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From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: paulmck@kernel.org, Feng Tang <feng.tang@intel.com>,
	kernel test robot <oliver.sang@intel.com>,
	0day robot <lkp@intel.com>, John Stultz <john.stultz@linaro.org>,
	Stephen Boyd <sboyd@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Marc Zyngier <maz@kernel.org>, Andi Kleen <ak@linux.intel.com>,
	Xing Zhengjun <zhengjun.xing@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	lkp@lists.01.org, kernel-team@fb.com, neeraju@codeaurora.org,
	zhengjun.xing@intel.com, x86@kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [clocksource]  8c30ace35d: WARNING:at_kernel/time/clocksource.c:#clocksource_watchdog
Date: Wed, 28 Apr 2021 19:00:15 +0200	[thread overview]
Message-ID: <871raumjj4.ffs@nanos.tec.linutronix.de> (raw)
In-Reply-To: <YImBlV8l7bjZ7Q6G@hirez.programming.kicks-ass.net>

On Wed, Apr 28 2021 at 17:39, Peter Zijlstra wrote:
> On Wed, Apr 28, 2021 at 03:34:52PM +0200, Thomas Gleixner wrote:
>> #4 is the easy case because we can check MSR_TSC_ADJUST to figure out
>>    whether something has written to MSR_TSC or MSR_TSC_ADJUST and undo
>>    the damage in a sane way.
>
> This is after the fact though; userspace (and kernel space) will have
> observed non-linear time and things will be broken in various subtle and
> hard to tell ways.

What I observed in the recent past is that _IF_ that happens it's a
small amount of cycles so it's not a given that this can be observed
accross CPUs. But yes, it's daft.

>> I can live with that and maybe we should have done that 15 years ago
>> instead of trying to work around it at the symptom level.
>
> Anybody that still has runtime BIOS wreckage will then silently suffer
> nonlinear time, doubly so for anybody not having TSC_ADJUST. Are we sure
> we can tell them all to bugger off and buy new hardware?
>
> At the very least we need something like tsc=broken, to explicitly mark
> TSC bad on machines, so that people that see TSC fail on their current
> kernels can continue to use the new kernels. This requires a whole lot
> of care on the part of users though, and will raise a ruckus, because I
> bet a fair number of these people are not even currently aware we're
> disabling TSC for them :/

I'm still allowed to dream, right? :)

Thanks,

        tglx

WARNING: multiple messages have this Message-ID (diff)
From: Thomas Gleixner <tglx@linutronix.de>
To: lkp@lists.01.org
Subject: Re: [clocksource] 8c30ace35d: WARNING:at_kernel/time/clocksource.c:#clocksource_watchdog
Date: Wed, 28 Apr 2021 19:00:15 +0200	[thread overview]
Message-ID: <871raumjj4.ffs@nanos.tec.linutronix.de> (raw)
In-Reply-To: <YImBlV8l7bjZ7Q6G@hirez.programming.kicks-ass.net>

[-- Attachment #1: Type: text/plain, Size: 1471 bytes --]

On Wed, Apr 28 2021 at 17:39, Peter Zijlstra wrote:
> On Wed, Apr 28, 2021 at 03:34:52PM +0200, Thomas Gleixner wrote:
>> #4 is the easy case because we can check MSR_TSC_ADJUST to figure out
>>    whether something has written to MSR_TSC or MSR_TSC_ADJUST and undo
>>    the damage in a sane way.
>
> This is after the fact though; userspace (and kernel space) will have
> observed non-linear time and things will be broken in various subtle and
> hard to tell ways.

What I observed in the recent past is that _IF_ that happens it's a
small amount of cycles so it's not a given that this can be observed
accross CPUs. But yes, it's daft.

>> I can live with that and maybe we should have done that 15 years ago
>> instead of trying to work around it at the symptom level.
>
> Anybody that still has runtime BIOS wreckage will then silently suffer
> nonlinear time, doubly so for anybody not having TSC_ADJUST. Are we sure
> we can tell them all to bugger off and buy new hardware?
>
> At the very least we need something like tsc=broken, to explicitly mark
> TSC bad on machines, so that people that see TSC fail on their current
> kernels can continue to use the new kernels. This requires a whole lot
> of care on the part of users though, and will raise a ruckus, because I
> bet a fair number of these people are not even currently aware we're
> disabling TSC for them :/

I'm still allowed to dream, right? :)

Thanks,

        tglx

  reply	other threads:[~2021-04-28 17:00 UTC|newest]

