linux-kernel - Re: [PATCH 19/21] clocksource: Improve comment explaining clocks_calc_max

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Date:	Thu, 2 Apr 2015 20:43:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH 19/21] clocksource: Improve comment explaining
 clocks_calc_max_nsecs()'s 50% safety margin

On Thu, Apr 02, 2015 at 11:41:15AM -0700, John Stultz wrote:
> On Thu, Apr 2, 2015 at 11:34 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Thu, Apr 02, 2015 at 10:30:18AM -0700, John Stultz wrote:
> >> > Should we make a further note that the tk_fast things rely on this
> >> > slack since they're not strongly serialized against this? That is, they
> >> > can end up using an older cycle_last value and therefore end up with a
> >> > larger delta than other code.
> >>
> >> Though, even with the tk_fast bits, we expect the update to happen
> >> regularly, its just that for the benefit of lock-free access we are ok
> >> with the possible slight inconsistencies (in the mono clock) that
> >> could happen if we use a slightly stale value mid-update. So I don't
> >> think the tk_fast bits are actually relying on the slack any more then
> >> the normal timekeeping code relies on the slack to handle slight
> >> delays in processing the updates.  If we deal with time deltas large
> >> enough to cause overflows, or time intervals larger then the hardware
> >> can represent, we're sunk in either case. This 50% margin just makes
> >> it easier to catch unexpected delays or issues.
> >
> > Right, so you're saying that even though the fast bits will see slightly
> > larger deltas than the normal code, they should still not get anywhere
> > near the 50% because we update much more frequently?
> 
> Well, they may get to 50% or slightly over (since 50% is the max nohz
> idle length), but that's likely rare, and we shouldn't get anywhere
> close to real failure edges (100% be it the mult-overflow or hardware
> mask limit).

Right, so the fast thing rely on there being slack. That was my point
rather. I know they'll not get to the end of slack.
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