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From: John Stultz <john.stultz@linaro.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	David Gibson <david@gibson.dropbear.id.au>,
	Liav Rehana <liavr@mellanox.com>,
	Chris Metcalf <cmetcalf@mellanox.com>,
	Richard Cochran <richardcochran@gmail.com>,
	Parit Bhargava <prarit@redhat.com>,
	Laurent Vivier <lvivier@redhat.com>,
	"Christopher S. Hall" <christopher.s.hall@intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math
Date: Thu, 8 Dec 2016 20:39:50 -0800	[thread overview]
Message-ID: <CALAqxLVxs-exUWFtTXV2aPEUU2=xZZ9_jfwSOgPmMKPYHmZYJw@mail.gmail.com> (raw)
In-Reply-To: <20161209042905.GA11001@gmail.com>

On Thu, Dec 8, 2016 at 8:29 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Ingo Molnar <mingo@kernel.org> wrote:
>
>>
>> * Thomas Gleixner <tglx@linutronix.de> wrote:
>>
>> > If the timekeeping CPU is scheduled out long enough by a hypervisor the
>> > clocksource delta multiplication can overflow and as a result time can go
>> > backwards. That's insane to begin with, but people already triggered a
>> > signed multiplication overflow, so a unsigned overflow is not necessarily
>> > impossible.
>> >
>> > Implement optional 128bit math which can be selected by a config option.
>>
>> What's the rough VM interruption time that would trigger an overflow? Given that
>> the clock shift tk_read_base::mult is often 1, isn't it 32-bit nsecs, i.e. 4
>> seconds?
>>
>> That doesn't sound 'insanely long'.
>>
>> Or some other value?
>
> Ok, wasn't fully awake yet: more realistic values of the scaling factor on x86
> would allow cycles input values of up to ~70 billion with 64-bit math, which would
> allow deltas of up to about 1 minute with 64-bit math.

So if I'm remembering properly, we pick mult/shift pairs such that the
mult shouldn't overflow from ~10 minutes worth of cycles.

> I think we should at least detect (and report?) the overflow and sanitize the
> effects to the max offset instead of generating random overflown values.

So with CONFIG_DEBUG_TIMEKEEPING, we do check to see if the cycle
value is larger then the max_cycles and will report a warning. But
this is done at interrupt time and not in the hotpath.

thanks
-john

  reply	other threads:[~2016-12-09  4:39 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-08 20:49 [patch 0/6] timekeeping: Cure the signed/unsigned wreckage Thomas Gleixner
2016-12-08 20:49 ` [patch 1/6] timekeeping: Force unsigned clocksource to nanoseconds conversion Thomas Gleixner
2016-12-08 23:38   ` David Gibson
2016-12-09 11:13   ` [tip:timers/core] timekeeping_Force_unsigned_clocksource_to_nanoseconds_conversion tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 2/6] timekeeping: Make the conversion call chain consistently unsigned Thomas Gleixner
2016-12-08 23:39   ` David Gibson
2016-12-09 11:13   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 3/6] timekeeping: Get rid of pointless typecasts Thomas Gleixner
2016-12-08 23:40   ` David Gibson
2016-12-09 11:14   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 4/6] timekeeping: Use mul_u64_u32_shr() instead of open coding it Thomas Gleixner
2016-12-08 23:41   ` David Gibson
2016-12-09 11:14   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 5/6] [RFD] timekeeping: Provide optional 128bit math Thomas Gleixner
2016-12-09  4:08   ` Ingo Molnar
2016-12-09  4:29     ` Ingo Molnar
2016-12-09  4:39       ` John Stultz [this message]
2016-12-09  4:48     ` Peter Zijlstra
2016-12-09  5:22       ` Ingo Molnar
2016-12-09  5:41         ` Peter Zijlstra
2016-12-09  5:11   ` Peter Zijlstra
2016-12-09  6:08     ` Peter Zijlstra
2016-12-09  5:26   ` Peter Zijlstra
2016-12-09  6:38     ` Peter Zijlstra
2016-12-09  8:30       ` Peter Zijlstra
2016-12-09  9:11         ` Peter Zijlstra
2016-12-09 10:01         ` Peter Zijlstra
2016-12-09 17:32         ` Chris Metcalf
2017-01-14 12:51         ` [tip:timers/core] math64, timers: Fix 32bit mul_u64_u32_shr() and friends tip-bot for Peter Zijlstra
2016-12-09 10:18       ` [patch 5/6] [RFD] timekeeping: Provide optional 128bit math Peter Zijlstra
2016-12-09 17:20         ` Chris Metcalf
2016-12-08 20:49 ` [patch 6/6] [RFD] timekeeping: Get rid of cycle_t Thomas Gleixner
2016-12-08 23:43   ` David Gibson
2016-12-09  4:52 ` [patch 0/6] timekeeping: Cure the signed/unsigned wreckage John Stultz
2016-12-09  5:30 ` Peter Zijlstra

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