Re: [PATCH 1/3] lib/vdso: Delay mask application in do_hres()

From: Thomas Gleixner
Date: Tue Jun 25 2019 - 14:27:22 EST

On Tue, 25 Jun 2019, Thomas Gleixner wrote:

> On Tue, 25 Jun 2019, Vincenzo Frascino wrote:
> CC+ Andy
> > do_hres() in the vDSO generic library masks the hw counter value
> > immediately after reading it.
> >
> > Postpone the mask application after checking if the syscall fallback is
> > enabled, in order to be able to detect a possible fallback for the
> > architectures that have masks smaller than ULLONG_MAX.
> Right. This only worked on x86 because the mask is there ULLONG_MAX for all
> VDSO capable clocksources, i.e. that ever worked just by chance.
> As we talked about that already yesterday, I tested this on a couple of
> machines and as expected the outcome is uarch dependent. Minimal deviations
> to both sides and some machines do not show any change at all. I doubt it's
> possible to come up with a solution which makes all uarchs go faster
> magically.
> Though, thinking about it, we could remove the mask operation completely on
> X86. /me runs tests

Unsurprisingly the results vary. Two uarchs do not care, but they did not
care about moving the mask either. The other two gain performance and the
last one falls back to the state before moving the mask. So in general it
looks like a worthwhile optimization.