linux-kernel - Re: [PATCH v4 1/3] perf: Use monotonic clock as a source for timestamps

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Date:	Wed, 21 Jan 2015 20:07:42 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Christopher Covington <cov@...eaurora.org>,
	Namhyung Kim <namhyung@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tomeu Vizoso <tomeu@...euvizoso.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v4 1/3] perf: Use monotonic clock as a source for
 timestamps

On Wed, 2015-01-21 at 19:48 +0000, Pawel Moll wrote:
> On Wed, 2015-01-21 at 15:52 +0000, Pawel Moll wrote:
> > On Mon, 2015-01-05 at 13:00 +0000, Peter Zijlstra wrote:
> > > On Thu, Nov 06, 2014 at 04:51:56PM +0000, Pawel Moll wrote:
> > > >  Documentation/kernel-parameters.txt |  9 +++++++++
> > > >  kernel/events/core.c                | 37 +++++++++++++++++++++++++++++++++++++
> > > >  2 files changed, 46 insertions(+)
> > > 
> > > > diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> > > > index 4c81a86..8ead8d8 100644
> > > > --- a/Documentation/kernel-parameters.txt
> > > > +++ b/Documentation/kernel-parameters.txt
> > > 
> > > > @@ -2763,6 +2764,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> > > >  			allocator.  This parameter is primarily	for debugging
> > > >  			and performance comparison.
> > > >  
> > > > +	perf_use_local_clock
> > > > +			[PERF]
> > > > +			Use local_clock() as a source for perf timestamps
> > > > +			generation. This was be the default behaviour and
> > > > +			this parameter can be used to maintain backward
> > > > +			compatibility or on older hardware with expensive
> > > > +			monotonic clock source.
> > > > +
> > > >  	pf.		[PARIDE]
> > > >  			See Documentation/blockdev/paride.txt.
> > > 
> > > So I'm always terminally confused on the naming of kernel parameters,
> > > sometimes things are modules (even when they're not actually =m capable)
> > > and get a module::foo naming or so and sometimes they're not.
> > 
> > I guess you mean module.foo?
> > 
> > > So we want to use the module naming thing or not?
> > 
> > Honestly, I don't mind either way. For one thing ftrace doesn't bother
> > and just uses __setup() as well.
> 
> There's one more thing to this - as far as I remember, the module name
> is actually derived from the compilation unit name (at some level of
> Kbuild). I may be wrong (will have to double check), but a module
> parameter defined in kernel/events/core.c may be called something like
> "core.parameter" ;-).

Ok, so it's possible to enforce "perf." prefix:

/* You can override this manually, but generally this should match the
   module name. */
#ifdef MODULE
#define MODULE_PARAM_PREFIX /* empty */
#else
#define MODULE_PARAM_PREFIX KBUILD_MODNAME "."
#endif

So, perf_use_local_clock or perf.use_local_clock? :-)

Pawel

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