linux-kernel - Re: [RFC PATCH 3/3] sched: introduce synchronized idle injection

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Message-ID: <20151105153625.25fbfe69@icelake>
Date:	Thu, 5 Nov 2015 15:36:25 -0800
From:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Paul Turner <pjt@...gle.com>, Len Brown <len.brown@...el.com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <andi.kleen@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	jacob.jun.pan@...ux.intel.com
Subject: Re: [RFC PATCH 3/3] sched: introduce synchronized idle injection

On Thu, 5 Nov 2015 14:59:52 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Nov 03, 2015 at 02:31:20PM +0100, Peter Zijlstra wrote:
> > > @@ -5136,6 +5148,16 @@ pick_next_task_fair(struct rq *rq, struct
> > > task_struct *prev) struct task_struct *p;
> > >  	int new_tasks;
> > >  
> > > +#ifdef CONFIG_CFS_IDLE_INJECT
> > > +	if (cfs_rq->force_throttled &&
> > > +		!idle_cpu(cpu_of(rq)) &&
> > > +		!unlikely(local_softirq_pending())) {
> > > +		/* forced idle, pick no task */
> > > +		trace_sched_cfs_idle_inject(cpu_of(rq), 1);
> > > +		update_curr(cfs_rq);
> > > +		return NULL;
> > > +	}
> > > +#endif
> > >  again:
> > >  #ifdef CONFIG_FAIR_GROUP_SCHED
> > >  	if (!cfs_rq->nr_running)
> > 
> > So this is horrible...
> 
> So this isn't ideal either (I rather liked the previous approach of a
> random task assuming idle, but tglx hated that). This should at least
> not touch extra cachelines in the hot paths, although it does add a
> few extra instructions :/
> 
> Very limited testing didn't show anything horrible.
> 
I did some testing with the code below, it shows random
[  150.442597] NOHZ: local_softirq_pending 02
[  153.032673] NOHZ: local_softirq_pending 202
[  153.203785] NOHZ: local_softirq_pending 202
[  153.206486] NOHZ: local_softirq_pending 282
I recalled that was why i checked for local_softirq_pending in the
initial patch, still trying to find out how we can avoid that. These
also causes non stop sched ticks in the inner idle loop.

> Your throttle would:
> 
> 	raw_spin_lock_irqsave(&rq->lock, flags);
> 	rq->cfs.forced_idle = true;
> 	resched = rq->cfs.runnable;
> 	rq->cfs.runnable = false;
> 	raw_spin_unlock_irqrestore(&rq->lock, flags);
> 	if (resched)
> 	  resched_cpu(cpu_of(rq));
> 
> And your unthrottle:
> 
> 	raw_spin_lock_irqsave(&rq->lock, flags);
> 	rq->cfs.forced_idle = false;
> 	resched = rq->cfs.runnable = !!rq->cfs.nr_running;
> 	raw_spin_unlock_irqrestore(&rq->lock, flags);
> 	if (resched)
> 	  resched_cpu(cpu_of(rq));
> 
> ---
>  kernel/sched/fair.c  |   13 +++++++++----
>  kernel/sched/sched.h |    1 +
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 824aa9f..1f0c809 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2341,7 +2341,8 @@ account_entity_enqueue(struct cfs_rq *cfs_rq,
> struct sched_entity *se) list_add(&se->group_node, &rq->cfs_tasks);
>  	}
>  #endif
> -	cfs_rq->nr_running++;
> +	if (!cfs_rq->nr_running++ && !cfs_rq->forced_idle)
> +		cfs_rq->runnable = true;
>  }
>  
>  static void
> @@ -2354,7 +2355,8 @@ account_entity_dequeue(struct cfs_rq *cfs_rq,
> struct sched_entity *se) account_numa_dequeue(rq_of(cfs_rq),
> task_of(se)); list_del_init(&se->group_node);
>  	}
> -	cfs_rq->nr_running--;
> +	if (!--cfs_rq->nr_running && !cfs_rq->forced_idle)
> +		cfs_rq->runnable = false;
>  }
>  
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> @@ -5204,7 +5206,7 @@ pick_next_task_fair(struct rq *rq, struct
> task_struct *prev) 
>  again:
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> -	if (!cfs_rq->nr_running)
> +	if (!cfs_rq->runnable)
>  		goto idle;
>  
>  	if (prev->sched_class != &fair_sched_class)
> @@ -5283,7 +5285,7 @@ simple:
>  	cfs_rq = &rq->cfs;
>  #endif
>  
> -	if (!cfs_rq->nr_running)
> +	if (!cfs_rq->runnable)
>  		goto idle;
>  
>  	put_prev_task(rq, prev);
> @@ -5302,6 +5304,9 @@ simple:
>  	return p;
>  
>  idle:
> +	if (cfs_rq->forced_idle)
> +		return NULL;
> +
>  	/*
>  	 * This is OK, because current is on_cpu, which avoids it
> being picked
>  	 * for load-balance and preemption/IRQs are still disabled
> avoiding diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index efd3bfc..33d355d 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -347,6 +347,7 @@ struct cfs_bandwidth { };
>  struct cfs_rq {
>  	struct load_weight load;
>  	unsigned int nr_running, h_nr_running;
> +	unsigned int runnable, forced_idle;
>  
>  	u64 exec_clock;
>  	u64 min_vruntime;

[Jacob Pan]
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