Re: [PATCH v4 3/3] x86/time: avoid reading the platform timer in rendezvous functions

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Apr 01, 2021 at 11:55:10AM +0200, Jan Beulich wrote:
> Reading the platform timer isn't cheap, so we'd better avoid it when the
> resulting value is of no interest to anyone.
> 
> The consumer of master_stime, obtained by
> time_calibration_{std,tsc}_rendezvous() and propagated through
> this_cpu(cpu_calibration), is local_time_calibration(). With
> CONSTANT_TSC the latter function uses an early exit path, which doesn't
> explicitly use the field. While this_cpu(cpu_calibration) (including the
> master_stime field) gets propagated to this_cpu(cpu_time).stamp on that
> path, both structures' fields get consumed only by the !CONSTANT_TSC
> logic of the function.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v4: New.
> ---
> I realize there's some risk associated with potential new uses of the
> field down the road. What would people think about compiling time.c a
> 2nd time into a dummy object file, with a conditional enabled to force
> assuming CONSTANT_TSC, and with that conditional used to suppress
> presence of the field as well as all audited used of it (i.e. in
> particular that large part of local_time_calibration())? Unexpected new
> users of the field would then cause build time errors.

Wouldn't that add quite a lot of churn to the file itself in the form
of pre-processor conditionals?

Could we instead set master_stime to an invalid value that would make
the consumers explode somehow?

I know there might be new consumers, but those should be able to
figure whether the value is sane by looking at the existing ones.

Also, since this is only done on the BSP on the last iteration I
wonder if it really makes such a difference performance-wise to
warrant all this trouble.

Thanks, Roger.