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=-0.9 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 11771C67839 for ; Tue, 11 Dec 2018 23:46:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C6BEA2084E for ; Tue, 11 Dec 2018 23:46:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C6BEA2084E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726243AbeLKXqE (ORCPT ); Tue, 11 Dec 2018 18:46:04 -0500 Received: from mga14.intel.com ([192.55.52.115]:29907 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726158AbeLKXqE (ORCPT ); Tue, 11 Dec 2018 18:46:04 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by fmsmga103.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 11 Dec 2018 15:46:03 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.56,343,1539673200"; d="scan'208";a="117577481" Received: from cli6-desk1.ccr.corp.intel.com (HELO [10.239.161.118]) ([10.239.161.118]) by orsmga002.jf.intel.com with ESMTP; 11 Dec 2018 15:46:01 -0800 Subject: Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks To: Dave Hansen , Aubrey Li , tglx@linutronix.de, mingo@redhat.com, peterz@infradead.org, hpa@zytor.com Cc: ak@linux.intel.com, tim.c.chen@linux.intel.com, arjan@linux.intel.com, linux-kernel@vger.kernel.org References: <20181211002448.3520-1-aubrey.li@intel.com> From: "Li, Aubrey" Message-ID: <143a5451-c7cf-16a5-a7ef-8c4b0633b689@linux.intel.com> Date: Wed, 12 Dec 2018 07:46:01 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.1.1 MIME-Version: 1.0 In-Reply-To: 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 2018/12/12 1:18, Dave Hansen wrote: > On 12/10/18 4:24 PM, Aubrey Li wrote: >> The tracking turns on the usage flag at the next context switch of >> the task, but requires 3 consecutive context switches with no usage >> to clear it. This decay is required because well-written AVX-512 >> applications are expected to clear this state when not actively using >> AVX-512 registers. > > One concern about this: Given a HZ=1000 system, this means that the > flag needs to get scanned every ~3ms. That's a pretty good amount of > scanning on a system with hundreds or thousands of tasks running around. > > How many tasks does this scale to until you're eating up an entire CPU > or two just scanning /proc? > Do we have a real requirement to do this in practical environment? AFAIK, 1s or even 5s is good enough in some customers environment. Thanks, -Aubrey