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=-7.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 8D7F4C83007 for ; Wed, 29 Apr 2020 11:30:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6BDAD2074A for ; Wed, 29 Apr 2020 11:30:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588159831; bh=75gzaDRNPp/zEslpydJPxXfqRCz7ox9duCIptsCmW7Q=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=so1bLBabKBbAAboi1unidf1Vdd06jZ1wAI7pmahCBDu9sv+TfUFhwhH69nU9Fa7mO tC6SHMlGOAovBHRczjLjUmhOtttOckj8fEYSbmIU474VleBTIn46QpMrnXGyYEg8hg SHRzm8wA/e54xSptn28TJGcrQydhYfxME+0vhNNo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726817AbgD2Laa (ORCPT ); Wed, 29 Apr 2020 07:30:30 -0400 Received: from mail-ot1-f67.google.com ([209.85.210.67]:45398 "EHLO mail-ot1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726620AbgD2Laa (ORCPT ); Wed, 29 Apr 2020 07:30:30 -0400 Received: by mail-ot1-f67.google.com with SMTP id e20so1333337otk.12; Wed, 29 Apr 2020 04:30:29 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=mJLQaIat2tDbEwACly2x5coQW2dyQ71GmCW1Pk/+Pe0=; b=lu6Wh/M+LRHLWNg4cR8b9fCM0GIKOZfVDKacY39nnCoWwOXTDcSFwjzMg82RfQGPuo 1aZHs/P9FeaBec98E/HlbzSpXaiJw94wb84edCPlyNzPKvjADGPopsxP/Jdu9OIu3WJP Ak5eZTj1iLyYJYSMpbX3tPuqinUsaiS/j+L34DqBNd8PJFVGtY5f/o5T0A/gBYiyYIb4 OKL8jT3mViTzzC23hhqnVog6UtgAYmF1POo68rYyS1XYrCZx8cBvHbzQXB/x2VIg8i5x NGXorNi14ZheGKkrjQW2qZQTq0Aie9WIWZ66kfQ3IL4LhLgd+THo5Vs5uEAwg4CxW+8N Dgfw== X-Gm-Message-State: AGi0PuanipB5Cmh8tlI0wrQTE44UC9QC6vGOC6JKEnII5ycA6wOboLT0 QtDfpOPNVdvesaENYeHcMN980M0FrC4Z86TTUj8= X-Google-Smtp-Source: APiQypIxdnHfH8fsytx0SXUL7Yzk0wRF6oZwhYIlSygbrTH5FgtTjQI6UbJwZ6dhKKE3HNiTqMY6ekE7V/Zic4Xznmc= X-Received: by 2002:a9d:1d07:: with SMTP id m7mr25072932otm.167.1588159829033; Wed, 29 Apr 2020 04:30:29 -0700 (PDT) MIME-Version: 1.0 References: <20200428132450.24901-1-ggherdovich@suse.cz> <20200428132450.24901-2-ggherdovich@suse.cz> In-Reply-To: <20200428132450.24901-2-ggherdovich@suse.cz> From: "Rafael J. Wysocki" Date: Wed, 29 Apr 2020 13:30:17 +0200 Message-ID: Subject: Re: [PATCH 1/2] x86, sched: Prevent divisions by zero in frequency invariant accounting To: Giovanni Gherdovich Cc: Srinivas Pandruvada , Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Borislav Petkov , Len Brown , "Rafael J . Wysocki" , "the arch/x86 maintainers" , Linux PM , Linux Kernel Mailing List , Ricardo Neri , Linus Torvalds Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 28, 2020 at 3:25 PM Giovanni Gherdovich wrote: > > The product mcnt * arch_max_freq_ratio could be zero if it overflows u64. > > For context, a large value for arch_max_freq_ratio would be 5000, > corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like > 1500-2000). A large increment frequency for the MPERF counter would be 5GHz > (the base clock of all CPUs on the market today is less than that). With > these figures, a CPU would need to go without a scheduler tick for around 8 > days for the u64 overflow to happen. It is unlikely, but the check is > warranted. > > In that case it's also appropriate to disable frequency invariant > accounting: the feature relies on measures of the clock frequency done at > every scheduler tick, which need to be "fresh" to be at all meaningful. > > Signed-off-by: Giovanni Gherdovich > Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance") Acked-by: Rafael J. Wysocki > --- > arch/x86/kernel/smpboot.c | 16 ++++++++++++++-- > 1 file changed, 14 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c > index 8c89e4d9ad28..4718f29a3065 100644 > --- a/arch/x86/kernel/smpboot.c > +++ b/arch/x86/kernel/smpboot.c > @@ -2039,6 +2039,14 @@ static void init_freq_invariance(bool secondary) > } > } > > +static void disable_freq_invariance_workfn(struct work_struct *work) > +{ > + static_branch_disable(&arch_scale_freq_key); > +} > + > +static DECLARE_WORK(disable_freq_invariance_work, > + disable_freq_invariance_workfn); > + > DEFINE_PER_CPU(unsigned long, arch_freq_scale) = SCHED_CAPACITY_SCALE; > > void arch_scale_freq_tick(void) > @@ -2055,14 +2063,18 @@ void arch_scale_freq_tick(void) > > acnt = aperf - this_cpu_read(arch_prev_aperf); > mcnt = mperf - this_cpu_read(arch_prev_mperf); > - if (!mcnt) > - return; > > this_cpu_write(arch_prev_aperf, aperf); > this_cpu_write(arch_prev_mperf, mperf); > > acnt <<= 2*SCHED_CAPACITY_SHIFT; > mcnt *= arch_max_freq_ratio; > + if (!mcnt) { > + pr_warn("Scheduler tick missing for long time, disabling scale-invariant accounting.\n"); > + /* static_branch_disable() acquires a lock and may sleep */ > + schedule_work(&disable_freq_invariance_work); > + return; > + } > > freq_scale = div64_u64(acnt, mcnt); > > -- > 2.16.4 >