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

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Message-ID: <4C509B6F.8000200@linux.intel.com>
Date:	Wed, 28 Jul 2010 14:04:47 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 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....

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


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