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

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Date:	Tue, 8 Jul 2014 08:08:40 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	bsegall@...gle.com
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
	linux-kernel@...r.kernel.org, rafael.j.wysocki@...el.com,
	arjan.van.de.ven@...el.com, len.brown@...el.com,
	alan.cox@...el.com, mark.gross@...el.com, pjt@...gle.com,
	fengguang.wu@...el.com
Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average
 tracking

Thanks, Ben,

On Mon, Jul 07, 2014 at 03:25:07PM -0700, bsegall@...gle.com wrote:
> 
> Yeah, while this is technically limited to 1/us (per cpu), it is still
> much higher - the replaced code would do updates generally only on
> period overflow (1ms) and even then only with nontrivial delta.
>

Will update it in "batch" mode as I replied to Peter. Whether or not set
up a threshold to not update trivial delta, will see.

> Also something to note is that cfs_rq->load_avg just takes samples of
> load.weight every 1us, which seems unfortunate. We thought this was ok
> for p->se.load.weight, because it isn't really likely for userspace to
> be calling nice(2) all the time, but wake/sleeps are more frequent,
> particularly on newer cpus. Still, it might not be /that/ bad.

The sampling of cfs_rq->load.weight should be equivalent to the current
code in that at the end of day cfs_rq->load.weight worth of runnable would 
contribute to runnable_load_avg/blocked_load_avg for both the current and
the rewrite.

> Also, as a nitpick/annoyance this does a lot of
>  if (entity_is_task(se)) __update_load_avg(... se ...)
>  __update_load_avg(... cfs_rq_of(se) ...)
> which is just a waste of the avg struct on every group se, and all it
> buys you is the ability to not have a separate rq->avg struct (removed
> by patch 1) instead of rq->cfs.avg.

I actually struggled on this issue. As we only need a sched_avg for task (not
entity), and a sched_avg for cfs_rq, I planned to move entity avg to task. Good?

So left are the migrate_task_rq_fair() not holding lock issue and cfs_rq->avg.load_avg
overflow issue. I need some time to study them.

Overall, I think none of these issues are originally caused by combination/split
of runnable and blocked. It is just a matter of how synchronized we want to be
(this rewrite is the most synchronized), and the workaround I need to borrow from the
current codes. 

Yuyang
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