Re: [PATCH v2 4/4] clock: add perf_clock posix clock

From: David Ahern
Date: Wed Feb 18 2015 - 13:11:22 EST

On 2/18/15 11:00 AM, John Stultz wrote:
I'd still strongly recommend against exposing the perf clock to
userspace this way. The time domain isn't clearly different from
something like CLOCK_MONOTONIC_RAW and doesn't really have well
defined behavior. We're just exporting "whatever the kernel does
internally" to userspace, and in the past similar internal use clocks
like the sched_clock have changed their behavior, so I'm not confident
the perf clock is really baked enough (including cross architectures)
to make it part of the ABI.

Pawel and others have continued to work on other approaches that allow
for perf events to be interpolated to, or use CLOCK_MONOTONIC itself,
which I don't object to, so you might want to follow up on those?

AFAIK Stephane is not proposing this patch for inclusion but rather it is an unfortunate necessary evil. The module exposes perf_clock (ie., local_clock) to userspace and allows in this case the generation of samples with a perf timestamp which is required for proper sorting.

I understand this solution is not liked, but it works, requires no kernel modifications to achieve the end goal and can be used for kernels going back to at least 2.6.38 (perhaps earlier, have not checked).


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