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

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Date:	Fri, 22 Aug 2008 08:28:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	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>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather
	than rcu


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> RCU can only control the lifetime of allocated memory blocks, which 
> forces all the call structures to be allocated.  This is expensive 
> compared to allocating them on the stack, which is the common case for 
> synchronous calls.
> 
> This patch takes a different approach.  Rather than using RCU, the 
> queues are managed under rwlocks.  Adding or removing from the queue 
> requires holding the lock for writing, but multiple CPUs can walk the 
> queues to process function calls under read locks.  In the common 
> case, where the structures are stack allocated, the calling CPU need 
> only wait for its call to be done, take the lock for writing and 
> remove the call structure.
> 
> Lock contention - particularly write vs read - is reduced by using 
> multiple queues.

hm, is there any authorative data on what is cheaper on a big box, a 
full-blown MESI cache miss that occurs for every reader in this new 
fastpath, or a local SLAB/SLUB allocation+free that occurs with the 
current RCU approach?

	Ingo
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