linux-kernel - Re: [PATCH 01/16] math128: Introduce various 128bit primitives

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Date:	Fri, 26 Oct 2012 11:12:33 -0700
From:	Juri Lelli <juri.lelli@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, rostedt@...dmis.org, oleg@...hat.com,
	fweisbec@...il.com, darren@...art.com, johan.eker@...csson.com,
	p.faure@...tech.ch, linux-kernel@...r.kernel.org,
	claudio@...dence.eu.com, michael@...rulasolutions.com,
	fchecconi@...il.com, tommaso.cucinotta@...up.it,
	nicola.manica@...i.unitn.it, luca.abeni@...tn.it,
	dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com,
	jkacur@...hat.com, harald.gustafsson@...csson.com,
	vincent.guittot@...aro.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 01/16] math128: Introduce various 128bit primitives

Hi,
first of all thanks to everybody for all this comments!

On 10/26/2012 05:56 AM, Peter Zijlstra wrote:
> On Fri, 2012-10-26 at 12:44 +0200, Peter Zijlstra wrote:
>>> We can still have the user space interface handing in the information
>>> in nsec resolution, but it's reasonable to scale it down to something
>>> useful. Just shift the incoming information right by 10, so you're in
>>> the 1us resolution for all the internal math and all your limitation
>>> problems are gone. A shift by ten for converting back and forth to
>>> nsecs is not a real performance issue.
>>
>> I'm fine with that.. all I wanted was to not have the undefined overflow
>> we initially had.
>
> Note that we still need the constraint checking with this, although with
> both values shifted right 10 bits the range is now much bigger and
> shouldn't be a practical limit anymore.
>

I'll try to recap what seems to me you agreed and what will be the
changes for the next iteration.

- remove first two patches (u128 math) [and keep them in a safe place
   just in case following constraints will annoy future generation users
   :P]

- scale down (right by 10) incoming parameters as to do internal
   math with ~1us resolution (and scale up outgoing params)

- insert new constraints on -dl entities parameters:

     o since we have - dl_period >= dl_deadline >= dl_runtime - the
       only constraint we have to add for the overflow problem should
       be dl_period * dl_runtime < U64_MAX

     o to rule out problems with <= 1000ns parameters just force the user
       to pass > 1000ns parameters (in the end its our real resolution)

- WARN_ONCE() in proper places

- properly document all this (comments and Documentation)

What you think?

Thanks a lot and Regards,

- Juri
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