Message-ID: <44EEFFC8.40202@linux.intel.com>
Date: Fri, 25 Aug 2006 15:48:56 +0200
From: Arjan van de Ven <arjan@linux.intel.com>
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To: Jan Kiszka <jan.kiszka@googlemail.com>
CC: linux-kernel@vger.kernel.org, len.brown@intel.com, mingo@elte.hu,
       akpm@osdl.org, jbarnes@virtuousgeek.org, dwalker@mvista.com,
       nickpiggin@yahoo.com.au, rpm@xenomai.org
Subject: Re: [RFC] maximum latency tracking infrastructure (version 2)
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Jan Kiszka wrote:
>> The patch below adds infrastructure to track "maximum allowable 
>> latency" for
>> power saving policies.
> 
> Very interesting approach. I wonder if it could be used to cover
> another problematic source of latencies as well: asynchronous SMIs.
> They quickly cause delays reaching from a few 100 us up to
> milliseconds.
> 
> Hard-RT extension like Xenomai work around this on several Intel
> chipsets by disabling SMI unconditionally 


I would consider that a mistake. SMI's are used to do things like emergency thermal protections etc etc.
Disabling them unconditionally is going to risk you your hardware.

> I guess an interface to let also applications / the sysadmin specifiy
> a latency constraint would be useful as well. sysfs?

I thought about this a lot but decided against. There are already ways to do things like disable specific C states etc,
and if we expose this it'll mostly get abused by certain desktop applications who have no idea what they are doing ;=(
What makes anyone think that userspace could make a better decision than the drivers?
Video / Audio playback are not good examples since these both already would work automatically correct with only in-kernel
infrastructure. Hard-RT systems are also not a good example since those should use the existing boot parameters. I couldn't
come up with other scenarios, and until we have a good one I'm against exposing crap to sysfs "just because we can".
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