linux-kernel - Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average tracking

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Date:	Mon, 28 Jul 2014 09:58:19 -0700
From:	bsegall@...gle.com
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Yuyang Du <yuyang.du@...el.com>, mingo@...hat.com,
	linux-kernel@...r.kernel.org, pjt@...gle.com,
	arjan.van.de.ven@...el.com, len.brown@...el.com,
	rafael.j.wysocki@...el.com, alan.cox@...el.com,
	mark.gross@...el.com, fengguang.wu@...el.com
Subject: Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average tracking

Peter Zijlstra <peterz@...radead.org> writes:

>> @@ -4551,18 +4382,34 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
>>  {
>>  	struct sched_entity *se = &p->se;
>>  	struct cfs_rq *cfs_rq = cfs_rq_of(se);
>> +	u64 last_update_time;
>>  
>>  	/*
>> +	 * Task on old CPU catches up with its old cfs_rq, and subtract itself from
>> +	 * the cfs_rq (task must be off the queue now).
>>  	 */
>> +#ifndef CONFIG_64BIT
>> +	u64 last_update_time_copy;
>> +
>> +	do {
>> +		last_update_time_copy = cfs_rq->load_last_update_time_copy;
>> +		smp_rmb();
>> +		last_update_time = cfs_rq->avg.last_update_time;
>> +	} while (last_update_time != last_update_time_copy);
>> +#else
>> +	last_update_time = cfs_rq->avg.last_update_time;
>> +#endif
>> +	__update_load_avg(last_update_time, &se->avg, 0);
>> +	atomic_long_add(se->avg.load_avg, &cfs_rq->removed_load_avg);
>> +
>> +	/*
>> +	 * We are supposed to update the task to "current" time, then its up to date
>> +	 * and ready to go to new CPU/cfs_rq. But we have difficulty in getting
>> +	 * what current time is, so simply throw away the out-of-date time. This
>> +	 * will result in the wakee task is less decayed, but giving the wakee more
>> +	 * load sounds not bad.
>> +	 */
>> +	se->avg.last_update_time = 0;
>>  
>>  	/* We have migrated, no longer consider this task hot */
>>  	se->exec_start = 0;
>
>
> And here we try and make good on that assumption. The thing I worry
> about is what happens if the machine is entirely idle...
>
> What guarantees an semi up-to-date cfs_rq->avg.last_update_time.

update_blocked_averages I think should do just as good a job as the old
code, which isn't perfect but is about as good as you can get worst case.
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