Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Nov 09, 2012 at 05:35:38PM +0100, Oleg Nesterov wrote:
> 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.

Indeed!  But this_cpu_add() just does the preempt_enable() under the
covers, so not much difference from a latency viewpoint.

> > > 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.

I doubt if it makes a measurable difference for either throughput or
latency.  One could argue that rcu_read_lock() would be better for
readability, but making sure that the preempt_disable() is clearly
commented as starting an RCU-sched read-side critical section would
be just as good.

So I am OK with the current preempt_disable() approach.

							Thanx, Paul