Messages in this thread |  | | | Date | Mon, 13 Feb 2006 15:44:03 +0100 | | From | Ingo Molnar <> | | Subject | Re: [PATCH 01/13] hrtimer: round up relative start time |
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* Roman Zippel <zippel@linux-m68k.org> wrote:
> Hi, > > On Mon, 13 Feb 2006, Ingo Molnar wrote: > > > but there is no 'old behavior' to restore to. The +1 to itimer intervals > > caused artifacts that were hitting users and caused 2.4 -> 2.6 itimer > > regressions, which hrtimers fixed. E.g.: > > > > http://bugzilla.kernel.org/show_bug.cgi?id=3289 > > Ingo, please read correctly, this is mainly about interval timers, > which my patch doesn't change. My patch only fixes the initial start > time.
Yeah, i know it's about the start time - what else could it possibly be about? As i wrote:
> > so i dont think restoring the first timeout of an interval timer to > > be increased by resolution [which your patch does] has any meaning. > > It 'restores' to half of what 2.6 did prior hrtimers. Doing that > > would be inconsistent and would push the 'sum-up' errors observed > > for interval timers above to be again observable in user-space (if > > user-space does a series of timeouts). What's the point?
Your change changes the initial start time to be longer by +1 jiffy. My "restores to half of what 2.6 did" observation was in reference to the start time. The other half is the interval time between timeouts. If you add a +1 jiffy to the start time, you ought to do it for the interval time too. Or do it for neither - which is what we chose to do.
Yes, the 2.6 regression in the bugzilla was _mainly_ about the intervals adding a comulative +1, but obviously the behavior should be symmetric: if we use our higher resolution for intervals, we should use it for the start time too.
In other words: your patch re-introduces half of the bug on low-res platforms. Users doing a series of one-shot itimer calls would be exposed to the same kind of (incorrect and unnecessary) summing-up errors. What's the point?
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