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Date: Fri, 9 Nov 2012 17:35:38 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Anton Arapov <anton@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not
	block the readers unnecessarily
Message-ID: <20121109163538.GB26134@redhat.com>
References: <20121031194158.GB504@redhat.com> <CA+55aFx0yyke6V1+wgMPBN4QZ0w=YoV7yBRqp0uy6aGKbcmC5g@mail.gmail.com> <20121102180606.GA13255@redhat.com> <20121108134805.GA23870@redhat.com> <20121108134849.GB23870@redhat.com> <20121108120700.42d438f2.akpm@linux-foundation.org> <20121108210843.GF2519@linux.vnet.ibm.com> <Pine.LNX.4.64.1211081836180.7134@file.rdu.redhat.com> <20121109004136.GH2519@linux.vnet.ibm.com> <20121109032310.GA2438@linux.vnet.ibm.com>
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On 11/08, Paul E. McKenney wrote:
>
> On Thu, Nov 08, 2012 at 04:41:36PM -0800, Paul E. McKenney wrote:
> > On Thu, Nov 08, 2012 at 06:41:10PM -0500, Mikulas Patocka wrote:
> > >
> > > On Thu, 8 Nov 2012, Paul E. McKenney wrote:
> > >
> > > > On Thu, Nov 08, 2012 at 12:07:00PM -0800, Andrew Morton wrote:
> > > > > On Thu, 8 Nov 2012 14:48:49 +0100
> > > > > Oleg Nesterov <oleg@redhat.com> wrote:
> > > > >
> > > >
> > > > The algorithm would work given rcu_read_lock()/rcu_read_unlock() and
> > > > synchronize_rcu() in place of preempt_disable()/preempt_enable() and
> > > > synchronize_sched().  The real-time guys would prefer the change
> > > > to rcu_read_lock()/rcu_read_unlock() and synchronize_rcu(), now that
> > > > you mention it.
> > > >
> > > > Oleg, Mikulas, any reason not to move to rcu_read_lock()/rcu_read_unlock()
> > > > and synchronize_rcu()?
> > >
> > > preempt_disable/preempt_enable is faster than
> > > rcu_read_lock/rcu_read_unlock for preemptive kernels.

Yes, I chose preempt_disable() because it is the fastest/simplest
primitive and the critical section is really tiny.

But:

> > Significantly faster in this case?  Can you measure the difference
> > from a user-mode test?

I do not think rcu_read_lock() or rcu_read_lock_sched() can actually
make a measurable difference.

> Actually, the fact that __this_cpu_add() will malfunction on some
> architectures is preemption is not disabled seems a more compelling
> reason to keep preempt_enable() than any performance improvement.  ;-)

Yes, but this_cpu_add() should work.

> > Careful.  The real-time guys might take the same every-little-bit approach
> > to latency that you seem to be taking for CPU cycles.  ;-)

Understand...


So I simply do not know. Please tell me if you think it would be
better to use rcu_read_lock/synchronize_rcu or rcu_read_lock_sched,
and I'll send the patch.

Oleg.