linux-kernel - Re: [PATCH v2 2/2] sched/fair: Update blocked load from newly idle balance

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Date:   Thu, 21 Dec 2017 10:19:22 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Brendan Jackman <brendan.jackman@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v2 2/2] sched/fair: Update blocked load from newly idle
 balance

On 12/20/2017 03:22 PM, Peter Zijlstra wrote:
> On Fri, Dec 01, 2017 at 06:01:57PM +0000, Brendan Jackman wrote:
>> @@ -7913,6 +7928,29 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>>   	if (child && child->flags & SD_PREFER_SIBLING)
>>   		prefer_sibling = 1;
>>   
>> +#ifdef CONFIG_NO_HZ_COMMON
>> +	if (env->idle == CPU_NEWLY_IDLE) {
>> +		int cpu;
>> +
>> +		/* Update the stats of NOHZ idle CPUs in the sd */
>> +		for_each_cpu_and(cpu, sched_domain_span(env->sd),
>> +				 nohz.idle_cpus_mask) {
>> +			struct rq *rq = cpu_rq(cpu);
>> +
>> +			/* ... Unless we've already done since the last tick */
>> +			if (time_after(jiffies, rq->last_blocked_load_update_tick))
>> +				update_blocked_averages(cpu);
>> +		}
>> +	}
>> +	/*
>> +	 * If we've just updated all of the NOHZ idle CPUs, then we can push
>> +	 * back the next nohz.next_update, which will prevent an unnecessary
>> +	 * wakeup for the nohz stats kick
>> +	 */
>> +	if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd)))
>> +		nohz.next_update = jiffies + LOAD_AVG_PERIOD;
>> +#endif
>> +
>>   	load_idx = get_sd_load_idx(env->sd, env->idle);
>>   
>>   	do {
> 
> We're already going to be iterating all those CPUs through
> update_sg_lb_stats(), why not push it all the way down there and avoid
> the double iteration?

Brendan doesn't work for ARM anymore although he might still follow this 
discussion.

So you think that we can do the time_after(jiffies, 
rq->last_blocked_load_update_tick) check in update_sg_lb_stats() and 
possibly the update_blocked_averages() for all the cpus in the sd and 
then only update nohz.next_update in update_sd_lb_stats(). Let me try 
this ...