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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1ED67C7EE2C for ; Mon, 5 Jun 2023 15:47:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235211AbjFEPrr (ORCPT ); Mon, 5 Jun 2023 11:47:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48954 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235020AbjFEPrR (ORCPT ); Mon, 5 Jun 2023 11:47:17 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5DAEB10F0 for ; Mon, 5 Jun 2023 08:46:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1685979938; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Fz6CKIUO/zInL0kXiLp1OqxboZ4F1Hx1Dqes19h0t8M=; b=dmUQdWnPnstxl5trZ5sR06K82v2cDHRadXCt2r8v090jjmJ+TpsG9uwJj+C6bcQ2aaNkOm T6iu9q87gHVQR4DCaPWqvmg2uMw/Mn2xTPYgSpmeRT+KRk6Dd9Do2vDYkb6PkoY3mWqYMI LjjY+AE2j29Cps7sh04T8nhwdNiYBkg= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-297-Exj74wdqPj6lwC7-q16Ohg-1; Mon, 05 Jun 2023 11:45:35 -0400 X-MC-Unique: Exj74wdqPj6lwC7-q16Ohg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 64A0E101A53B; Mon, 5 Jun 2023 15:45:34 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-3.gru2.redhat.com [10.97.112.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2B27AC154D1; Mon, 5 Jun 2023 15:45:34 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 884C140178ACF; Mon, 5 Jun 2023 12:43:24 -0300 (-03) Date: Mon, 5 Jun 2023 12:43:24 -0300 From: Marcelo Tosatti To: Michal Hocko Cc: Christoph Lameter , Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Vlastimil Babka Subject: Re: [PATCH v2 3/3] mm/vmstat: do not refresh stats for nohz_full CPUs Message-ID: References: <20230602185757.110910188@redhat.com> <20230602190115.545766386@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 05, 2023 at 09:59:57AM +0200, Michal Hocko wrote: > On Fri 02-06-23 15:58:00, Marcelo Tosatti wrote: > > The interruption caused by queueing work on nohz_full CPUs > > is undesirable for certain aplications. > > This is not a proper changelog. I am not going to write a changelog for > you this time. Please explain why this is really needed and why this > approach is desired. > E.g. why don't you prevent userspace from > refreshing stats if interference is not desirable. Michal, Can you please check if the following looks better, as a changelog? thanks --- schedule_work_on API uses the workqueue mechanism to queue a work item on a queue. A kernel thread, which runs on the target CPU, executes those work items. Therefore, when using the schedule_work_on API, it is necessary for the kworker kernel thread to be scheduled in, for the work function to be executed. Time sensitive applications such as SoftPLCs (https://tum-esi.github.io/publications-list/PDF/2022-ETFA-How_Real_Time_Are_Virtual_PLCs.pdf), have their response times affected by such interruptions. The /proc/sys/vm/stat_refresh file was originally introduced by commit 52b6f46bc163eef17ecba4cd552beeafe2b24453 Author: Hugh Dickins Date: Thu May 19 17:12:50 2016 -0700 mm: /proc/sys/vm/stat_refresh to force vmstat update Provide /proc/sys/vm/stat_refresh to force an immediate update of per-cpu into global vmstats: useful to avoid a sleep(2) or whatever before checking counts when testing. Originally added to work around a bug which left counts stranded indefinitely on a cpu going idle (an inaccuracy magnified when small below-batch numbers represent "huge" amounts of memory), but I believe that bug is now fixed: nonetheless, this is still a useful knob. Other than the potential interruption to a time sensitive application, if using SCHED_FIFO or SCHED_RR priority on the isolated CPU, then system hangs can occur: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=978688 To avoid the problems above, do not schedule the work to synchronize per-CPU mm counters on isolated CPUs. Given the possibility for breaking existing userspace applications, avoid changing behaviour of access to /proc/sys/vm/stat_refresh, such as returning errors to userspace. --- > Also would it make some sense to reduce flushing to cpumask > of the calling process? (certainly a daring thought but have > you even considered it?) Fail to see the point here ?