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

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Date:	Wed, 18 Feb 2015 10:18:30 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Rose Belcher <cel@...ibm.com>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	Sonny Rao <sonnyrao@...omium.org>,
	John Mccutchan <johnmccutchan@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Paweł Moll <mail@...elmoll.com>
Subject: Re: [PATCH v2 4/4] clock: add perf_clock posix clock

On Wed, Feb 18, 2015 at 10:11 AM, David Ahern <dsahern@...il.com> wrote:
> 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).

Yep. And I'm sympathetic to the fact that an alternative solution
hasn't made it upstream yet. I'm hopeful Pawel's recent approach will
make it in (it seems like it hasn't raised any flags w/ scheduler
folks - but I've not always been able to follow the discussion
closely).

thanks
-john
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