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index fbfc3f1d368a..f75a4169cd47 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2568,13 +2568,41 @@ void __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
>  	dl_se->dl_density = to_ratio(dl_se->dl_deadline, dl_se->dl_runtime);
>  }
>  
> -void __getparam_dl(struct task_struct *p, struct sched_attr *attr)
> +void __getparam_dl(struct task_struct *p, struct sched_attr *attr,
> +		   unsigned int flags)
>  {
>  	struct sched_dl_entity *dl_se = &p->dl;
>  
>  	attr->sched_priority = p->rt_priority;
> -	attr->sched_runtime = dl_se->dl_runtime;
> -	attr->sched_deadline = dl_se->dl_deadline;
> +
> +	if (flags & SCHED_GETATTR_FLAGS_DL_ABSOLUTE) {
> +		/*
> +		 * If the task is not running, its runtime is already
> +		 * properly accounted. Otherwise, update clocks and the
> +		 * statistics for the task.
> +		 */
> +		if (task_running(task_rq(p), p)) {
> +			struct rq_flags rf;
> +			struct rq *rq;
> +
> +			rq = task_rq_lock(p, &rf);
> +			sched_clock_tick();

This isn't required here. The reason it is used elsewhere is because
those are interrupts, but this is a system call, the clock state should
be good.

> +			update_rq_clock(rq);
> +			task_tick_dl(rq, p, 0);

Do we really want task_tick_dl() here, or update_curr_dl()? Also, who
says the task still is dl ? :-)

> +			task_rq_unlock(rq, p, &rf);
> +		}
> +
> +		/*
> +		 * If the task is throttled, this value could be negative,
> +		 * but sched_runtime is unsigned.
> +		 */
> +		attr->sched_runtime = dl_se->runtime <= 0 ? 0 : dl_se->runtime;
> +		attr->sched_deadline = dl_se->deadline;

This is all very racy..

Even if the task wasn't running when you did the task_running() test, it
could be running now. And if it was running, it might not be running
anymore by the time you've acquired the rq->lock.

On 32bit reading these numbers without locks is broken to boot. And even
on 64bit, I suppose you can a consistent snapshot of runtime and
deadline together, which isn't possible without the locks.

And of course, by the time we get back to userspace, the returned values
will be out-of-date anyway. But that isn't to be helped I suppose.


> +	} else {
> +		attr->sched_runtime = dl_se->dl_runtime;
> +		attr->sched_deadline = dl_se->dl_deadline;
> +	}
> +
>  	attr->sched_period = dl_se->dl_period;
>  	attr->sched_flags = dl_se->flags;
>  }