Re: [patch 11/18] sched: prevent interactions with throttled entities

[Kamalesh Babulal]

* Paul Turner <pjt@xxxxxxxxxx> [2011-07-21 09:43:36]:

> >From the perspective of load-balance and shares distribution, throttled
> entities should be invisible.
> 
> However, both of these operations work on 'active' lists and are not
> inherently aware of what group hierarchies may be present.  In some cases this
> may be side-stepped (e.g. we could sideload via tg_load_down in load balance) 
> while in others (e.g. update_shares()) it is more difficult to compute without
> incurring some O(n^2) costs.
> 
> Instead, track hierarchicaal throttled state at time of transition.  This
> allows us to easily identify whether an entity belongs to a throttled hierarchy
> and avoid incorrect interactions with it.
> 
> Also, when an entity leaves a throttled hierarchy we need to advance its
> time averaging for shares averaging so that the elapsed throttled time is not
> considered as part of the cfs_rq's operation.
> 
> We also use this information to prevent buddy interactions in the wakeup and
> yield_to() paths.
> 
> Signed-off-by: Paul Turner <pjt@xxxxxxxxxx>
> Reviewed-by: Hidetoshi Seto <seto.hidetoshi@xxxxxxxxxxxxxx>
> 
> ---
>  kernel/sched.c      |    2 -
>  kernel/sched_fair.c |   99 ++++++++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 94 insertions(+), 7 deletions(-)
> 
> Index: tip/kernel/sched_fair.c
> ===================================================================
> --- tip.orig/kernel/sched_fair.c
> +++ tip/kernel/sched_fair.c
> @@ -725,6 +725,8 @@ account_entity_dequeue(struct cfs_rq *cf
>  }
> 
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> +/* we need this in update_cfs_load and load-balance functions below */
> +static inline int throttled_hierarchy(struct cfs_rq *cfs_rq);
>  # ifdef CONFIG_SMP
>  static void update_cfs_rq_load_contribution(struct cfs_rq *cfs_rq,
>  					    int global_update)
> @@ -747,7 +749,7 @@ static void update_cfs_load(struct cfs_r
>  	u64 now, delta;
>  	unsigned long load = cfs_rq->load.weight;
> 
> -	if (cfs_rq->tg == &root_task_group)
> +	if (cfs_rq->tg == &root_task_group || throttled_hierarchy(cfs_rq))
>  		return;
> 
>  	now = rq_of(cfs_rq)->clock_task;
> @@ -856,7 +858,7 @@ static void update_cfs_shares(struct cfs
> 
>  	tg = cfs_rq->tg;
>  	se = tg->se[cpu_of(rq_of(cfs_rq))];
> -	if (!se)
> +	if (!se || throttled_hierarchy(cfs_rq))
>  		return;
>  #ifndef CONFIG_SMP
>  	if (likely(se->load.weight == tg->shares))
> @@ -1425,6 +1427,65 @@ static inline int cfs_rq_throttled(struc
>  	return cfs_rq->throttled;
>  }
> 
> +/* check whether cfs_rq, or any parent, is throttled */
> +static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
> +{
> +	return cfs_rq->throttle_count;
> +}
> +
> +/*
> + * Ensure that neither of the group entities corresponding to src_cpu or
> + * dest_cpu are members of a throttled hierarchy when performing group
> + * load-balance operations.
> + */
> +static inline int throttled_lb_pair(struct task_group *tg,
> +				    int src_cpu, int dest_cpu)
> +{
> +	struct cfs_rq *src_cfs_rq, *dest_cfs_rq;
> +
> +	src_cfs_rq = tg->cfs_rq[src_cpu];
> +	dest_cfs_rq = tg->cfs_rq[dest_cpu];
> +
> +	return throttled_hierarchy(src_cfs_rq) ||
> +	       throttled_hierarchy(dest_cfs_rq);
> +}
> +
> +/* updated child weight may affect parent so we have to do this bottom up */
> +static int tg_unthrottle_up(struct task_group *tg, void *data)
> +{
> +	struct rq *rq = data;
> +	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> +
> +	cfs_rq->throttle_count--;
