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

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Date:	Thu, 24 Aug 2006 23:20:01 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
CC:	linux-kernel@...r.kernel.org, len.brown@...el.com
Subject: Re: [RFC] maximum latency tracking infrastructure

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...)

> Should there be a callback so 
> they can be woken up?  A callback would also allow ACPI to tell the 
> user "disabling C3 because of device <foo>" or somesuch, which might be 
> nice.

printk'ing would be evil, changes like this will be "semi frequent", like every time you start
or stop playing audio. What ACPI could easily do is indicate in /proc/acpi/processor/*/power
that a state will not be reachable because it violates the latency constraints. That would
be entirely reasonable.

> Also, should subsystems have the ability to set a lower bound on  
> latency?  That would mean set_acceptable_latency() could fail, 
> indicating that the user should buy a better device or a system with 
> better realtime guarantees, which is also valuable info.

While it's valuable info.. there is nothing you can DO about it...
While the kernel can even do a latency of 1us by just not going into C1 even... so the kernel
CAN honor it, even if it thinks it might not be a good idea. Can you give a more concrete example
of a situation where you think your idea would be useful?

Greetings,
    Arjan van de Ven
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