linux-kernel - Re: [PATCH 06/10] time: Cap clocksource reads to the clocksource max

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Date:	Mon, 12 Jan 2015 21:30:07 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Prarit Bhargava <prarit@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 06/10] time: Cap clocksource reads to the clocksource
 max_cycles value

On Mon, Jan 12, 2015 at 10:54:50AM -0800, John Stultz wrote:
> On Sun, Jan 11, 2015 at 4:41 AM, Richard Cochran
> <richardcochran@...il.com> wrote:
> > On Fri, Jan 09, 2015 at 04:34:24PM -0800, John Stultz wrote:
> >> When calculating the current delta since the last tick, we
> >> currently have no hard protections to prevent a multiplciation
> >> overflow from ocurring.
> >
> > This is just papering over the problem. The "hard protection" should
> > be having a tick scheduled before the range of the clock source is
> > exhausted.
> 
> So I disagree this is papering over the problem.
> 
> You say the tick should be scheduled before the clocksource wraps -
> but we have logic to do that.

Well that is a shame.  To my way of thinking, having a reliable
watchdog (clock readout) at half the period would be a real solution.
Yes, I do mean providing some sort of "soft real time" guarantee.

What is the use case here?  I thought we are trying to fix unreliable
clocks with random jumps.  It is hard to see how substituting
MAX_DURATION for RANDOM_JUMP_VALUE is helping to catch bad hardware.
 
> However there are many ways that can still go wrong.  Virtualization
> can delay interrupts for long periods of time,

fixable with some soft RT?

> the timer/irq code isn't the simplest and there can be bugs,

simplify and fix?

> or timer hardware itself can have issues.

for this we can have a compile time timer validation module, just like
we have for locks, mutexs, rcu, etc.

> The difficulty is that when something has gone wrong, the
> only thing we have to measure the problem may become corrupted.  And
> worse, once the timekeeping code is having problems,  that can result
> in bugs that manifest in all sorts of strange ways that are very
> difficult to debug (you can't trust your log timestamps, etc).

But this this patch make the timestamps trustworthy?  Not really.
 
Thanks,
Richard
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