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

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Message-ID: <20121024151100.GA16793@redhat.com>
Date:	Wed, 24 Oct 2012 17:11:00 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

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.

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