On Fri, Nov 29, 2013 at 9:37 AM, Peter Zijlstra <pet...@infradead.org> wrote: > Use the 'latch' data structure for cyc2ns. > > This is a data structure first proposed by me and later named by > Mathieu. If anybody's got a better name; do holler.
That structure must exist in the literature, but I have no idea what it's called. It's a multi-word lock-free atomic (I think -- maybe it's just regular) register. I even published a considerably fancier version of much the same thing a few years ago. :) > > Its a multi-version thing which allows always having a coherent > object; we use this to avoid having to disable IRQs while reading > sched_clock() and avoids a problem when getting an NMI while changing > the cyc2ns data. > > The patch should have plenty comments actually explaining the thing. > > The hope is that the extra logic is offset by no longer requiring the > sti;cli around reading the clock. I've occasionally wondered whether it would be possible to make a monotonicity-preserving version of this and use it for clock_gettime. One approach: have the writer set the time for the update to be a bit in the future and have the reader compare the current raw time to the cutoff to see which set of frequency/offset to use. (This requires having some kind of bound on how long it takes to update the data structures.) The advantage: clock_gettime would never block. The disadvantage: complicated, potentially nasty to implement, and it would get complicated if anyone tried to allow multiple updates in rapid succession. Anyway, this is mostly irrelevant to your patches. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
