From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods Date: Mon, 18 May 2009 17:42:41 +0200 Message-ID: <20090518154241.GA27047@elte.hu> References: <20090517191141.GA25915@linux.vnet.ibm.com> <20090518075630.GA10687@elte.hu> <20090518151421.GB6768@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, netfilter-devel@vger.kernel.org, akpm@linux-foundation.org, torvalds@linux-foundation.org, davem@davemloft.net, dada1@cosmosbay.com, zbr@ioremap.net, jeff.chua.linux@gmail.com, paulus@samba.org, laijs@cn.fujitsu.com, jengelh@medozas.de, r000n@r000n.net, benh@kernel.crashing.org, mathieu.desnoyers@polymtl.ca To: "Paul E. McKenney" Return-path: Received: from mx2.mail.elte.hu ([157.181.151.9]:48679 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752432AbZERPnf (ORCPT ); Mon, 18 May 2009 11:43:35 -0400 Content-Disposition: inline In-Reply-To: <20090518151421.GB6768@linux.vnet.ibm.com> Sender: netfilter-devel-owner@vger.kernel.org List-ID: * Paul E. McKenney wrote: > > i might be missing something fundamental here, but why not just > > have per CPU helper threads, all on the same waitqueue, and wake > > them up via a single wake_up() call? That would remove the SMP > > cross call (wakeups do immediate cross-calls already). > > My concern with this is that the cache misses accessing all the > processes on this single waitqueue would be serialized, slowing > things down. In contrast, the bitmask that smp_call_function() > traverses delivers on the order of a thousand CPUs' worth of bits > per cache miss. I will give it a try, though. At least if you go via the migration threads, you can queue up requests to them locally. But there's going to be cachemisses _anyway_, since you have to access them all from a single CPU, and then they have to fetch details about what to do, and then have to notify the originator about completion. Ingo