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

[david]

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. ÂGood 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