From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752168Ab2FKJAi (ORCPT ); Mon, 11 Jun 2012 05:00:38 -0400 Received: from mail-wg0-f44.google.com ([74.125.82.44]:44288 "EHLO mail-wg0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750905Ab2FKJAg (ORCPT ); Mon, 11 Jun 2012 05:00:36 -0400 Date: Mon, 11 Jun 2012 11:00:31 +0200 From: Ingo Molnar To: Vlad Zolotarov 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 Message-ID: <20120611090031.GM31556@gmail.com> References: <1744141.b5asW8k6jC@vlad> <5555210.ryZGheQWJb@vlad> <1415723.ntIHEoaamI@vlad> <6799312.MzsZpBJbVB@vlad> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6799312.MzsZpBJbVB@vlad> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * 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? Thanks, Ingo