Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

[Oleg Nesterov <oleg@redhat.com>]

On 10/23, Peter Zijlstra wrote:
>
> On Tue, 2012-10-23 at 21:23 +0200, Oleg Nesterov wrote:
> >
> >         static void mb_ipi(void *arg)
> >         {
> >                 smp_mb(); /* unneeded ? */
> >         }
> >
> >         static void force_mb_on_each_cpu(void)
> >         {
> >                 smp_mb();
> >                 smp_call_function(mb_ipi, NULL, 1);
> >         }
>
> You know we're spending an awful lot of time and effort to get rid of
> such things, right? RT and HPC people absolutely hate these random IPI
> things.

No I do not know ;) but I am not suprized.

And,

> > I have to admit, I have
> > no idea how much cli/sti is slower compared to preempt_disable/enable.
> >
> A lot.. esp on stupid hardware (insert pentium-4 reference), but I think
> its more expensive for pretty much all hardware,

Thanks Peter, this alone answers my question.

Oleg.