Re: [PATCH 1/4] timer: Added usleep[_range] timer

From: Andrew Morton
Date: Wed Jul 28 2010 - 17:23:13 EST

On Wed, 28 Jul 2010 14:04:47 -0700
Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:

> On 7/28/2010 1:58 PM, Andrew Morton wrote:
> >
> > My main concern is that someone will type usleep(50) and won't realise
> > that it goes and sleeps for 100 usecs and their code gets slow as a
> > result. This sort of thing takes *years* to discover and fix. If we'd
> > forced them to type usleep_range() instead, it would never have happened.
> >
> >
> >
> > Another question: what is the typical overhead of a usleep()? IOW, at
> > what delay value does it make more sense to use udelay()? Another way
> > of asking that would be "how long does a usleep(1) take"? If it
> > reliably consumes 2us CPU time then we shouldn't do it.
> >
> > But it's not just CPU time, is it? A smart udelay() should put the CPU
> > into a lower power state, so a udelay(3) might consume less energy than
> > a usleep(2), because the usleep() does much more work in schedule() and
> > friends?
> >
> for very low values of udelay() you're likely right.... but we could and
> should catch that inside usleep imo and fall back to a udelay
> it'll likely be 10 usec or so where we'd cut off.
> now there is no such thing as a "low power udelay", not really anyway....

Yup. I can't find any arch which tries to do anything fancy.

x86's rep_nop() tries to save a bit of juice, doesn't it? Should we be
using that?

Because we use udelay() in many places - it wouldn't surprise me if
some people's machines were consuming significant amounts of
time/energy in there, if they have suitably broken hardware or drivers.

> but the opposite is true; the cpu idle code will effectively do the
> equivalent of udelay() if you're asking for a very short delay, so
> short that any power saving thing isn't giong to be worth it. ( +
> hitting scheduler overhead

hm, point.
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