linux-kernel - Re: [patch v6 13/21] sched: using avg

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Date:	Wed, 03 Apr 2013 13:41:10 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	mingo@...hat.com, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, arjan@...ux.intel.com, bp@...en8.de,
	pjt@...gle.com, efault@....de, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v6 13/21] sched: using avg_idle to detect bursty wakeup

	struct rq *rq = cpu_rq(i);
>> +
>> +		if (burst && rq->nr_running > 1)
>> +			/* use nr_running as instant utilization */
>> +			sgs->group_util += rq->nr_running;
> 
> I guess multiplying FULL_UTIL to rq->nr_running here will remove
> special-casing the burst in is_sd_full().  Also moving this logic to
> max_rq_util() looks better IMHO.

Yes, right! Thanks a lot!
> 
> 

>> +	if (cpu_rq(cpu)->avg_idle < sysctl_sched_burst_threshold)
>> +		burst = 1;
> 
> Sorry, I don't understand this.
> 
> Given that sysctl_sched_burst_threshold is twice of
> sysctl_sched_migration_cost which is max value of rq->avg_idle, the
> avg_idle will be almost always less than the threshold, right?

In fact, lots of time we have avg_idle at the max value. so won't always
have burst.
> 
> So how does it find out the burst case?  I thought it's the case of a
> cpu is in idle for a while and then wakes number of tasks at once.

Yes.
  If
> so, shouldn't it check whether the avg_idle is *longer* than certain
> threshold?  What am I missing?

avg_idle is smaller when many wakeup happens. update_avg() is not always
increase it.
> 
> Thanks,
> Namhyung
> 

-- 
Thanks Alex
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