From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ecfrec.frec.bull.fr (ecfrec.frec.bull.fr [129.183.4.8]) by ozlabs.org (Postfix) with ESMTP id D06F9DDEFB for ; Fri, 25 Jul 2008 18:33:48 +1000 (EST) Date: Fri, 25 Jul 2008 10:34:18 +0200 From: Sebastien Dugue To: Peter Zijlstra Subject: Re: [PATCH 2/2][RT] powerpc - Make the irq reverse mapping radix tree lockless Message-ID: <20080725103418.2ae898f5@bull.net> In-Reply-To: <1216972177.7257.351.camel@twins> References: <20080724122352.3bc76bda@bull.net> <20080724125044.53b604cb@bull.net> <200807242111.35338.nickpiggin@yahoo.com.au> <20080724141839.477de30b@bull.net> <1216972177.7257.351.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: Nick Piggin , Tim Chavez , Linux-rt , linux-kernel , Jean Pierre Dion , linux-ppc , Paul Mackerras , Gilles Carry List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Hi Peter, On Fri, 25 Jul 2008 09:49:37 +0200 Peter Zijlstra wr= ote: > On Thu, 2008-07-24 at 14:18 +0200, Sebastien Dugue wrote: > > On Thu, 24 Jul 2008 21:11:34 +1000 Nick Piggin wrote: > >=20 > > > On Thursday 24 July 2008 20:50, Sebastien Dugue wrote: > > > > From: Sebastien Dugue > > > > Date: Tue, 22 Jul 2008 11:56:41 +0200 > > > > Subject: [PATCH][RT] powerpc - Make the irq reverse mapping radix t= ree > > > > lockless > > > > > > > > The radix tree used by interrupt controllers for their irq reverse > > > > mapping (currently only the XICS found on pSeries) have a complex l= ocking > > > > scheme dating back to before the advent of the concurrent radix tre= e on > > > > preempt-rt. > > > > > > > > Take advantage of this and of the fact that the items of the tree= are > > > > pointers to a static array (irq_map) elements which can never go un= der us > > > > to simplify the locking. > > > > > > > > Concurrency between readers and writers are handled by the intrin= sic > > > > properties of the concurrent radix tree. Concurrency between the tr= ee > > > > initialization which is done asynchronously with readers and writer= s access > > > > is handled via an atomic variable (revmap_trees_allocated) set when= the > > > > tree has been initialized and checked before any reader or writer a= ccess > > > > just like we used to check for tree.gfp_mask !=3D 0 before. > > >=20 > > > Hmm, RCU radix tree is in mainline too for quite a while. I thought > > > Ben had already converted this code over ages ago... > >=20 > > Mainline does not have the concurrent radix tree which this patch > > is based on, but maybe it's overkill and the RCU radix tree is enough. > > Not sure, will have to think about it a bit more. >=20 > Should be. The model of the concurrent radix tree can be mapped to > spinlock + rcu radix tree. >=20 > So instead of: >=20 > > =EF=BB=BF+ DEFINE_RADIX_TREE_CONTEXT(ctx, tree); > > + radix_tree_lock(&ctx); > > + radix_tree_insert(ctx.tree, hwirq, &irq_map[virq]); > > + radix_tree_unlock(&ctx); >=20 >=20 > you then write: >=20 > spin_lock(&host->revmap_data.tree_lock); > radix_tree_insert(&host->revmap_data.tree, hwirq, &irq_map[virq]); > spin_unlock(&host->revmap_data.tree_lock); >=20 Cool, that will indeed makes it much easier to have something applicable to mainline which works with preempt-rt. >=20 > The only advantage of the concurrent radix tree over this model is that > it can potentially do multiple modification operations at the same time. Well in theory that can happen if a module is loaded which creates a mapp= ing while another one is unloaded at the same time. The time window is pretty n= arrow, but still present nonetheless. That's why I chose to use the concurrent ver= sion. >=20 > Still, cool that you used it ;-) Yep, looked like what was needed until I realized it was not available in mainline. Nice work though and good paper for explaining it all. Sebastien.