From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 09B9C309EEC for ; Thu, 2 Apr 2026 17:07:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775149674; cv=none; b=QnKe++lU9iGsEg7IvFVDet/OoTqM+p0WdPX+gXh9ZhUz5hP3oql6YZhAZQRguOeFesu8MjvdX5vhCLc/o6+kV0cqm7bLFawCOCnyv35Ke6kk+l5aoiwowNHexgNUM0uZqmpGOTXR8iHBY/haekwdWp+cqKwv6FIYQ9T7gA/uqCs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775149674; c=relaxed/simple; bh=2PqYdwTb1Efa7xhAQJaPEI5V9jOqJhJwkqip4PT/HbQ=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=Cr2cVdjg4d1OfHfFYnVrYd9w9FHQN2+8C2rbdBtwfXpwj2x1qiqSaHtkDEnp0NEsq9yCB6Jun5svz/sy+Wi81YOtV0Vdnr3yr2GEFC/UrL2pKZdbKtQeeC0mQOYq2UJe3W7h7YG8s7qCxgheQURjYR1xXVe+wTFXKW2wzwIM2UQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Ffbfc6SC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Ffbfc6SC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F3DDFC2BCAF; Thu, 2 Apr 2026 17:07:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775149672; bh=2PqYdwTb1Efa7xhAQJaPEI5V9jOqJhJwkqip4PT/HbQ=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=Ffbfc6SCPRrfAU4T2DXgOq3GKOy04a8acp5Xy9dOkCavx4fJ/tfWQU+Jxws6nViaZ cvMbxbRTO6lMMTXniREuC/IfDUmB/lFTXGpGLNbDr79+GgkNwxah2g2uJrUoILHSUJ Q493tzhjGI3eXJPLRMjG0YuOxGJggJ2Ol69qI7Hiw7yRB5mhPyqnn6xWlzvv+W7R9q 6aaFJ0zGxNbh9bYT3KdTVTAH4Q4eqohZTGjm/yx6cPxIoafkNf+oT+591UX9+ucpxO PDOINFyXjU1Bo/WPShs9/W6783G4KmKcfie8nEz9NwUezuzEagHO27UQVYOA9JpTzZ 0N90z45B9LR+A== From: Thomas Gleixner To: Borislav Petkov , Calvin Owens Cc: Petr Mladek , linux-kernel@vger.kernel.org, arighi@nvidia.com, yaozhenguo1@gmail.com, tj@kernel.org, feng.tang@linux.alibaba.com, lirongqing@baidu.com, realwujing@gmail.com, hu.shengming@zte.com.cn, dianders@chromium.org, joel.granados@kernel.org, Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Frederic Weisbecker , Anna-Maria Behnsen , x86@kernel.org Subject: [PATCH] clockevents: Prevent timer interrupt starvation In-Reply-To: <20260401163435.GGac1JG42tWmsCKL37@fat_crate.local> References: <87v7ejetl1.ffs@tglx> <875x6a913n.ffs@tglx> <20260401163435.GGac1JG42tWmsCKL37@fat_crate.local> Date: Thu, 02 Apr 2026 19:07:49 +0200 Message-ID: <87jyup70ka.ffs@tglx> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Calvin reported an odd NMI watchdog lockup which claims that the CPU locked up in user space. He provided a reproducer, which set's up a timerfd based timer and then rearms it in a loop with an absolute expiry time of 1ns. As the expiry time is in the past, the timer ends up as the first expiring timer in the per CPU hrtimer base and the clockevent device is programmed with the minimum delta value. If the machine is fast enough, this ends up in a endless loop of programming the delta value to the minimum value defined by the clock event device, before the timer interrupt can fire, which starves the interrupt and consequently triggers the lockup detector because the hrtimer callback of the lockup mechanism is never invoked. As a first step to prevent this, avoid reprogramming the clock event device when: - a forced minimum delta event is pending - the new expiry delta is less then or equal to the minimum delta Thanks to Calvin for providing the reproducer and to Borislav for testing and providing data from his Zen5 machine. The problem is not limited to Zen5, but depending on the underlying clock event device (e.g. TSC deadline timer on Intel) and the CPU speed not necessarily observable. This change serves only as the last resort and further changes will be made to prevent this scenario earlier in the call chain. Reported-by: Calvin Owens Signed-off-by: Thomas Gleixner --- P.S: I'm working on the other changes, but wanted to get this out ASAP for testing. --- include/linux/clockchips.h | 2 ++ kernel/time/clockevents.c | 37 +++++++++++++++++++++++-------------- 2 files changed, 25 insertions(+), 14 deletions(-) --- a/include/linux/clockchips.h +++ b/include/linux/clockchips.h @@ -80,6 +80,7 @@ enum clock_event_state { * @shift: nanoseconds to cycles divisor (power of two) * @state_use_accessors:current state of the device, assigned by the core code * @features: features + * @next_event_forced: True if the last programming was a forced event * @retries: number of forced programming retries * @set_state_periodic: switch state to periodic * @set_state_oneshot: switch state to oneshot @@ -108,6 +109,7 @@ struct clock_event_device { u32 shift; enum clock_event_state state_use_accessors; unsigned int features; + unsigned int next_event_forced; unsigned long retries; int (*set_state_periodic)(struct clock_event_device *); --- a/kernel/time/clockevents.c +++ b/kernel/time/clockevents.c @@ -172,6 +172,7 @@ void clockevents_shutdown(struct clock_e { clockevents_switch_state(dev, CLOCK_EVT_STATE_SHUTDOWN); dev->next_event = KTIME_MAX; + dev->next_event_forced = 0; } /** @@ -224,13 +225,7 @@ static int clockevents_increase_min_delt return 0; } -/** - * clockevents_program_min_delta - Set clock event device to the minimum delay. - * @dev: device to program - * - * Returns 0 on success, -ETIME when the retry loop failed. - */ -static int clockevents_program_min_delta(struct clock_event_device *dev) +static int __clockevents_program_min_delta(struct clock_event_device *dev) { unsigned long long clc; int64_t delta; @@ -263,13 +258,7 @@ static int clockevents_program_min_delta #else /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */ -/** - * clockevents_program_min_delta - Set clock event device to the minimum delay. - * @dev: device to program - * - * Returns 0 on success, -ETIME when the retry loop failed. - */ -static int clockevents_program_min_delta(struct clock_event_device *dev) +static int __clockevents_program_min_delta(struct clock_event_device *dev) { unsigned long long clc; int64_t delta = 0; @@ -293,6 +282,21 @@ static int clockevents_program_min_delta #endif /* CONFIG_GENERIC_CLOCKEVENTS_MIN_ADJUST */ /** + * clockevents_program_min_delta - Set clock event device to the minimum delay. + * @dev: device to program + * + * Returns 0 on success, -ETIME when the retry loop failed. + */ +static int clockevents_program_min_delta(struct clock_event_device *dev) +{ + if (dev->next_event_forced) + return 0; + + dev->next_event_forced = 1; + return __clockevents_program_min_delta(dev); +} + +/** * clockevents_program_event - Reprogram the clock event device. * @dev: device to program * @expires: absolute expiry time (monotonic clock) @@ -324,6 +328,11 @@ int clockevents_program_event(struct clo return dev->set_next_ktime(expires, dev); delta = ktime_to_ns(ktime_sub(expires, ktime_get())); + + /* Don't reprogram when a forced event is pending */ + if (dev->next_event_forced && delta <= (int64_t)dev->min_delta_ns) + return 0; + if (delta <= 0) return force ? clockevents_program_min_delta(dev) : -ETIME;