linux-kernel - Re: [PATCH 2/2] percpu-rw-semaphores: use rcu_read_lock

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Date:	Thu, 25 Oct 2012 08:07:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	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 2/2] percpu-rw-semaphores: use rcu_read_lock_sched

On Thu, Oct 25, 2012 at 10:54:11AM -0400, Mikulas Patocka wrote:
> 
> 
> On Wed, 24 Oct 2012, Paul E. McKenney wrote:
> 
> > On Mon, Oct 22, 2012 at 07:39:16PM -0400, Mikulas Patocka wrote:
> > > Use rcu_read_lock_sched / rcu_read_unlock_sched / synchronize_sched
> > > instead of rcu_read_lock / rcu_read_unlock / synchronize_rcu.
> > > 
> > > This is an optimization. The RCU-protected region is very small, so
> > > there will be no latency problems if we disable preempt in this region.
> > > 
> > > So we use rcu_read_lock_sched / rcu_read_unlock_sched that translates
> > > to preempt_disable / preempt_disable. It is smaller (and supposedly
> > > faster) than preemptible rcu_read_lock / rcu_read_unlock.
> > > 
> > > Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> > 
> > OK, as promised/threatened, I finally got a chance to take a closer look.
> > 
> > The light_mb() and heavy_mb() definitions aren't doing much for me,
> > the code would be cleared with them expanded inline.  And while the
> > approach of pairing barrier() with synchronize_sched() is interesting,
> > it would be simpler to rely on RCU's properties.  The key point is that
> > if RCU cannot prove that a given RCU-sched read-side critical section
> > is seen by all CPUs to have started after a given synchronize_sched(),
> > then that synchronize_sched() must wait for that RCU-sched read-side
> > critical section to complete.
> 
> Also note that you can define both light_mb() and heavy_mb() to be 
> smp_mb() and slow down the reader path a bit and speed up the writer path.
> 
> On architectures with in-order memory access (and thus smp_mb() equals 
> barrier()), it doesn't hurt the reader but helps the writer, for example:
> #ifdef ARCH_HAS_INORDER_MEMORY_ACCESS
> #define light_mb()      smp_mb()
> #define heavy_mb()      smp_mb()
> #else
> #define light_mb()      barrier()
> #define heavy_mb()      synchronize_sched()
> #endif

Except that there are no systems running Linux with in-order memory
access.  Even x86 and s390 require a barrier instruction for smp_mb()
on SMP=y builds.

							Thanx, Paul

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