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 2AB7DC433F5 for ; Thu, 24 Feb 2022 12:27:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234353AbiBXM2X (ORCPT ); Thu, 24 Feb 2022 07:28:23 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39100 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234303AbiBXM2V (ORCPT ); Thu, 24 Feb 2022 07:28:21 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id CDA721728A0 for ; Thu, 24 Feb 2022 04:27:50 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1645705670; 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=oAPSavxMw1gj5XJMFm/UX3hZtPsS8GU5YjgnhG2ojNg=; b=K+2DHu/Gz8BjlIoK0l4leBNaCgY1PN+GqgFVN25iyovtGbK532YthDFQF7fW4vqnKwvrpU iwWikUFyZHz+AmkLQyJ5qiDS6Hh+EvYA78+AiY7D31Mj4heJA/kyxzVezeAg4idDKafCSw VJiYvHeKTx/0UPAXit3j4sWL3JdbjRY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-629-M6vh_8m6M5mAyCCpJ2S4KQ-1; Thu, 24 Feb 2022 07:27:48 -0500 X-MC-Unique: M6vh_8m6M5mAyCCpJ2S4KQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id ED8AC1854E21; Thu, 24 Feb 2022 12:27:46 +0000 (UTC) Received: from fuller.cnet (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0CD8483287; Thu, 24 Feb 2022 12:27:39 +0000 (UTC) Received: by fuller.cnet (Postfix, from userid 1000) id 0F204416D862; Thu, 24 Feb 2022 09:27:14 -0300 (-03) Date: Thu, 24 Feb 2022 09:27:14 -0300 From: Marcelo Tosatti To: Aaron Tomlin Cc: Frederic Weisbecker , Christoph Lameter , tglx@linutronix.de, mingo@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Phil Auld Subject: Re: [RFC PATCH] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too Message-ID: References: <20220203214339.1889971-1-atomlin@redhat.com> <20220217124729.GA743618@lothringen> <20220217142615.xqtiydixvnumyvei@ava.usersys.com> <20220217163205.GA748087@lothringen> <20220218125454.utlgmuhijklzr3if@ava.usersys.com> <20220219154616.pwsvh445x3vn7ltf@ava.usersys.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220219154616.pwsvh445x3vn7ltf@ava.usersys.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Feb 19, 2022 at 03:46:16PM +0000, Aaron Tomlin wrote: > On Fri 2022-02-18 12:54 +0000, Aaron Tomlin wrote: > > On Thu 2022-02-17 17:32 +0100, Frederic Weisbecker wrote: > > > > If I understand correctly, in the context of nohz_full, since such work is > > > > deferred, it will only be handled in a scenario when the periodic/or > > > > scheduling-clock tick is enabled i.e. the timer was reprogrammed on exit > > > > from idle. > > > > > > Oh I see, it's a deferrable delayed work... > > > Then I can see two other issues: > > > > > > 1) Can an interrupt in idle modify the vmstat and thus trigger the need to > > > flush it? Yes. Page allocation and page freeing for example. 6 3730 ../mm/page_alloc.c <> __mod_zone_freepage_state(zone, -(1 << order), 4 1096 ../mm/page_alloc.c <<__free_one_page>> __mod_zone_freepage_state(zone, -(1 << order), > > > I believe it's the case and then the problem goes beyond nohz_full > > > because if the idle interrupt fired while the tick is stopped and didn't set > > > TIF_RESCHED, we go back to sleep without calling quiet_vmstat(). > > > > Yes: e.g. a nohz_full CPU, in idle code, could indeed receive a reschedule > > IPI; re-enable local IRQs and generic idle code sees the TIF_NEED_RESCHED > > flag against the idle task. Additionally, the selected task could > > indirectly released a few pages [to satisfy a low-memory condition] and > > modify CPU-specific vmstat data i.e. vm_stat_diff[NR_FREE_PAGES]. > > > > > > > 2) What if we are running task A in kernel mode while the tick is stopped > > > (nohz_full). Task A modifies the vmstat and goes to userspace for a long > > > while. > > > Your patch fixes case 1) but not case 2). The problem is that TIMER_DEFERRABLE > > > should really be about dynticks-idle only and not dynticks-full. I've always > > > been afraid about enforcing that rule though because that would break old > > > noise-free setups. But perhaps I should... > > > > If I understand correctly, I agree. For the latter case, nothing can be > > done unfortunately since the scheduling-clock tick is stopped. > > Hi Frederic, > > As far vmstat_updateas I understand, in the context of nohz_full, options are indeed > limited; albeit, if we can ensure CPU-specific vmstat data is folded on > return to idle [when required] then this should be good enough. I suppose the desired behaviour, with the deferred timer for vmstat_sync, is: "Allow the per-CPU vmstats to be out of sync, but for a maximum of sysctl_stat_interval". But Aaron, vmstat_shepherd should be ensuring that per-CPU vmstat_update work are queued, if the per-CPU vmstat are out of sync. And: static void trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer) { if (!is_timers_nohz_active()) return; /* * TODO: This wants some optimizing similar to the code below, but we * will do that when we switch from push to pull for deferrable timers. */ if (timer->flags & TIMER_DEFERRABLE) { if (tick_nohz_full_cpu(base->cpu)) wake_up_nohz_cpu(base->cpu); return; } * @TIMER_DEFERRABLE: A deferrable timer will work normally when the * system is busy, but will not cause a CPU to come out of idle just * to service it; instead, the timer will be serviced when the CPU * eventually wakes up with a subsequent non-deferrable timer. You'd want that vmstat_update to execute regardless of whether there are armed non-deferrable timers. Should fix both 1 and 2 AFAICS.