Re: some thoughts about TSC based delay_tsc()

Re: some thoughts about TSC based delay_tsc()

[Marin Mitov]

On Wednesday 21 November 2007 09:27:54 pm you wrote:
> * Marin Mitov <mitov@issp.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

Re: some thoughts about TSC based delay_tsc()

[Ingo Molnar]

* Marin Mitov <mitov@issp.bas.bg> wrote:

> On Wednesday 21 November 2007 09:27:54 pm you wrote:
> > * Marin Mitov <mitov@issp.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?).

on non-PREEMPT kernels there's no real difference between old and and 
new code because the kernel is not preemptible. So we can use the new 
code unconditionally.

	Ingo