* Robin Holt <h...@sgi.com> wrote:
> On Thu, Apr 11, 2013 at 07:03:58AM -0500, Robin Holt wrote:
> > On Thu, Apr 11, 2013 at 02:00:27PM +0200, Ingo Molnar wrote:
> > >
> > > * Robin Holt <h...@sgi.com> wrote:
> > >
> > > > > Ok, so it looks profilable.
> > > > >
> > > > > The result above is not surprising: most CPUs sit in idle and don't
> > > > > do anything,
> > > > > while the loop goes on, right?
> > > > >
> > > > > The interesting thing to profile would be the parallel bring-down,
> > > > > with the
> > > > > simplest global lock solution you mentioned. In that case most CPUs
> > > > > should be
> > > > > doing 'something' all the time - maybe spinning on the lock, maybe
> > > > > something else,
> > > > > right?
> > > >
> > > > Again, mostly looks idle.
> > >
> > > Forgot to suggest:
> > >
> > > perf record -a /sbin/reboot
> >
> > I used perf record -a /sbin/reboot -f -d -n
>
> OK. Looking at Russ' patch, I understand now why it is looking idle.
> We are still serially doing the DOWN_PREPARE, etc. All those other cpus
> are still sitting idle.
>
> Can we call the __cpu_down functions from an smp_call_function()?
I think the kthread_park() will generally schedule.
But ... whether it's an IPI or a wakeup should matter little: wakeups are IPI
based (sometimes faster, mwait based).
So the main overhead is the serial loop - if that's done in parallel, and then
all
CPUs are waited for in a second loop, then much of the work can go on in
parallel.
Thanks,
Ingo
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