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 17:51:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	paulmck@...ux.vnet.ibm.com, 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 Mon, 2008-08-25 at 10:46 -0500, Christoph Lameter wrote:
> Peter Zijlstra wrote:
> >
> > If we combine these two cases, and flip the counter as soon as we've
> > enqueued one callback, unless we're already waiting for a grace period
> > to end - which gives us a longer window to collect callbacks.
> > 
> > And then the rcu_read_unlock() can do:
> > 
> >   if (dec_and_zero(my_counter) && my_index == dying)
> >     raise_softirq(RCU)
> > 
> > to fire off the callback stuff.
> > 
> > /me ponders - there must be something wrong with that...
> > 
> > Aaah, yes, the dec_and_zero is non trivial due to the fact that its a
> > distributed counter. Bugger..
> 
> Then lets make it per cpu. If we get the cpu ops in then dec_and_zero would be
> very cheap.

Hmm, perhaps that might work for classic RCU, as that disables
preemption and thus the counters should always be balanced.

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