linux-kernel - Re: [CFS Bandwidth Control v4 6/7] sched: hierarchical task accounting for SCHED

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Date:	Wed, 23 Feb 2011 14:32:11 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Turner <pjt@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>
Subject: Re: [CFS Bandwidth Control v4 6/7] sched: hierarchical task
 accounting for SCHED_OTHER

On Tue, 2011-02-15 at 19:18 -0800, Paul Turner wrote:

> @@ -1846,6 +1846,11 @@ static const struct sched_class rt_sched
>  
>  #include "sched_stats.h"
>  
> +static void mod_nr_running(struct rq *rq, long delta)
> +{
> +	rq->nr_running += delta;
> +}

I personally don't see much use in such trivial wrappers.. if you're
going to rework all the nr_running stuff you might as well remove all of
that.

> Index: tip/kernel/sched_fair.c
> ===================================================================
> --- tip.orig/kernel/sched_fair.c
> +++ tip/kernel/sched_fair.c
> @@ -81,6 +81,8 @@ unsigned int normalized_sysctl_sched_wak
>  
>  const_debug unsigned int sysctl_sched_migration_cost = 500000UL;
>  
> +static void account_hier_tasks(struct sched_entity *se, int delta);
> +
>  /*
>   * The exponential sliding  window over which load is averaged for shares
>   * distribution.
> @@ -933,6 +935,40 @@ static inline void update_entity_shares_
>  }
>  #endif /* CONFIG_FAIR_GROUP_SCHED */
>  
> +#ifdef CONFIG_CFS_BANDWIDTH
> +/* maintain hierarchal task counts on group entities */
> +static void account_hier_tasks(struct sched_entity *se, int delta)
> +{
> +	struct rq *rq = rq_of(cfs_rq_of(se));
> +	struct cfs_rq *cfs_rq;
> +
> +	for_each_sched_entity(se) {
> +		/* a throttled entity cannot affect its parent hierarchy */
> +		if (group_cfs_rq(se) && cfs_rq_throttled(group_cfs_rq(se)))
> +			break;
> +
> +		/* we affect our queuing entity */
> +		cfs_rq = cfs_rq_of(se);
> +		cfs_rq->h_nr_tasks += delta;
> +	}
> +
> +	/* account for global nr_running delta to hierarchy change */
> +	if (!se)
> +		mod_nr_running(rq, delta);
> +}
> +#else
> +/*
> + * In the absence of group throttling, all operations are guaranteed to be
> + * globally visible at the root rq level.
> + */
> +static void account_hier_tasks(struct sched_entity *se, int delta)
> +{
> +	struct rq *rq = rq_of(cfs_rq_of(se));
> +
> +	mod_nr_running(rq, delta);
> +}
> +#endif

While Balbir suggested expanding the _hier_ thing, I'd suggest to simply
drop it altogether, way too much typing ;-), but that is if you cannot
get rid of the extra hierarchy iteration, see below.

> +
>  static void enqueue_sleeper(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
>  #ifdef CONFIG_SCHEDSTATS
> @@ -1428,6 +1464,7 @@ enqueue_task_fair(struct rq *rq, struct 
>  		update_cfs_shares(cfs_rq);
>  	}
>  
> +	account_hier_tasks(&p->se, 1);
>  	hrtick_update(rq);
>  }
>  
> @@ -1461,6 +1498,7 @@ static void dequeue_task_fair(struct rq 
>  		update_cfs_shares(cfs_rq);
>  	}
>  
> +	account_hier_tasks(&p->se, -1);
>  	hrtick_update(rq);
>  }
>  
> @@ -1488,6 +1526,8 @@ static u64 tg_request_cfs_quota(struct t
>  	return delta;
>  }
>  
> +static void account_hier_tasks(struct sched_entity *se, int delta);
> +
>  static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
>  {
>  	struct sched_entity *se;
> @@ -1507,6 +1547,7 @@ static void throttle_cfs_rq(struct cfs_r
>  	if (!se->on_rq)
>  		goto out_throttled;
>  
> +	account_hier_tasks(se, -cfs_rq->h_nr_tasks);
>  	for_each_sched_entity(se) {
>  		struct cfs_rq *cfs_rq = cfs_rq_of(se);
>  
> @@ -1541,6 +1582,7 @@ static void unthrottle_cfs_rq(struct cfs
>  	cfs_rq->load_stamp = cfs_rq->load_last = rq->clock_task;
>  
>  	cfs_rq->throttled = 0;
> +	account_hier_tasks(se, cfs_rq->h_nr_tasks);
>  	for_each_sched_entity(se) {
>  		if (se->on_rq)
>  			break;

All call-sites are right next to a for_each_sched_entity() iteration, is
there really no way to fold those loops?

> Index: tip/kernel/sched_rt.c
> ===================================================================
> --- tip.orig/kernel/sched_rt.c
> +++ tip/kernel/sched_rt.c
> @@ -906,6 +906,8 @@ enqueue_task_rt(struct rq *rq, struct ta
>  
>  	if (!task_current(rq, p) && p->rt.nr_cpus_allowed > 1)
>  		enqueue_pushable_task(rq, p);
> +
> +	inc_nr_running(rq);
>  }
>  
>  static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
> @@ -916,6 +918,8 @@ static void dequeue_task_rt(struct rq *r
>  	dequeue_rt_entity(rt_se);
>  
>  	dequeue_pushable_task(rq, p);
> +
> +	dec_nr_running(rq);
>  }
>  
>  /*
> @@ -1783,4 +1787,3 @@ static void print_rt_stats(struct seq_fi
>  	rcu_read_unlock();
>  }
>  #endif /* CONFIG_SCHED_DEBUG */


You mentioned something about -rt having the same problem, yet you don't
fix it.. tskkk :-)

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