linux-kernel - Re: [RFC PATCH v3 3/5] sched: nominate preferred wakeup cpu

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Date:	Tue, 11 Nov 2008 22:57:50 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	David Collier-Brown <davecb@....com>,
	Tim Connors <tconnors@...ro.swin.edu.au>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"gregory.haskins" <gregory.haskins@...il.com>
Subject: Re: [RFC PATCH v3 3/5] sched: nominate preferred wakeup cpu

* Balbir Singh <balbir@...ux.vnet.ibm.com> [2008-11-11 22:19:46]:

> Vaidyanathan Srinivasan wrote:
> > * Peter Zijlstra <a.p.zijlstra@...llo.nl> [2008-11-11 14:43:39]:
> > 
> >> On Tue, 2008-11-11 at 00:03 +0530, Vaidyanathan Srinivasan wrote:
> >>> When the system utilisation is low and more cpus are idle,
> >>> then the process waking up from sleep should prefer to
> >>> wakeup an idle cpu from semi-idle cpu package (multi core
> >>> package) rather than a completely idle cpu package which
> >>> would waste power.
> >>>
> >>> Use the sched_mc balance logic in find_busiest_group() to
> >>> nominate a preferred wakeup cpu.
> >>>
> >>> This info can be sored in appropriate sched_domain, but
> >>> updating this info in all copies of sched_domain is not
> >>> practical.  For now lets try with a per-cpu variable
> >>> pointing to a common storage in partition sched domain
> >>> attribute.  Global variable may not work in partitioned
> >>> sched domain case.
> >> Would it make sense to place the preferred_wakeup_cpu stuff in the
> >> root_domain structure we already have?
> > 
> > Yep, that will be a good idea.  We can get to root_domain from each
> > CPU's rq and we can get rid of the per-cpu pointers for
> > preferred_wakeup_cpu as well. I will change the implementation and
> > re-post.
> 
> Did you see Vatsa's comments? root_domain will no work if you have more than one
> preferred_wakeup_cpu per domain.

Hi Balbir,

I just saw Vatsa's comments.  We have similar limitation with the
current implementation also.  sched_domain_attr dattr is also per
partitioned domain and not per numa node.  

In the current implementation we can get rid of the per-cpu variables
and use root_domain.  Later we can have an array in root_domain and
index it based on the cpu's node.

Thanks,
Vaidy
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