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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A51D0C433EF for ; Tue, 26 Oct 2021 14:11:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 887926103C for ; Tue, 26 Oct 2021 14:11:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236429AbhJZON2 (ORCPT ); Tue, 26 Oct 2021 10:13:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:40594 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236419AbhJZON0 (ORCPT ); Tue, 26 Oct 2021 10:13:26 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 498436108D; Tue, 26 Oct 2021 14:11:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1635257462; bh=SJ9PUp+cdVFoBYAia41yQGLaN3QtG7WGf7eTk7JzeJM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=c3iEmlSfxhtiCWm5qSLhY/maSOjp9JPtt3PVHbujJKPqhC9083xzaLZGzC/679CCF yweywoBovGeEMd98nocxuHl12jSUkjiaHZQ0C+pEWN1JpYOEv65UpkF6bGu3Gx+7jM rXB40yc9Yk7XFCYlIixbTQPjE9czfIY/TeTB8jZvUfrsXoqIOv3QNGr2dsAPsW76LL Deum4v+Q6JI7BYs7NvKSaAw5R2LsMRhy9bUNIUgMmSKGJzSFTp1xOWko6gJm4UKsSB LMSQ2fgdIWOtzCuiJwNM1UGPsS+IrjC9iSNaxUBKGC9mAEXrJ9NzMXimkYil/Prld8 B8PHXwijCxTKA== From: Frederic Weisbecker To: Thomas Gleixner Cc: LKML , Frederic Weisbecker , "Paul E . McKenney" , Peter Zijlstra , Hasegawa Hitomi Subject: [PATCH 1/2 RESEND] timers/nohz: Last resort update jiffies on nohz_full IRQ entry Date: Tue, 26 Oct 2021 16:10:54 +0200 Message-Id: <20211026141055.57358-2-frederic@kernel.org> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20211026141055.57358-1-frederic@kernel.org> References: <20211026141055.57358-1-frederic@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When at least one CPU runs in nohz_full mode, a dedicated timekeeper CPU is guaranteed to stay online and to never stop its tick. Meanwhile on some rare case, the dedicated timekeeper may be running with interrupts disabled for a while, such as in stop_machine. If jiffies stop being updated, a nohz_full CPU may end up endlessly programming the next tick in the past, taking the last jiffies update monotonic timestamp as a stale base, resulting in an tick storm. Here is a scenario where it matters: 0) CPU 0 is the timekeeper and CPU 1 a nohz_full CPU. 1) A stop machine callback is queued to execute somewhere. 2) CPU 0 reaches MULTI_STOP_DISABLE_IRQ while CPU 1 is still in MULTI_STOP_PREPARE. Hence CPU 0 can't do its timekeeping duty. CPU 1 can still take IRQs. 3) CPU 1 receives an IRQ which queues a timer callback one jiffy forward. 4) On IRQ exit, CPU 1 schedules the tick one jiffy forward, taking last_jiffies_update as a base. But last_jiffies_update hasn't been updated for 2 jiffies since the timekeeper has interrupts disabled. 5) clockevents_program_event(), which relies on ktime_get(), observes that the expiration is in the past and therefore programs the min delta event on the clock. 6) The tick fires immediately, goto 3) 7) Tick storm, the nohz_full CPU is drown and takes ages to reach MULTI_STOP_DISABLE_IRQ, which is the only way out of this situation. Solve this with unconditionally updating jiffies if the value is stale on nohz_full IRQ entry. IRQs and other disturbances are expected to be rare enough on nohz_full for the unconditional call to ktime_get() to actually matter. Reported-and-tested-by: Paul E. McKenney Cc: Peter Zijlstra Signed-off-by: Frederic Weisbecker --- kernel/softirq.c | 3 ++- kernel/time/tick-sched.c | 7 +++++++ 2 files changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/softirq.c b/kernel/softirq.c index 322b65d45676..41f470929e99 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -595,7 +595,8 @@ void irq_enter_rcu(void) { __irq_enter_raw(); - if (is_idle_task(current) && (irq_count() == HARDIRQ_OFFSET)) + if (tick_nohz_full_cpu(smp_processor_id()) || + (is_idle_task(current) && (irq_count() == HARDIRQ_OFFSET))) tick_irq_enter(); account_hardirq_enter(current); diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 6bffe5af8cb1..17a283ce2b20 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -1375,6 +1375,13 @@ static inline void tick_nohz_irq_enter(void) now = ktime_get(); if (ts->idle_active) tick_nohz_stop_idle(ts, now); + /* + * If all CPUs are idle. We may need to update a stale jiffies value. + * Note nohz_full is a special case: a timekeeper is guaranteed to stay + * alive but it might be busy looping with interrupts disabled in some + * rare case (typically stop machine). So we must make sure we have a + * last resort. + */ if (ts->tick_stopped) tick_nohz_update_jiffies(now); } -- 2.25.1