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 4E04AECAAD8 for ; Wed, 14 Sep 2022 11:04:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229766AbiINLEh (ORCPT ); Wed, 14 Sep 2022 07:04:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32806 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229502AbiINLEe (ORCPT ); Wed, 14 Sep 2022 07:04:34 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5B82FFD7 for ; Wed, 14 Sep 2022 04:04:32 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 0B27DB81A3E for ; Wed, 14 Sep 2022 11:04:31 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 244FBC433D6; Wed, 14 Sep 2022 11:04:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1663153469; bh=tbx0ApNcuaaHnoKpUSFbEFs9vNYZFNnjuUmTNGNSz84=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=N6tuKtMG5Wmi2TiNPlqXNBlsCCM7q7I4weRyHU3u7SOpc+dpRiZWg/K4KmkjzcpRg BCMAtkSNyStcTOmwtbpvDianxv2Te0aJjOemrRB3hCdd5piRmtOFvbr39TdCoxmphd ytgetmLNAQ80hgq5GRNAjPy8rNeidiEdPsXEZVUeTR9PChFvfVrSyoU6rek9ZHflYe JRQElf5dlxjXAHMTwF23DENQRw0hZTJTLAFn0Fxl1f3Cz8vFLYj0QnvlCVo4yhhLOF U1hR0bUvrYkCgWQVZRJS7LMX7oNnAlwEacKl0afM8jxz4mvhIpomZoTNh+0c1ua5w4 7PZKF9/UM9V8g== Date: Wed, 14 Sep 2022 13:04:26 +0200 From: Frederic Weisbecker To: Aaron Tomlin Cc: Marcelo Tosatti , cl@linux.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v7 2/3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too Message-ID: <20220914110426.GD1936@lothringen> References: <20220817191346.287594886@redhat.com> <20220817191524.201253713@redhat.com> <20220909121224.GA220905@lothringen> <20220912143822.irn6xhs2etmumqlt@ava.usersys.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220912143822.irn6xhs2etmumqlt@ava.usersys.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 12, 2022 at 03:38:23PM +0100, Aaron Tomlin wrote: > On Fri 2022-09-09 16:35 -0300, Marcelo Tosatti wrote: > Hi Frederic, > > Sorry about that. How about the following: > > - Note: CPU X is part of 'tick_nohz_full_mask' > > 1. CPU Y migrated running task A to CPU X that > was in an idle state i.e. waiting for an IRQ; > marked the current task on CPU X to need/or > require a reschedule i.e., set TIF_NEED_RESCHED > and invoked a reschedule IPI to CPU X > (see sched_move_task()) > > 2. CPU X acknowledged the reschedule IPI. Generic > idle loop code noticed the TIF_NEED_RESCHED flag > against the idle task and attempts to exit of the > loop and calls the main scheduler function i.e. > __schedule(). > > Since the idle tick was previously stopped no > scheduling-clock tick would occur. > So, no deferred timers would be handled > > 3. Post transition to kernel execution Task A > running on CPU X, indirectly released a few pages > (e.g. see __free_one_page()); CPU X's > 'vm_stat_diff[NR_FREE_PAGES]' was updated and zone > specific 'vm_stat[]' update was deferred as per the > CPU-specific stat threshold > > 4. Task A does invoke exit(2) and the kernel does > remove the task from the run-queue; the idle task > was selected to execute next since there are no > other runnable tasks assigned to the given CPU > (see pick_next_task() and pick_next_task_idle()) > > 5. On return to the idle loop since the idle tick > was already stopped and can remain so (see [1] > below) e.g. no pending soft IRQs, no attempt is > made to zero and fold CPU X's vmstat counters > since reprogramming of the scheduling-clock tick > is not required/or needed (see [2]) Much better thanks. Please cut the patch in two patches: one that fixes the stuff in the idle path and another one that fixes the return to user path. The first one is definetly a fix, the second one is rather a feature that is definetly wanted as well but I need to think it through further. > > > > Kind regards, > > -- > Aaron Tomlin >