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

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Message-ID: <20140710200831.GB12984@intel.com>
Date:	Fri, 11 Jul 2014 04:08:32 +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

On Thu, Jul 10, 2014 at 10:06:27AM -0700, bsegall@...gle.com wrote:
 
> So, sched_clock(_cpu) can be arbitrarily far off of cfs_rq_clock_task, so you
> can't really do that. Ideally, yes, you would account for any time since
> the last update and account that time as !runnable. However, I don't
> think there is any good way to do that, and the current code doesn't.

Yeah. We only catch up the migrating task to its cfs_rq and substract. No catching
up to "current" time.
 
> >
> > I made another mistake. Should not only track task entity load, group entity
> > (as an entity) is also needed. Otherwise, task_h_load can't be done correctly...
> > Sorry for the messup. But this won't make much change in the codes.
> 
> This will increase it to 2x __update_load_avg per cgroup per
> enqueue/dequeue. What does this (and this patch in general) do to
> context switch cost at cgroup depth 1/2/3?
 
We can update cfs_rq load_avg, and let the cfs_rq's own se take a ride in that update.
These two should get exactly synchronized anyway (group se's load is only usefull for
task_h_load calc, and group cfs_rq's load is useful for task_h_load and update_cfs_share
calc). And technically, it looks easy:

To update cfs_rq, the update weight is cfs_rq->load.weight
To update its se, the update weight is cfs_rq->tg->se[cpu]->load.weight * on_rq

So the it will not increase to 2x, but 1.05x, maybe, :)

Thanks,
Yuyang
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