Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751128AbWHXLWA (ORCPT ); Thu, 24 Aug 2006 07:22:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751130AbWHXLWA (ORCPT ); Thu, 24 Aug 2006 07:22:00 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:31928 "EHLO mx2.mail.elte.hu") by vger.kernel.org with ESMTP id S1751128AbWHXLV7 (ORCPT ); Thu, 24 Aug 2006 07:21:59 -0400 Date: Thu, 24 Aug 2006 13:14:40 +0200 From: Ingo Molnar To: Gautham R Shenoy Cc: rusty@rustcorp.com.au, torvalds@osdl.org, akpm@osdl.org, linux-kernel@vger.kernel.org, arjan@intel.linux.com, davej@redhat.com, vatsa@in.ibm.com, dipankar@in.ibm.com, ashok.raj@intel.com Subject: Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking" Message-ID: <20060824111440.GA19248@elte.hu> References: <20060824103233.GD2395@in.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20060824103233.GD2395@in.ibm.com> User-Agent: Mutt/1.4.2.1i X-ELTE-SpamScore: -2.9 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.9 required=5.9 tests=ALL_TRUSTED,AWL,BAYES_50 autolearn=no SpamAssassin version=3.0.3 -3.3 ALL_TRUSTED Did not pass through any untrusted hosts 0.5 BAYES_50 BODY: Bayesian spam probability is 40 to 60% [score: 0.4990] -0.1 AWL AWL: From: address is in the auto white-list X-ELTE-VirusStatus: clean Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1217 Lines: 30 * Gautham R Shenoy wrote: > void lock_cpu_hotplug(void) > { > + DECLARE_WAITQUEUE(wait, current); > + spin_lock(&cpu_hotplug.lock); > + cpu_hotplug.reader_count++; this should be per-CPU - lock_cpu_hotplug() should _not_ be a globally synchronized event. CPU removal is such a rare event that we can easily do something like a global read-mostly 'CPU is locked for writes' flag (plus a completion queue) that the 'write' side takes atomically - combined with per-CPU refcount and a waitqueue that the read side increases/decreases and wakes. Read-locking of the CPU is much more common and should be fundamentally scalable: it should increase the per-CPU refcount, then check the global 'writer active' flag, and if the writer flag is set, it should wait on the global completion queue. When a reader drops the refcount it should wake up the per-CPU waitqueue. [in which a writer might be waiting for the refcount to go down to 0.] Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/