linux-kernel - Re: [PATCH v7 0/8] Tunable sched_mc_power

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Date:	Wed, 7 Jan 2009 16:56:39 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Mike Galbraith <efault@....de>
Cc:	balbir@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v7 0/8] Tunable sched_mc_power_savings=n

* Mike Galbraith <efault@....de> [2009-01-07 09:59:15]:

> I have a couple questions if you don't mind.
> 
> In wake_idle() there's an odd looking check for kthreads.  Why does it
> matter if a kbuild task wakes kernel or userland helper thread?

Only user space tasks/threads should be biased at wakeup and
consolidated for the following reasons:

* kernel threads generally perform housekeeping tasks and its rate of
  execution and cpu utilisation is far less than that of user task in
  an under utilised system.  
* Biasing at wakeup help consolidate bursty user space tasks. Long
  running user space tasks will be consolidated by load balancer.
  kthreads are not bursty, they are generally very short running.

> I also don't see why sched_mc overrides domain tunings.  You can turn
> NEWIDLE off and sched_mc remains as set, so it's a one-way override.  If
> NEWIDLE is a requirement for sched_mc > 0, it seems only logical to set
> sched_mc to 0 if the user explicitly disables NEWIDLE.

SD_BALANCE_NEWIDLE is required for sched_mc=2 and power savings while
not mandated for sched_mc=0.  We can remove NEWIDLE balance at
sched_mc=0 if that helps baseline performance.  Ingo had identified
that removing NEWIDLE balance help performance in certain benchmarks.

I do not get what you have mentioned by NEWIDLE off?  Is there
a separate user space control to independently set or reset
SD_BALANCE_NEWIDLE at a give sched_domain level?

Thanks for the detailed review.

--Vaidy

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