linux-kernel - Re: [PATCH 1/4] timer: Added usleep[

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Message-Id: <20100728142239.d8dd468b.akpm@linux-foundation.org>
Date:	Wed, 28 Jul 2010 14:22:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Patrick Pannuto <ppannuto@...eaurora.org>,
	linux-kernel@...r.kernel.org, apw@...onical.com, corbet@....net,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Akinobu Mita <akinobu.mita@...il.com>
Subject: Re: [PATCH 1/4] timer: Added usleep[_range] timer

On Wed, 28 Jul 2010 14:04:47 -0700
Arjan van de Ven <arjan@...ux.intel.com> 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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