linux-kernel - Re: some thoughts about TSC based delay

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Message-Id: <200711212206.34339.mitov@issp.bas.bg>
Date:	Wed, 21 Nov 2007 22:06:34 +0200
From:	Marin Mitov <mitov@...p.bas.bg>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: some thoughts about TSC based delay_tsc()

On Wednesday 21 November 2007 09:27:54 pm you wrote:
> * Marin Mitov <mitov@...p.bas.bg> wrote:
> > Hi Ingo,
> >
> > The patch is quite good ;-) but we forget when it is needed :-( In
> > fact we need it only for PREEMPT SMP kernels - it could hurt PREEMPT
> > UP kernels (no migration possible), so no need for
> > preempt_disable()/preempt_enable().
> >
> > In short the old version of delay_tsc() is good for UP kernels and NON
> > PREEMPT SMP kernels too.
>
> please reply to the public list, so that discussions do not get lost.
>
> i dont think there's any problem: udelay() is about _wasting_ cycles -
> it's what drivers use for short delays.

Sure for the thread executing udelay(), but not for the other ready threads 
which should also wait till preempt_enable() to grab the same cpu even 
for PREEMPT (UP or SMP) kernels (or I misunderstand something?).

Marin 
>
> 	Ingo


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