linux-kernel - Re: [PATCH v2 4/4] clock: add perf

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Message-ID: <54E4D5C4.1070005@gmail.com>
Date:	Wed, 18 Feb 2015 11:11:16 -0700
From:	David Ahern <dsahern@...il.com>
To:	John Stultz <john.stultz@...aro.org>,
	Stephane Eranian <eranian@...gle.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	acme@...hat.com, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, ak@...ux.intel.com,
	Jiri Olsa <jolsa@...hat.com>, namhyung@...nel.org,
	cel@...ibm.com, sukadev@...ux.vnet.ibm.com, sonnyrao@...omium.org,
	johnmccutchan@...gle.com, adrian.hunter@...el.com,
	pawell.moll@....com
Subject: Re: [PATCH v2 4/4] clock: add perf_clock posix clock

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).

David

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