Thread overview: 72+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-25 22:45 [PATCH v10 clocksource 0/7] Do not mark clocks unstable due to delays for v5.13 Paul E. McKenney
2021-04-25 22:47 ` [PATCH v10 clocksource 1/7] clocksource: Provide module parameters to inject delays in watchdog Paul E. McKenney
2021-04-26  4:07   ` Andi Kleen
2021-04-26  7:13     ` Thomas Gleixner
2021-04-26 15:28     ` Paul E. McKenney
2021-04-26 16:00       ` Andi Kleen
2021-04-26 16:14         ` Paul E. McKenney
2021-04-26 17:56           ` Andi Kleen
2021-04-26 18:24             ` Paul E. McKenney
2021-04-28  4:49               ` Luming Yu
2021-04-28 13:57                 ` Paul E. McKenney
2021-04-28 14:24                   ` Luming Yu
2021-04-28 14:37                     ` Thomas Gleixner
2021-04-25 22:47 ` [PATCH v10 clocksource 2/7] clocksource: Retry clock read if long delays detected Paul E. McKenney
2021-04-27  1:44   ` Feng Tang
2021-04-25 22:47 ` [PATCH v10 clocksource 3/7] clocksource: Check per-CPU clock synchronization when marked unstable Paul E. McKenney
2021-04-26  4:12   ` Andi Kleen
2021-04-26  7:16     ` Thomas Gleixner
2021-04-25 22:47 ` [PATCH v10 clocksource 4/7] clocksource: Provide a module parameter to fuzz per-CPU clock checking Paul E. McKenney
2021-04-25 22:47 ` [PATCH v10 clocksource 5/7] clocksource: Limit number of CPUs checked for clock synchronization Paul E. McKenney
2021-04-25 22:47 ` [PATCH v10 clocksource 6/7] clocksource: Forgive tsc_early pre-calibration drift Paul E. McKenney
2021-04-26 15:01   ` Feng Tang
2021-04-26 15:25     ` Paul E. McKenney
2021-04-26 15:36       ` Feng Tang
2021-04-26 18:26         ` Paul E. McKenney
2021-04-27  1:13           ` Feng Tang
2021-04-27  3:46             ` Paul E. McKenney
2021-04-27  4:16               ` Feng Tang
2021-04-26 15:28     ` Thomas Gleixner
2021-04-27 21:03     ` Thomas Gleixner
2021-04-27  7:27   ` [clocksource] 8c30ace35d: WARNING:at_kernel/time/clocksource.c:#clocksource_watchdog kernel test robot
2021-04-27  7:27     ` kernel test robot
2021-04-27  8:45     ` Feng Tang
2021-04-27  8:45       ` Feng Tang
2021-04-27 13:37       ` Paul E. McKenney
2021-04-27 13:37         ` Paul E. McKenney
2021-04-27 17:50         ` Paul E. McKenney
2021-04-27 17:50           ` Paul E. McKenney
2021-04-27 21:09           ` Thomas Gleixner
2021-04-27 21:09             ` Thomas Gleixner
2021-04-28  1:48             ` Paul E. McKenney
2021-04-28  1:48               ` Paul E. McKenney
2021-04-28 10:14               ` Thomas Gleixner
2021-04-28 10:14                 ` Thomas Gleixner
2021-04-28 18:31                 ` Paul E. McKenney
2021-04-28 18:31                   ` Paul E. McKenney
2021-04-28 13:34             ` Thomas Gleixner
2021-04-28 13:34               ` Thomas Gleixner
2021-04-28 15:39               ` Peter Zijlstra
2021-04-28 15:39                 ` Peter Zijlstra
2021-04-28 17:00                 ` Thomas Gleixner [this message]
2021-04-28 17:00                   ` Thomas Gleixner
2021-04-29  7:38                   ` Feng Tang
2021-04-29  7:38                     ` Feng Tang
2021-04-28 18:31               ` Paul E. McKenney
2021-04-28 18:31                 ` Paul E. McKenney
2021-04-29  8:27                 ` Thomas Gleixner
2021-04-29  8:27                   ` Thomas Gleixner
2021-04-29 14:26                   ` Paul E. McKenney
2021-04-29 14:26                     ` Paul E. McKenney
2021-04-29 17:30                     ` Thomas Gleixner
2021-04-29 17:30                       ` Thomas Gleixner
2021-04-29 23:04                       ` Andi Kleen
2021-04-29 23:04                         ` Andi Kleen
2021-04-30  0:24                         ` Paul E. McKenney
2021-04-30  0:24                           ` Paul E. McKenney
2021-04-30  0:59                           ` Paul E. McKenney
2021-04-30  0:59                             ` Paul E. McKenney
2021-04-30  5:08                       ` Paul E. McKenney
2021-04-30  5:08                         ` Paul E. McKenney
2021-04-25 22:47 ` [PATCH v9 clocksource 6/6] clocksource: Reduce WATCHDOG_THRESHOLD Paul E. McKenney
2021-04-25 22:47 ` [PATCH v10 clocksource 7/7] " Paul E. McKenney

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