From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D793CC3F68F for ; Fri, 20 Dec 2019 15:12:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ADEA12465E for ; Fri, 20 Dec 2019 15:12:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576854765; bh=HnilqKXVsRAGliLqZJR0UJzrWAk0nKhD/l9sJKYGuVY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=IEOvGOFVSjO0ZtFGg1XNe1OgSqGyG0B/cAUGc9vaOma3/1+O9YU4WdXRFqb16sJHo wpqSuvkv4JHLZOBEcQvMwLgcrxmjbUqdp/p/xIGM705buVqDd8Iq0Ij0q/Xc17i60x a5DVEr0nOhC/Rvr0wRiL+4utWtotKnMcnxhdzwkU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727426AbfLTPMo (ORCPT ); Fri, 20 Dec 2019 10:12:44 -0500 Received: from mail-qk1-f174.google.com ([209.85.222.174]:46265 "EHLO mail-qk1-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727233AbfLTPMo (ORCPT ); Fri, 20 Dec 2019 10:12:44 -0500 Received: by mail-qk1-f174.google.com with SMTP id r14so7839683qke.13; Fri, 20 Dec 2019 07:12:43 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=IHi61ZO4N2TxTXDRb0qVYOix5FZH5i77yLyvReH1I6A=; b=BbT7FCDT4odjAl0Zy7AQJmuklSDWt+1P4hqlkWz+0lL4aODJcNceUnRqUpzlQlocAg QTwzQrI1ypcSmTIY2zO9sQdwEo+VMvSLFvcYb8sfmkXymsaKH1puK3srRU/cbFC/nrAA sNWF/HifIOYVqEbv7K6lrUklsR7rFHDKQHTyWaKHd2plhLrJUTVxjsTTpARVaT9uM/ve SFBCklhFBcmQkn8RMSDCYt7DPgMMM4i3XZUSurTfPzIFFK5w++iI679W6Cc7xdumjP7y ixdoDC32Q/Ned0ACKQUQo+I/R9z/ISPYQBFdX61X4ZWvp+QOLG+GErbrIJiE4sZwTB0q gT7g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to:user-agent; bh=IHi61ZO4N2TxTXDRb0qVYOix5FZH5i77yLyvReH1I6A=; b=tQTu6PjOcWrre9Mzx0LMxvSvi+tZCqPaXzLWrceiv42elkgkoNBoTcfSVg5IRTzZrw 480YAGHzW0Tv7LS2i+kZqF9RyLaUEYSptj8l/MhKXDMiW1rZxQvy/ovLICGKJyceMf+b CV0M9xQY0Xv4O0jtG510z1NtG2VleLcolRfbJu36pNULABD5qIzCQqdIjtbJgdjXvhcw 63AMIWiqz8pvmXubTqCG0R+8KdCPgNChynh/tqvg1pVdwJ9DaDb3CA1CqMA0yeM5rQKL 7B2Ukri5vU52aC3MSF/nzBjmzGgNZn8tDO6d2Ycts83emxpvX5m0zwFwhq/kYIfa4n+V Xqgg== X-Gm-Message-State: APjAAAX1fW01rRLAzjuJgMJzWDh/5kSrHHXuUL7f1Yjs2XwhS5/9B0Zg 08fT9t/64z/pAXBFvlqVUCk= X-Google-Smtp-Source: APXvYqzTbo9nrvCWuKzXMQFWRRYJMQDSkWc3FRa0b4eUgurK7Mv3sZSw4ccP1LjqE3PNfRehmXNMeA== X-Received: by 2002:a05:620a:782:: with SMTP id 2mr13970138qka.169.1576854762744; Fri, 20 Dec 2019 07:12:42 -0800 (PST) Received: from localhost ([2620:10d:c091:480::11e7]) by smtp.gmail.com with ESMTPSA id v125sm2973124qka.47.2019.12.20.07.12.41 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 20 Dec 2019 07:12:42 -0800 (PST) Date: Fri, 20 Dec 2019 07:12:39 -0800 From: Tejun Heo To: Jesper Dangaard Brouer Cc: =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , bpf , LKML , Christoph Lameter , Dennis Zhou Subject: Re: Percpu variables, benchmarking, and performance weirdness Message-ID: <20191220151239.GE2914998@devbig004.ftw2.facebook.com> References: <20191220103420.6f9304ab@carbon> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191220103420.6f9304ab@carbon> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 20, 2019 at 10:34:20AM +0100, Jesper Dangaard Brouer wrote: > > So, my question to the uarch/percpu folks out there: Why are percpu > > accesses (%gs segment register) more expensive than regular global > > variables in this scenario. > > I'm also VERY interested in knowing the answer to above question!? > (Adding LKML to reach more people) No idea. One difference is that percpu accesses are through vmap area which is mapped using 4k pages while global variable would be accessed through the fault linear mapping. Maybe you're getting hit by tlb pressure? Thanks. -- tejun