linux-kernel - Re: [PATCH] sched: Fix PI handling vs sched

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Message-ID: <20160120100839.GL8573@e106622-lin>
Date:	Wed, 20 Jan 2016 10:08:39 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Andrea Parri <parri.andrea@...il.com>
Subject: Re: [PATCH] sched: Fix PI handling vs sched_setscheduler()

Hi,

On 19/01/16 12:18, Peter Zijlstra wrote:

[...]

> @@ -4097,15 +4099,14 @@ static int __sched_setscheduler(struct t
>  	if (running)
>  		p->sched_class->set_curr_task(rq);
>  	if (queued) {
> -		int enqueue_flags = ENQUEUE_RESTORE;
>  		/*
>  		 * We enqueue to tail when the priority of a task is
>  		 * increased (user space view).
>  		 */
> -		if (oldprio <= p->prio)
> -			enqueue_flags |= ENQUEUE_HEAD;
> +		if (oldprio < p->prio)
> +			queue_flags |= ENQUEUE_HEAD;

Was this condition broken before or it needs to be changed now with this
patch?

[...]

> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1130,18 +1130,35 @@ static inline void finish_lock_switch(st
>  extern const int sched_prio_to_weight[40];
>  extern const u32 sched_prio_to_wmult[40];
>  
> +/*
> + * {de,en}queue flags:
> + *
> + * SAVE/RESTORE - an otherwise spurious dequeue/enqueue, done to ensure tasks
> + *                are in a known state which allows modification. Such pairs
> + *                should preserve as much state as possible.
> + *
> + * MOVE - paired with SAVE/RESTORE, explicitly does not preserve the location
> + *        in the runqueue.
> + *
> + * ENQUEUE_HEAD - place at front of runqueue (tail if not specificed)
> + *
> + */

Do we want to document all the flags while we are at it? :)

Best,

- Juri

> +
> +#define DEQUEUE_SLEEP		0x01
> +#define DEQUEUE_SAVE		0x02 /* matches ENQUEUE_RESTORE */
> +#define DEQUEUE_MOVE		0x04 /* matches ENQUEUE_MOVE */
> +
>  #define ENQUEUE_WAKEUP		0x01
> -#define ENQUEUE_HEAD		0x02
> +#define ENQUEUE_RESTORE		0x02
> +#define ENQUEUE_MOVE		0x04
> +
> +#define ENQUEUE_HEAD		0x08
> +#define ENQUEUE_REPLENISH	0x10
>  #ifdef CONFIG_SMP
> -#define ENQUEUE_WAKING		0x04	/* sched_class::task_waking was called */
> +#define ENQUEUE_WAKING		0x20	/* sched_class::task_waking was called */
>  #else
>  #define ENQUEUE_WAKING		0x00
>  #endif
> -#define ENQUEUE_REPLENISH	0x08
> -#define ENQUEUE_RESTORE	0x10
> -
> -#define DEQUEUE_SLEEP		0x01
> -#define DEQUEUE_SAVE		0x02
>  
>  #define RETRY_TASK		((void *)-1UL)
>  
>