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 80607C636D3 for ; Wed, 1 Feb 2023 12:02:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231509AbjBAMCs (ORCPT ); Wed, 1 Feb 2023 07:02:48 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33154 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229597AbjBAMCq (ORCPT ); Wed, 1 Feb 2023 07:02:46 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 874E327D72 for ; Wed, 1 Feb 2023 04:02:45 -0800 (PST) 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 dfw.source.kernel.org (Postfix) with ESMTPS id 2148361776 for ; Wed, 1 Feb 2023 12:02:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EB852C433D2; Wed, 1 Feb 2023 12:02:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1675252964; bh=41Xyg3jeP8WJJmfHvsoU4JXbVCVGlTLKPezm4OBpJK4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=H6MtAi9X147T7iYnUcH9rmp5gsgYxpihzHT+65ttexPl2R3YLw8AKfGRk5NUyB+Dp 9oaSGdwtrvhJ9CVgjhB2RZbZMZyOHsOwr1DPBapqmQ/e68Efv+swiJrHBoPFQYhcjY d+dC/JagWSz0hqMpdCW06GAe/5s1fjdeDUy46XiNabZne3duroJRdeS66f7bL7bYOn UyiAhiJGCPhASPE3BHIm00hz/05cLBi2oZcertJ2g7hOxiV9+HUZzhtpBoH+RMS3Z0 Qj6r4PUVGusg+Z0drEiaxh45M1bHsNksGh7xh6zcbryZGXsQfhU3ZLjTZw4hG9XQUD ZnAw1Ng4jCX6g== Date: Wed, 1 Feb 2023 13:02:41 +0100 From: Frederic Weisbecker To: Hillf Danton Cc: Thomas Gleixner , Yu Liao , fweisbec@gmail.com, mingo@kernel.org, liwei391@huawei.com, adobriyan@gmail.com, mirsad.todorovac@alu.unizg.hr, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Peter Zijlstra Subject: Re: [PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us() Message-ID: References: <20230128020051.2328465-1-liaoyu15@huawei.com> <20230201045302.316-1-hdanton@sina.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230201045302.316-1-hdanton@sina.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 01, 2023 at 12:53:02PM +0800, Hillf Danton wrote: > On Tue, 31 Jan 2023 15:44:00 +0100 Thomas Gleixner > > > > Seriously this procfs accuracy is the least of the problems and if this > > would be the only issue then we could trivially fix it by declaring that > > the procfs output might go backwards. It's an estimate after all. If > > there would be a real reason to ensure monotonicity there then we could > > easily do that in the readout code. > > > > But the real issue is that both get_cpu_idle_time_us() and > > get_cpu_iowait_time_us() can invoke update_ts_time_stats() which is way > > worse than the above procfs idle time going backwards. > > > > If update_ts_time_stats() is invoked concurrently for the same CPU then > > ts->idle_sleeptime and ts->iowait_sleeptime are turning into random > > numbers. > > > > This has been broken 12 years ago in commit 595aac488b54 ("sched: > > Introduce a function to update the idle statistics"). > > [...] > > > > > P.S.: I hate the spinlock in the idle code path, but I don't have a > > better idea. > > Provided the percpu rule is enforced, the random numbers mentioned above > could be erased without another spinlock added. > > Hillf > +++ b/kernel/time/tick-sched.c > @@ -640,13 +640,26 @@ static void tick_nohz_update_jiffies(kti > /* > * Updates the per-CPU time idle statistics counters > */ > -static void > -update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, u64 *last_update_time) > +static u64 update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, > + int io, u64 *last_update_time) > { > ktime_t delta; > > + if (last_update_time) > + *last_update_time = ktime_to_us(now); > + > if (ts->idle_active) { > delta = ktime_sub(now, ts->idle_entrytime); > + > + /* update is only expected on the local CPU */ > + if (cpu != smp_processor_id()) { Why not just updating it only on idle exit then? > + if (io) I fear it's not up to the caller to decides if the idle time is IO or not. > + delta = ktime_add(ts->iowait_sleeptime, delta); > + else > + delta = ktime_add(ts->idle_sleeptime, delta); > + return ktime_to_us(delta); > + } > + > if (nr_iowait_cpu(cpu) > 0) > ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta); > else But you kept the old update above. So if this is not the local CPU, what do you do? You'd need to return (without updating iowait_sleeptime): ts->idle_sleeptime + ktime_sub(ktime_get(), ts->idle_entrytime) Right? But then you may race with the local updater, risking to return the delta added twice. So you need at least a seqcount. But in the end, nr_iowait_cpu() is broken because that counter can be decremented remotely and so the whole thing is beyond repair: CPU 0 CPU 1 CPU 2 ----- ----- ------ //io_schedule() TASK A current->in_iowait = 1 rq(0)->nr_iowait++ //switch to idle // READ /proc/stat // See nr_iowait_cpu(0) == 1 return ts->iowait_sleeptime + ktime_sub(ktime_get(), ts->idle_entrytime) //try_to_wake_up(TASK A) rq(0)->nr_iowait-- //idle exit // See nr_iowait_cpu(0) == 0 ts->idle_sleeptime += ktime_sub(ktime_get(), ts->idle_entrytime) Thanks.