Re: [patch] reduce IPI noise due to /dev/cdrom open/close

[Nick Piggin <nickpiggin@yahoo.com.au>]

Jes Sorensen wrote:
> Nick Piggin wrote:
>
> > Andrew Morton wrote:
> >
> > > I expect raw_smp_processor_id() is used here as a a microoptimisation -
> > > avoid a might_sleep() which obviously will never trigger.
> >
> > A microoptimisation because they've turned on DEBUG_PREEMPT and found
> > that smp_processor_id slows down? ;) Wouldn't it be better to just stick
> > to the normal rules (ie. what Keith said)?
> >
> > It may be obvious in this case (though that doesn't help people who make
> > obvious mistakes, or mismerge patches) but this just seems like a nasty
> > precedent to set (or has it already been?).
>
>
> I suspect the real reason here is that there's now so many ways to get
> the processor ID that I cannot keep track of which one to use. Paul's
> mention of __raw_get_cpu_var() just confuses me even more.
>
> So if anyone can give me a conclusive answer of which one to use, I'm
> happy to go there.
>
> Granted I have a bias to avoid anything involving the preempt crap, but
> thats just me :)

Use smp_processor_id() unless you explicitly want a lazy CPU number,
and in that case use the raw_ version. Turning off preempt or preempt
debug options does the rest for you.

If you're just using the number to feed into per_cpu, then use the
appropriate get_cpu variant.

--
SUSE Labs, Novell Inc.
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