linux-kernel - Re: [PATCH v2] sched/freq: move call to cpufreq_update

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Date:   Tue, 12 Nov 2019 16:05:44 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, dietmar.eggemann@....com,
        juri.lelli@...hat.com, rostedt@...dmis.org, mgorman@...e.de,
        dsmythies@...us.net
Cc:     linux-pm@...r.kernel.org, torvalds@...ux-foundation.org,
        tglx@...utronix.de, sargun@...gun.me, tj@...nel.org,
        xiexiuqi@...wei.com, xiezhipeng1@...wei.com,
        srinivas.pandruvada@...ux.intel.com
Subject: Re: [PATCH v2] sched/freq: move call to cpufreq_update_util

Le Tuesday 12 Nov 2019 à 15:48:13 (+0100), Vincent Guittot a écrit :
> update_cfs_rq_load_avg() calls cfs_rq_util_change() everytime pelt decays,
> which might be inefficient when cpufreq driver has rate limitation.
> 
> When a task is attached on a CPU, we have call path:
> 
> update_blocked_averages()
>   update_cfs_rq_load_avg()
>     cfs_rq_util_change -- > trig frequency update
>   attach_entity_load_avg()
>     cfs_rq_util_change -- > trig frequency update
> 
> The 1st frequency update will not take into account the utilization of the
> newly attached task and the 2nd one might be discard because of rate
> limitation of the cpufreq driver.
> 
> update_cfs_rq_load_avg() is only called by update_blocked_averages()
> and update_load_avg() so we can move the call to
> {cfs_rq,cpufreq}_util_change() into these 2 functions. It's also
> interesting to notice that update_load_avg() already calls directly
> cfs_rq_util_change() for !SMP case.
> 
> This changes will also ensure that cpufreq_update_util() is called even
> when there is no more CFS rq in the leaf_cfs_rq_list to update but only
> irq, rt or dl pelt signals.
> 
> Reported-by: Doug Smythies <dsmythies@...us.net>
> Fixes: 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load balancing path")
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> 
> ---
> 
> I have just rebased the patch on latest tip/sched/core and made it a proper
> patchset after Doug reported that the problem has diseappeared according to
> his 1st results but tests results are not all based on the same v5.4-rcX
> and with menu instead of teo governor.
> 
>  kernel/sched/fair.c | 33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e458f52..c93d534 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3508,9 +3508,6 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
>  	cfs_rq->load_last_update_time_copy = sa->last_update_time;
>  #endif
>  
> -	if (decayed)
> -		cfs_rq_util_change(cfs_rq, 0);
> -
>  	return decayed;
>  }
>  
> @@ -3620,8 +3617,12 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>  		attach_entity_load_avg(cfs_rq, se, SCHED_CPUFREQ_MIGRATION);
>  		update_tg_load_avg(cfs_rq, 0);
>  
> -	} else if (decayed && (flags & UPDATE_TG))
> -		update_tg_load_avg(cfs_rq, 0);
> +	} else if (decayed) {
> +		cfs_rq_util_change(cfs_rq, 0);
> +
> +		if (flags & UPDATE_TG)
> +			update_tg_load_avg(cfs_rq, 0);
> +	}
>  }
>  
>  #ifndef CONFIG_64BIT
> @@ -7484,6 +7485,7 @@ static void update_blocked_averages(int cpu)
>  	const struct sched_class *curr_class;
>  	struct rq_flags rf;
>  	bool done = true;
> +	int decayed = 0;
>  
>  	rq_lock_irqsave(rq, &rf);
>  	update_rq_clock(rq);
> @@ -7493,9 +7495,9 @@ static void update_blocked_averages(int cpu)
>  	 * that RT, DL and IRQ signals have been updated before updating CFS.
>  	 */
>  	curr_class = rq->curr->sched_class;
> -	update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> -	update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> -	update_irq_load_avg(rq, 0);
> +	decayed |= update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> +	decayed |= update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> +	decayed |= update_irq_load_avg(rq, 0);
>  
>  	/* Don't need periodic decay once load/util_avg are null */
>  	if (others_have_blocked(rq))
> @@ -7529,6 +7531,9 @@ static void update_blocked_averages(int cpu)
>  	}
>  
>  	update_blocked_load_status(rq, !done);
> +
> +	if (decayed)
> +		cpufreq_update_util(rq, 0);
>  	rq_unlock_irqrestore(rq, &rf);
>  }
>  
> @@ -7585,6 +7590,7 @@ static inline void update_blocked_averages(int cpu)
>  	struct cfs_rq *cfs_rq = &rq->cfs;
>  	const struct sched_class *curr_class;
>  	struct rq_flags rf;
> +	int decayed = 0;
>  
>  	rq_lock_irqsave(rq, &rf);
>  	update_rq_clock(rq);
> @@ -7594,13 +7600,16 @@ static inline void update_blocked_averages(int cpu)
>  	 * that RT, DL and IRQ signals have been updated before updating CFS.
>  	 */
>  	curr_class = rq->curr->sched_class;
> -	update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> -	update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> -	update_irq_load_avg(rq, 0);
> +	decayed |= update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> +	decayed |= update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> +	decayed |= update_irq_load_avg(rq, 0);
>  
> -	update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
> +	decayed |= update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
>  
>  	update_blocked_load_status(rq, cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));
> +
> +	if (decayed)
> +		cpufreq_update_util(rq, 0);
>  	rq_unlock_irqrestore(rq, &rf);
>  }
>  
> -- 
> 2.7.4
>