> +#ifdef CONFIG_SMP
> +	if (!cfs_rq->throttle_count) {
> +		u64 delta = rq->clock_task - cfs_rq->load_stamp;
> +
> +		/* leaving throttled state, advance shares averaging windows */
> +		cfs_rq->load_stamp += delta;
> +		cfs_rq->load_last += delta;
> +
> +		/* update entity weight now that we are on_rq again */
> +		update_cfs_shares(cfs_rq);
> +	}
> +#endif
> +
> +	return 0;
> +}
> +
> +static int tg_throttle_down(struct task_group *tg, void *data)
> +{
> +	struct rq *rq = data;
> +	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> +
> +	/* group is entering throttled state, record last load */
> +	if (!cfs_rq->throttle_count)
> +		update_cfs_load(cfs_rq, 0);
> +	cfs_rq->throttle_count++;
> +
> +	return 0;
> +}
> +
>  static __used void throttle_cfs_rq(struct cfs_rq *cfs_rq)
>  {
>  	struct rq *rq = rq_of(cfs_rq);
> @@ -1435,7 +1496,9 @@ static __used void throttle_cfs_rq(struc
>  	se = cfs_rq->tg->se[cpu_of(rq_of(cfs_rq))];
> 
>  	/* account load preceding throttle */
> -	update_cfs_load(cfs_rq, 0);
> +	rcu_read_lock();
> +	walk_tg_tree_from(cfs_rq->tg, tg_throttle_down, tg_nop, (void *)rq);
> +	rcu_read_unlock();
> 
>  	task_delta = cfs_rq->h_nr_running;
>  	for_each_sched_entity(se) {
> @@ -1476,6 +1539,10 @@ static void unthrottle_cfs_rq(struct cfs
>  	list_del_rcu(&cfs_rq->throttled_list);
>  	raw_spin_unlock(&cfs_b->lock);
> 
> +	update_rq_clock(rq);
> +	/* update hierarchical throttle state */
> +	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);
> +
>  	if (!cfs_rq->load.weight)
>  		return;
> 
> @@ -1620,6 +1687,17 @@ static inline int cfs_rq_throttled(struc
>  {
>  	return 0;
>  }
> +
> +static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
> +{
> +	return 0;
> +}
> +
> +static inline int throttled_lb_pair(struct task_group *tg,
> +				    int src_cpu, int dest_cpu)
> +{
> +	return 0;
> +}
>  #endif
> 
>  /**************************************************
> @@ -2519,6 +2597,9 @@ move_one_task(struct rq *this_rq, int th
> 
>  	for_each_leaf_cfs_rq(busiest, cfs_rq) {
>  		list_for_each_entry_safe(p, n, &cfs_rq->tasks, se.group_node) {
> +			if (throttled_lb_pair(task_group(p),
> +					      busiest->cpu, this_cpu))
> +				break;
> 
>  			if (!can_migrate_task(p, busiest, this_cpu,
>  						sd, idle, &pinned))
> @@ -2630,8 +2711,13 @@ static void update_shares(int cpu)
>  	struct rq *rq = cpu_rq(cpu);
> 
>  	rcu_read_lock();
> -	for_each_leaf_cfs_rq(rq, cfs_rq)
> +	for_each_leaf_cfs_rq(rq, cfs_rq) {
> +		/* throttled entities do not contribute to load */
> +		if (throttled_hierarchy(cfs_rq))
> +			continue;
> +
>  		update_shares_cpu(cfs_rq->tg, cpu);
> +	}
>  	rcu_read_unlock();
>  }
> 
> @@ -2655,9 +2741,10 @@ load_balance_fair(struct rq *this_rq, in
>  		u64 rem_load, moved_load;
> 
>  		/*
> -		 * empty group
> +		 * empty group or part of a throttled hierarchy
>  		 */
> -		if (!busiest_cfs_rq->task_weight)
> +		if (!busiest_cfs_rq->task_weight ||
> +		    throttled_lb_pair(tg, busiest_cpu, this_cpu))

tip commit 9763b67fb9f30 removes both tg and busiest_cpu from
load_balance_fair.

>  			continue;
> 
>  		rem_load = (u64)rem_load_move * busiest_weight;
> Index: tip/kernel/sched.c
> ===================================================================
> --- tip.orig/kernel/sched.c
> +++ tip/kernel/sched.c
> @@ -399,7 +399,7 @@ struct cfs_rq {
>  	u64 runtime_expires;
>  	s64 runtime_remaining;
> 
> -	int throttled;
> +	int throttled, throttle_count;
>  	struct list_head throttled_list;
>  #endif
>  #endif
> 
> 
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