linux-kernel - Re: [PATCH] nohz: delay going tickless under CPU load to favor deeper C states

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Date:	Thu, 07 Apr 2011 12:57:06 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Kevin Hilman <khilman@...com>
CC:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-omap@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	Len Brown <lenb@...nel.org>,
	Nicole Chalhoub <n-chalhoub@...com>,
	Vincent Bour <v-bour@...com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] nohz: delay going tickless under CPU load to favor deeper
 C states

On 4/7/2011 11:18 AM, Kevin Hilman wrote:
> From: Nicole Chalhoub<n-chalhoub@...com>
>
> While there is CPU load, continue the periodic tick in order to give
> CPUidle another opportunity to pick a deeper C-state instead of
> spending potentially long i

so I don't really like this patch. It's actually a pretty bad hack (I'm 
sure it'll work somewhat)
[and I mean that in the most positive sense of the word ;-) ]

what we really need instead, and this is inside cpuidle, is the option 
to set a timer when we enter the non-deepest C state,
so that if that timer fires we then reevaluate.
The duration of that timer will be dependent on the C state (so should 
come from the C state structure of the state we pick).

For the most shallow one this will be a relatively short time, but for 
the deepest-but-one this might be a lot longer time.

your patch abuses a completely different, unrelated timer for this, with 
a pretty much unspecified frequency, that also has other side effects 
that we probably don't want.

it shouldn't be hard to do the right thing instead and make it a 
separate timer with a per C state timeout.

(and I would say a default timeout of 10x the break even time that we 
already have in the structure)

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