linux-kernel - Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

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Date:	Mon, 25 Aug 2008 12:31:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andi Kleen <andi@...stfloor.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather
	than rcu

On Fri, 2008-08-22 at 13:53 -0700, Paul E. McKenney wrote:
> On Fri, Aug 22, 2008 at 03:03:13PM -0500, Christoph Lameter wrote:
> > Paul E. McKenney wrote:
> > 
> > > I was indeed thinking in terms of the free from RCU being specially marked.
> > 
> > Isnt there some way to shorten the rcu periods significantly? Critical
> > sections do not take that long after all.
> 
> In theory, yes.  However, the shorter the grace period, the greater the
> per-update overhead of grace-period detection -- the general approach
> is to use a per-CPU high-resolution timer to force RCU grace period
> processing every 100 microseconds or so.  

You could of course also drive the rcu state machine from
rcu_read_unlock().

> Also, by definition, the RCU
> grace period can be no shorter than the longest active RCU read-side
> critical section.  Nevertheless, I have designed my current hierarchical
> RCU patch with expedited grace periods in mind, though more for the
> purpose of reducing latency of long strings of operations that involve
> synchronize_rcu() than for cache locality.

Another thing that could be done is more often force a grace period by
flipping the counters.

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