linux-kernel - Re: [patch 02/16] sched: maintain per-rq runnable averages

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Date:	Mon, 29 Oct 2012 10:38:15 -0700
From:	Benjamin Segall <bsegall@...gle.com>
To:	Preeti Murthy <preeti.lkml@...il.com>
Cc:	pjt@...gle.com, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Venki Pallipadi <venki@...gle.com>,
	Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [patch 02/16] sched: maintain per-rq runnable averages

Preeti Murthy <preeti.lkml@...il.com> writes:

> Hi Paul, Ben,
>
> A few queries regarding this patch:
>
> 1.What exactly is the significance of introducing sched_avg structure
> for a runqueue? If I have
>    understood correctly, sched_avg keeps track of how long a task has
> been active,
>    how long has it been serviced by the processor and its lifetime.How
> does this apply analogously
>    to the runqueue?

Remember that sched_avg's are not just for tasks, they're for any CFS
group entity (sched_entity), for which they track the time runnable and
the time used, which allows the system-wide per-task_group computation
of runnable and usage.

Computing these on the root has no usage in this patchset, but any
extensions of this using hierarchy-based fractional usage or runnable
time would need it, and retrofitting it afterwards would be a pain.
>
> 2.Is this a right measure to overwrite rq->load.weight because the
> rq->sched_avg does not seem to
>    take care of task priorities.IOW, what is the idea behind
> introducing this metric for the runqueue?
>    Why cant the run queue load be updated the same way as the cfs_rq
> load is updated:
>     cfs_rq->runnable_load_avg and cfs_rq->blocked_load_avg.

Loadwise you would indeed want the cfs_rq statistics, that is what they
are there for. The sched_avg numbers are only useful in computing the
parent's load (irrelevant on the root), or for extensions using raw
usage/runnable numbers.
>
> 3.What is the significance of passing rq->nr_running in
> enqueue_task_fair while updating
>    the run queue load? Because __update_entity_runnable_avg does not
> treat this argument
>    any differently if it is >1.

That could just as well be rq->nr_running != 0, it would behave the same.
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