linux-kernel - Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine call

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Message-ID: <1331583916.18960.56.camel@twins>
Date:	Mon, 12 Mar 2012 21:25:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Lai Jiangshan <eag0628@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org, tj@...nel.org
Subject: Re: [RFC PATCH 5/6] implement per-cpu&per-domain state machine
 call_srcu()

On Mon, 2012-03-12 at 11:32 -0700, Paul E. McKenney wrote:
> And another question I should have asked to begin with...  Would each
> VMA have its own SRCU domain, or are you thinking in terms of one
> SRCU domain for all VMAs globally?

The latter, single domain for all objects.

> If the latter, that pushes pretty strongly for per-CPU SRCU callback
> lists. 

Agreed. I was under the impression the proposed thing had this, but on
looking at it again it does not. Shouldn't be hard to add though.

>  Which brings up srcu_barrier() scalability (and yes, I am working
> on rcu_barrier() scalability).  One way to handle this at least initially
> is to have srcu_barrier() avoid enqueueing callbacks on CPUs whose
> callback lists are empty.  In addition, if the loop over all CPUs is
> preemptible, then there should not be much in the way of realtime issues.

Why do we have rcu_barrier() and how is it different from
synchronize_rcu()?

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