Re: [patch take 2] Re: [BUG] How to get real-time priority using idle priority

[Mike Galbraith <efault@gmx.de>]

Missing cc added

On Mon, 2009-01-12 at 21:46 +0100, Mike Galbraith wrote:
> On Mon, 2009-01-12 at 14:03 +0100, Mike Galbraith wrote:
> 
> > Would you mind trying the below?
> 
> Or better, this more complete version.
> 
> Impact: fix latency issues when SCHED_IDLE tasks are queued.
> 
> Exclude SCHED_IDLE tasks from wakeup preemption, ensure that same will
> always be wakeup preempted, and exclude them from being buddies so they
> will only be selected via their vruntime.  Per a tip from Peter Zijlstra,
> also scale migration vruntime adjustment to compensate for differences
> in min_vruntime advance rates.
> 
> Reported-by: Brian Rogers <brian@xyzw.org>
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> 
>  kernel/sched.c      |   19 +++++++++++++++++--
>  kernel/sched_fair.c |   22 ++++++++++++++++------
>  2 files changed, 33 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index deb5ac8..bdc0b38 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -1888,8 +1888,23 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>  			schedstat_inc(p, se.nr_forced2_migrations);
>  	}
>  #endif
> -	p->se.vruntime -= old_cfsrq->min_vruntime -
> -					 new_cfsrq->min_vruntime;
> +	if (old_cpu != new_cpu) {
> +		s64 delta = p->se.vruntime - old_cfsrq->min_vruntime;
> +
> +		/*
> +		 * min_vruntimes may be advancing at wildly different
> +		 * rates, so we must scale the delta accordingly.
> +		 */
> +		if (new_cfsrq->load.weight != old_cfsrq->load.weight) {
> +			int negative = delta < 0;
> +
> +			delta = negative ? -delta : delta;
> +			delta = calc_delta_mine(delta,
> +				new_cfsrq->load.weight, &old_cfsrq->load);
> +			delta = negative ? -delta : delta;
> +		}
> +		p->se.vruntime = new_cfsrq->min_vruntime + delta;
> +	}
>  
>  	__set_task_cpu(p, new_cpu);
>  }
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 8e1352c..500ed14 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1340,14 +1340,18 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>  
>  static void set_last_buddy(struct sched_entity *se)
>  {
> -	for_each_sched_entity(se)
> -		cfs_rq_of(se)->last = se;
> +	for_each_sched_entity(se) {
> +		if (likely(task_of(se)->policy != SCHED_IDLE))
> +			cfs_rq_of(se)->last = se;
> +	}
>  }
>  
>  static void set_next_buddy(struct sched_entity *se)
>  {
> -	for_each_sched_entity(se)
> -		cfs_rq_of(se)->next = se;
> +	for_each_sched_entity(se) {
> +		if (likely(task_of(se)->policy != SCHED_IDLE))
> +			cfs_rq_of(se)->next = se;
> +	}
>  }
>  
>  /*
> @@ -1393,12 +1397,18 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
>  		return;
>  
>  	/*
> -	 * Batch tasks do not preempt (their preemption is driven by
> +	 * Batch and idle tasks do not preempt (their preemption is driven by
>  	 * the tick):
>  	 */
> -	if (unlikely(p->policy == SCHED_BATCH))
> +	if (unlikely(p->policy != SCHED_NORMAL))
>  		return;
>  
> +	/* Idle tasks are by definition preempted by everybody. */
> +	if (unlikely(curr->policy == SCHED_IDLE)) {
> +		resched_task(curr);
> +		return;
> +	}
> +
>  	if (!sched_feat(WAKEUP_PREEMPT))
>  		return;
>  
>