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=-2.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,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 22AE9C2D0D2 for ; Fri, 20 Dec 2019 16:22:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ECD5B222C2 for ; Fri, 20 Dec 2019 16:22:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="SJuXFwvj" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727435AbfLTQWG (ORCPT ); Fri, 20 Dec 2019 11:22:06 -0500 Received: from mail-wr1-f51.google.com ([209.85.221.51]:46849 "EHLO mail-wr1-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726808AbfLTQWG (ORCPT ); Fri, 20 Dec 2019 11:22:06 -0500 Received: by mail-wr1-f51.google.com with SMTP id z7so9927693wrl.13; Fri, 20 Dec 2019 08:22:05 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=XwJJzR6QzlYFD6/ZYjJ0H72RiZX/I4Z7P7dfLpzKIXQ=; b=SJuXFwvjbiUIRxZkIVt+njC8able0gAD3XWHO69SVtatSS0MqPO+VPSXy96S0p2cI+ odr+aKhnTn6QMlDugEZSQXJmnL34By0zKWcu9Ni5nUQP0eIEo7nZvQevhjbvr2eC+aF+ OEfY309aoD7G88UbnBQmyLha+NY1t66TxGyXH4eq1hBWOCp+DbpBMCpXW0UQUz/RMomq 6ghM2oWwpsp4NtnIkf4yRP3TiCF4aK7Lccf+LKI6k9U3eVoM5amHtE8kUAS/WS/j/Vc4 SCy78EN025YHIKYVHKpvrAPWqBU1olivqkuyh+pyq2EJ92GpEqc+yU4gyeTo1ORpURkW KR4g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=XwJJzR6QzlYFD6/ZYjJ0H72RiZX/I4Z7P7dfLpzKIXQ=; b=JQJYSQBiKI442RISlfLz7fRe1ckUgh86vHg1Uw6d0QJlv6utgYY2bm7JcHQa0bl3rj pjpaMe1eSxlEevD1xUhh3xBSLLam1Ymlpsnkk6CCCiVAvJuEzYL3c3rVqB7UqFFYx1Kl 5NuX0OtXSmpAKget0mqK0EiKu4vKSZtqUkniFmb/tAc8X4fdP3CbQBEV4XtsFTBrTJzm i/g3bGblBPCEykq/nSzSn/XQ2PzJETMgdkSGWB/91UYt1VXkUTZFTHkvu1vUm7xUDHlR JSZau+iBiccfsjMdPLSkWANEkIF9H41Bd6PAT7gH0d0eAbhyyqbxPXmYlhvNI677MeCg WZ3w== X-Gm-Message-State: APjAAAXiRxZFxO/NVrM/gDxkY1lwAxZgoBrn3gqn7Z72BAKevaTpw1zo s6Nj43juPIqjnN9VrKs5PUs= X-Google-Smtp-Source: APXvYqyBGuIo3mtqHDF9loUVBWZbuDY+RxQ2FX1olpk5WamJiGLmvUTfrVnVeE5AobMfgFB2EHWcxQ== X-Received: by 2002:adf:e6c6:: with SMTP id y6mr16344627wrm.284.1576858924463; Fri, 20 Dec 2019 08:22:04 -0800 (PST) Received: from [192.168.8.147] (72.173.185.81.rev.sfr.net. [81.185.173.72]) by smtp.gmail.com with ESMTPSA id n3sm10488026wmc.27.2019.12.20.08.22.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Dec 2019 08:22:04 -0800 (PST) Subject: Re: Percpu variables, benchmarking, and performance weirdness To: Tejun Heo , Jesper Dangaard Brouer Cc: =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , bpf , LKML , Christoph Lameter , Dennis Zhou References: <20191220103420.6f9304ab@carbon> <20191220151239.GE2914998@devbig004.ftw2.facebook.com> From: Eric Dumazet Message-ID: Date: Fri, 20 Dec 2019 08:22:02 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <20191220151239.GE2914998@devbig004.ftw2.facebook.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/20/19 7:12 AM, Tejun Heo wrote: > 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? I definitely seen expensive per-cpu updates in the stack. (SNMP counters, or per-cpu stats for packets/bytes counters) It might be nice to have an option to use 2M pages. (I recall sending some patches in the past about using high-order pages for vmalloc, but this went nowhere)