Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()

[Brent Casavant <bcasavan@sgi.com>]

On Fri, 16 Jun 2006, Chase Venters wrote:

> On Fri, 16 Jun 2006, Andi Kleen wrote:
> 
> > 
> > > To this last point, it might be more reasonable to map in a page that
> > > contained a new structure with a stable ABI, which mirrored some of
> > > the task_struct information, and likely other useful information as
> > > needs are identified in the future.  In any case, it would be hard
> > > to beat a single memory read for performance.
> > 
> > That would mean making the context switch and possibly other
> > things slower.
> 
> Well, if every process had a page of its own, what would the context switch
> overhead be?

Mostly copying the useful information into the read-only mapped page.

However, this doesn't have to be all that expensive.  The particular
information we care about in this case only needs to be copied when a
task begins running on a CPU different from the one it last ran on.  In
fact, on ia64 we already have something very similar to handle certain
I/O pecularities on SN2.

http://marc.theaimsgroup.com/?l=linux-ia64&m\x113831137712197&w=2

That work could form the basis for a low-impact method of exporting
the current CPU to user space via a read-only mapped page.  I'll admit
to having zero knowledge of whether this would be workable on anything
other than ia64.

Thanks,
Brent

-- 
Brent Casavant                          All music is folk music.  I ain't
bcasavan@sgi.com                        never heard a horse sing a song.
Silicon Graphics, Inc.                    -- Louis Armstrong