linux-kernel - Re: [RFC PATCH v3 0/5] Tunable sched_mc_power

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Date:	Tue, 11 Nov 2008 10:22:40 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	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>,
	Balbir Singh <balbir@...ux.vnet.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>
Subject: Re: [RFC PATCH v3 0/5] Tunable sched_mc_power_savings=n

* Peter Zijlstra <a.p.zijlstra@...llo.nl> [2008-11-10 19:50:16]:

> 
> a quick response, I'll read them more carefully tomorrow:

Hi Peter,

Thanks for the quick review.

> 
>  - why are the preferred cpu things pointers? afaict using just the cpu
> number is both smaller and clearer to the reader.

I would need each cpu within a partitioned sched domain to point to
the _same_ preferred wakeup cpu.  The preferred CPU will be updated in
one place in find_busiest_group() and used by wake_idle.

If I have a per cpu value, then updating it for each cpu in the
partitioned sched domain will be slow.

The actual number of preferred_wakeup_cpu will be equal to the number
of partitions.  If there are no partitions in the sched domains, then
then all per-cpu pointers will point to the same variable.

>  - in patch 5/5 you do:
> 
> +               spin_unlock(&this_rq->lock);
> +               double_rq_lock(this_rq, busiest);
> 
> we call that double_lock_balance()

Will fix this. Did not look for such a routine :)

>  - comments go like:
> 
>  /*
>   * this is a multi-
>   * line comment
>   */

Will fix this too.

Thanks,
Vaidy

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