Rethinking multi-grain timestamps [LWN.net]

Rethinking multi-grain timestamps

Posted Oct 11, 2023 22:55 UTC (Wed) by nijhof (subscriber, #4034)
In reply to: Rethinking multi-grain timestamps by jlayton
Parent article: Rethinking multi-grain timestamps

How would that work up with multiple coarse - fine - coarse - fine... updates in close succession? If each would have to be later than the previous one, then each coarse timestamp would have to be advanced. And so you could end up with timestamps in the future?

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Rethinking multi-grain timestamps

Posted Oct 11, 2023 23:18 UTC (Wed) by jlayton (subscriber, #31672) [Link]

When we talk about a fine grained timestamp, what we mean is one that comes directly from the kernel's internal high-resolution timekeeping. That generally has very fine resolution (~100ns or better) and monotonically increases. We grab that value, calculate and fix up the wallclock time from it and give it out. The coarse grained timestamp is updated approximately every jiffy (once per timer tick) and is just a snapshot of the fine grained timestamp at that time.

So to answer your question, there should be no problem. The idea is to update the apparent coarse-grained timestamp _source_ before returning any fine-grained timestamp. Any later fetch from the timestamp source (coarse or fine), will always be later than or equal to the last fine grained timestamp handed out. That should be good enough to keep "make" happy.

(Note that it's a bit more complex with the way that times are tracked in the timekeeping code, so I'm glossing over some details here.)