Re: [PATCH] tick/common: Align tick period during sched_timer setup.

From: Mathias Krause <minipli@grsecurity.net>
To: Greg KH <gregkh@linuxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Luiz Capitulino <luizcap@amazon.com>,
	Sven-Haegar Koch <haegar@sdinet.de>,
	"Bhatnagar, Rishabh" <risbhat@amazon.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"sashal@kernel.org" <sashal@kernel.org>,
	abuehaze@amazon.com,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] tick/common: Align tick period during sched_timer setup.
Date: Mon, 19 Jun 2023 09:03:14 +0200	[thread overview]
Message-ID: <2d6def18-58d4-6a24-22ab-9fd065809e7f@grsecurity.net> (raw)
In-Reply-To: <2023061944-uprising-applaud-990a@gregkh>

On 19.06.23 08:18, Greg KH wrote:
> On Thu, Jun 15, 2023 at 11:18:30AM +0200, Sebastian Andrzej Siewior wrote:
>> From: Thomas Gleixner <tglx@linutronix.de>
>>
>> The tick period is aligned very early while the first clock_event_device
>> is registered. The system runs in periodic mode and switches later to
>> one-shot mode if possible.
>>
>> The next wake-up event is programmed based on aligned value
>> (tick_next_period) but the delta value, that is used to program the
>> clock_event_device, is computed based on ktime_get().
>>
>> With the subtracted offset, the devices fires in less than the exacted
>> time frame. With a large enough offset the system programs the timer for
>> the next wake-up and the remaining time left is too little to make any
>> boot progress. The system hangs.
>>
>> Move the alignment later to the setup of tick_sched timer. At this point
>> the system switches to oneshot mode and a highres clocksource is
>> available. It safe to update tick_next_period ktime_get() will now
>> return accurate (not jiffies based) time.
>>
>> [bigeasy: Patch description + testing].
>>
>> Reported-by: Mathias Krause <minipli@grsecurity.net>
>> Reported-by: "Bhatnagar, Rishabh" <risbhat@amazon.com>
>> Fixes: e9523a0d81899 ("tick/common: Align tick period with the HZ tick.")
>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> Link: https://lore.kernel.org/5a56290d-806e-b9a5-f37c-f21958b5a8c0@grsecurity.net
>> Link: https://lore.kernel.org/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@amazon.com
>> ---
>>  kernel/time/tick-common.c | 11 +----------
>>  kernel/time/tick-sched.c  | 13 ++++++++++++-
>>  2 files changed, 13 insertions(+), 11 deletions(-)
> 
> What's the status of this fix, I didn't see it in -rc7, am I looking in
> the wrong place?

It's in the tip tree since Friday, but yeah, no pull request yet:

https://git.kernel.org/tip/13bb06f8dd42

Thanks,
Mathias