linux-kernel - Re: [linux-pm] Higer latency with dynamic tick (need for an io-ondemand govenor?)

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Date:	Sat, 19 Apr 2008 15:49:23 -0700 (PDT)
From:	david@...g.hm
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	David Brownell <david-b@...bell.net>,
	linux-pm@...ts.linux-foundation.org,
	"Woodruff, Richard" <r-woodruff2@...com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [linux-pm] Higer latency with dynamic tick (need for an io-ondemand
 govenor?)

On Sat, 19 Apr 2008, Thomas Gleixner wrote:

> On Fri, 18 Apr 2008, David Brownell wrote:
>> On Friday 18 April 2008, Woodruff, Richard wrote:
>>> When capturing some traces with dynamic tick we were noticing the
>>> interrupt latency seems to go up a good amount. If you look at the trace
>>> the gpio IRQ is now offset a good amount.  Good news I guess is its
>>> pretty predictable.
>>
>> That is, about 24 usec on this CPU ... an ARM v7, which I'm guessing
>> is an OMAP34xx running fairly fast (order of 4x faster than most ARMs).
>>
>> Similar issues were noted, also using ETM trace, on an ARM920 core [1]
>> from Atmel.  There, the overhead of NO_HZ was observed to be more like
>> 150 usec of per-IRQ overhead, which is enough to make NO_HZ non-viable
>> in some configurations.
>>
>>
>>> I was wondering what thoughts of optimizing this might be.
>>
>> Cutting down the math implied by jiffies updates might help.
>> The 64 bit math for ktime structs isn't cheap; purely by eyeball,
>> that was almost 1/3 the cost of that 24 usec (mostly __do_div64).
>
> Hmm, I have no real good idea to avoid the div64 in the case of a long
> idle sleep. Any brilliant patches are welcome :)

how long is 'long idle sleep'? and how common are such sleeps? is it 
possibly worth the cost of a test in the hotpath to see if you need to do 
the 64 bit math or can get away with 32 bit math (at least on some 
platforms)

David Lang