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

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Date:	Wed, 18 Feb 2015 13:21:50 -0500
From:	Stephane Eranian <eranian@...gle.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	David Ahern <dsahern@...il.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>,
	Pawel Moll <pawel.moll@....com>
Subject: Re: [PATCH v2 4/4] clock: add perf_clock posix clock

On Wed, Feb 18, 2015 at 1:18 PM, John Stultz <john.stultz@...aro.org> wrote:
> 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).
>
Ok, let me try switching my series to Pawel's patch then. Should not take long.
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