Re: [RFC patch 15/15] LTTng timestamp x86

Linus Torvalds
2008-10-20T20:11:14Z

On Fri, 17 Oct 2008, Mathieu Desnoyers wrote:
>
> Hrm, on such systems
> - *large* amount of cpus
> - no synchronized TSCs
>
> What would be the best approach to order events ?

My strong opinion has been - for a longish while now, and independently of
any timestamping code - that we should be seriously looking at basically
doing essentially a "ntp" inside the kernel to give up the whole idiotic
notion of "synchronized TSCs". Yes, TSC's are often synchronized, but even
when they are, we might as well _think_ of them as not being so.

In other words, instead of expecting internal clocks to be synchronized,
just make the clock be a clock network of independent TSC domains. The
domains could in theory be per-package (assuming TSC is synchronized at
that level), but even if we _could_ do that, we'd probably still be better
off by simply always doing it per-core. If only because then the reading
would be per-core.

I think it's a mistake for us to maintain a single clock for
gettimeofday() (well, "getnstimeofday" and the whole "clocksource_read()"
crud to be technically correct). And sure, I bet clocksource_read() can do
various per-CPU things and try to do that, but it's complex and pretty
generic code, and as far as I know none of the clocksources have even
tried. The TSC clocksource read certainly does not (it just does a very
similar horrible "at least don't go backwards" crud that the LTTng patch
suggested).

So I think we should make "xtime" be a per-CPU thing, and add support for
per-CPU clocksources. And screw that insane "mark_tsc_unstable()" thing.

And if we did it well, we migth be able to get good timestamps that way
too.

Linus