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

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Message-ID: <4C50A054.8030503@linux.intel.com>
Date:	Wed, 28 Jul 2010 14:25:40 -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 2:22 PM, Andrew Morton wrote:
> 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?
>    

it doesn't save juice so much as it is a "yield to my hyperthreading 
brother"
(there is some power saved as well potentially...)

afaik we already use this in udelay()    (cpu_relax is rep_nop after all)

> 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.
>    

the only real place I've seen it used (based on profiles) is in libata 
in the intel piix sata driver
(the non-AHCI one)... and those are completly wrong, Alan Cox had 
patches to fix it but those somehow went nowhere


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