linux-kernel - Re: [RFC] maximum latency tracking infrastructure

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Date:	Fri, 25 Aug 2006 09:58:05 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-kernel@...r.kernel.org, len.brown@...el.com,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC] maximum latency tracking infrastructure

Nick Piggin wrote:
> Arjan van de Ven wrote:
>> Jesse Barnes wrote:
>>
>>> On Thursday, August 24, 2006 10:41 am, Arjan van de Ven wrote:
>>>
>>>> The reason for adding this infrastructure is that power management in
>>>> the idle loop needs to make a tradeoff between latency and power
>>>> savings (deeper power save modes have a longer latency to running code
>>>> again).
>>>
>>>
>>> What if a processor was already in a sleep state when a call to 
>>> set_acceptable_latency() latency occurs? 
>>
>>
>> there's nothing sane that can be done in that case; any wake up 
>> already will cause the unwanted latency!
>> A premature wakeup is only making it happen *now*, but now is as 
>> inconvenient a time as any...
>> (in fact it may be a worst case time scenario, say, an audio 
>> interrupt...)
> 
> Surely you would call set_acceptable_latency() *before* running such
> operation that requires the given latency? And that set_acceptable_latency
> would block the caller until all CPUs are set to wake within this latency.
> 
> That would be the API semantics I would expect, anyway.

but that means it blocks, and thus can't be used in irq context

(the usage model I imagine happens most is a set_acceptable_latency() which can block during device init,
with either no or a very course limit, and a modify_acceptable_latency(), which cannot block, from irq context or
device open)
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