linux-kernel - Re: [RFC] maximum latency tracking infrastructure (version 2)

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Date:	Fri, 25 Aug 2006 15:48:56 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Jan Kiszka <jan.kiszka@...glemail.com>
CC:	linux-kernel@...r.kernel.org, len.brown@...el.com, mingo@...e.hu,
	akpm@...l.org, jbarnes@...tuousgeek.org, dwalker@...sta.com,
	nickpiggin@...oo.com.au, rpm@...omai.org
Subject: Re: [RFC] maximum latency tracking infrastructure (version 2)

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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