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

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Date:	Wed, 14 Jan 2015 10:35:39 +0100
From:	Peter Zijlstra <peterz@...radead.org>
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>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 06/10] time: Cap clocksource reads to the clocksource
 max_cycles value

On Tue, Jan 13, 2015 at 01:33:29PM -0800, John Stultz wrote:
> On Tue, Jan 13, 2015 at 3:11 AM, Peter Zijlstra <peterz@...radead.org> 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 patch introduces such a cap that limits the read delta
> >> value to the max_cycles value, which is where an overflow would
> >> occur.
> >
> >> +++ b/kernel/time/timekeeping.c
> >> @@ -202,6 +202,9 @@ static inline s64 timekeeping_get_ns(struct tk_read_base *tkr)
> >>       /* calculate the delta since the last update_wall_time: */
> >>       delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask);
> >>
> >> +     /* Cap delta value to the max_cycles values to avoid mult overflows */
> >> +     delta = min(delta, tkr->clock->max_cycles);
> >> +
> >>       nsec = delta * tkr->mult + tkr->xtime_nsec;
> >>       nsec >>= tkr->shift;
> >>
> >
> > So while I appreciate stuff can be broken, should we not at least keep
> > track of this brokenness? That is, we all agree bad things happened IF
> > we actually hit this, right? So should we then not inform people that
> > bad things did happen?
> 
> So since this is a time reading function, this could be called
> anywhere. So I'm hesitant to try to printk anything in such a hot
> path. Though, if we catch such a large delta during the timekeeping
> update function, we will print a warning (which is done in an earlier
> patch in the series).
> 
> Were you thinking of something else maybe? I guess we could set a flag
> and then print later (if there is a later), but we'd lose much of the
> context of what went wrong.

Maybe a stats counter? In any case, keeping it silent seems the wrong
thing.
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