From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752590Ab2FKJIw (ORCPT ); Mon, 11 Jun 2012 05:08:52 -0400 Received: from orion.tchmachines.com ([208.76.84.200]:42625 "EHLO orion.tchmachines.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751812Ab2FKJIv (ORCPT ); Mon, 11 Jun 2012 05:08:51 -0400 From: Vlad Zolotarov To: Ingo Molnar Cc: linux-kernel , Shai@scalemp.com, Thomas Gleixner , "H. Peter Anvin" Subject: Re: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read_mostly section Date: Mon, 11 Jun 2012 12:08:51 +0300 Message-ID: <28500062.nHPFoziTL1@vlad> Organization: ScaleMP Ltd. User-Agent: KMail/4.8.3 (Linux/3.2.0-24-generic; KDE/4.8.3; i686; ; ) In-Reply-To: <20120611090031.GM31556@gmail.com> References: <1744141.b5asW8k6jC@vlad> <6799312.MzsZpBJbVB@vlad> <20120611090031.GM31556@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - orion.tchmachines.com X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - scalemp.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Monday 11 June 2012 11:00:31 Ingo Molnar wrote: > * Vlad Zolotarov wrote: > > On Wednesday 23 May 2012 12:16:29 Vlad Zolotarov wrote: > > > On Tuesday, May 22, 2012 18:55:41 Vlad Zolotarov wrote: > > > > > > I have no fundamental prefer to either approach, but the > > > > > > direction taken should be justified explicitly, with numbers, > > > > > > arguments, etc. - also a short blurb somewhere in the headers > > > > > > that explains when they should be used, so that others can be > > > > > > aware of vSMP's special needs here. > > > > > > > > > > I.e. *numbers* are needed: roughly how many percpu variables in > > > > > a defconfig of one type versus the other type. This settles the > > > > > question whether we want to identify read-mostly or > > > > > write-frequently variables, to address this particular problem > > > > > ... > > > > > > Ingo, here is the proposal to the patch (series) description: > > > > > > ------------------------------------------------------------------------ > > > ---- -------------------- Added "read-mostly" qualifier to the following > > > variables in smp.h: - cpu_sibling_map > > > > > > - cpu_core_map > > > - cpu_llc_shared_map > > > - cpu_llc_id > > > - cpu_number > > > - x86_cpu_to_apicid > > > - x86_bios_cpu_apicid > > > - x86_cpu_to_logical_apicid > > > > > > As long as all the variables above are only written during the > > > initialization, this change is meant to prevent the false sharing. More > > > specifically, on vSMP Foundation platform x86_cpu_to_apicid shared the > > > same > > > internode_cache_line with frequently written lapic_events. > > > > > > From the analysis of the first 33 per_cpu variables out of 219 > > > (memories > > > they describe, to be more specific) the 8 have read_mostly nature > > > (tlb_vector_offset, cpu_loops_per_jiffy, xen_debug_irq, etc.) and 25 are > > > frequently written (irq_stack_union, gdt_page, exception_stacks, > > > idt_desc, > > > etc.). Assuming that the spread of the rest of the per_cpu variables is > > > similar, identifying the read mostly memories will make more sense in > > > terms > > > of long-term code maintenance comparing to identifying frequently > > > written > > > memories. > > > ------------------------------------------------------------------------ > > > ---- ----------------------- > > > > > > Pls., tell me if the above looks satisfactory to u in light of all your > > > previous remarks. > > > > > > If yes - I'll respin the series with the description above. > > > > Ingo, sorry for bothering. Could u, pls., tell if the above > > description is ok? We'd like to move on with this patch > > series. > > Yeah, that description and analysis looks good and sensible. > > Mind resending the updated patches in a new thread? Sure. I'm on it right now, sir. :) thanks, vlad > > Thanks, > > Ingo