Re: [PATCH] clocksource: Allow toggling between runtime and persistent clocksource for idle

[John Stultz]

On Mon, Jul 6, 2015 at 12:12 AM, Tony Lindgren <tony@xxxxxxxxxxx> wrote:
> Some persistent clocksources can be on a slow external bus. For shorter
> latencies for RT use, let's allow toggling the clocksource during idle
> between a faster non-persistent runtime clocksource and a slower persistent
> clocksource.

So yea, as Thomas mentioned, this will cause problems for timekeeping
accuracy, since we end up resetting the ntp state when we change
clocksource (additionally you gain a bit of error each time you switch
clocksources). So you'll most likely see trouble w/ ntpd steering the
clock.

I'm not sure its quite clear in the description as to the need here.
Could you expand a bit as to the rational for why?  I assume it has to
do with you have a high-res clocksource that is good for fine-grained
clock_gettime() calls, but wraps quickly, making it poor for long idle
times. So you're trying to swap to a less fine grained clocksource
that won't wrap so fast?

The persistent-clock-like name muddies things further, since the
persistent-clock is used for suspend, while it looks like this is just
for idle times.

thanks
-john
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/