linux-kernel - Re: [PATCH V2 4/4] sh: Use the perf-events backend for oprofile

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Date:	Tue, 31 Aug 2010 13:28:41 +0200
From:	Robert Richter <robert.richter@....com>
To:	Matt Fleming <matt@...sole-pimps.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Deacon <will.deacon@....com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH V2 4/4] sh: Use the perf-events backend for oprofile

On 27.08.10 16:19:46, Matt Fleming wrote:
> On Fri, Aug 27, 2010 at 04:59:01PM +0200, Robert Richter wrote:
> > On 26.08.10 15:09:19, Matt Fleming wrote:
> > > Use the perf-events based wrapper for oprofile available in
> > > drivers/oprofile. This allows us to centralise the code to control
> > > performance counters.
> > > 
> > > Signed-off-by: Matt Fleming <matt@...sole-pimps.org>
> > > ---
> > > 
> > > Paul,
> > > 
> > > I dropped the CONFIG_PERF_EVENTS dependency from the Makefile in this
> > > version because to do anything useful we need perf events anyway.
> > 
> > Initialization should simply fail with a printk message for this case,
> > implement function stubs for the !CONFIG_PERF_EVENTS case instead in
> > the oprofile.h header file.
> 
> I didn't do this because I was hoping that eventually we'd make
> CONFIG_OPROFILE select PERF_EVENTS. Would you be OK making that change
> instead? Runtime failure is best avoided where possible, especially when
> we can sort this out at compile time.

Ok, we don't need it if we add architectural dependencies to Kconfig
for those architectures requiring perf.

> > > -static int op_sh_start(void)
> > > +static char *op_name_from_perf_name(const char *name)
> > >  {
> > > -	/* Enable performance monitoring for all counters.  */
> > > -	on_each_cpu(model->cpu_start, NULL, 1);
> > > +	if (!strcmp(name, "SH-4A"))
> > > +		return "sh/sh4a";
> > > +	if (!strcmp(name, "SH7750"))
> > > +		return "sh/sh7750";
> > 
> > With that implementation we always have to touch the code for new
> > cpus. Maybe we derive it from the perf name, e.g. making all lowercase
> > and removing dashes?
> 
> Is this code really that bad that we need to start playing string
> manipulation games?

No, but with that implementation we always have to update the cpu
string with each new cpu though nothing else changes. We may keep this
code. But, shouldn't we return a default string "sh/<name>" for all
other cases? We will then need to update only the oprofile userland
with new cpus.

> > > +	ops->setup		= oprofile_perf_setup;
> > > +	ops->create_files	= oprofile_perf_create_files;
> > > +	ops->start		= oprofile_perf_start;
> > > +	ops->stop		= oprofile_perf_stop;
> > > +	ops->cpu_type		= op_name_from_perf_name(sh_pmu_name());
> > >  
> > > -	model = lmodel;
> > > +	oprofile_perf_set_num_counters(sh_pmu_num_events());
> > >  
> > > -	ops->setup		= op_sh_setup;
> > > -	ops->create_files	= op_sh_create_files;
> > > -	ops->start		= op_sh_start;
> > > -	ops->stop		= op_sh_stop;
> > > -	ops->cpu_type		= lmodel->cpu_type;
> > > +	ret = oprofile_perf_init();
> > 
> > Instead of exporting all the functions above implement something like:
> > 
> > 	name = op_name_from_perf_name(sh_pmu_name());
> > 	num_events = sh_pmu_num_events();
> > 	ret = oprofile_perf_init(ops, name, num_events);
> > 
> > We will then have only oprofile_perf_init() and oprofile_perf_exit()
> > as interface which is much cleaner.
> 
> Well, the reason that I left it this way is so that architectures can
> choose to implement wrappers around the oprofile_perf_* functions. I
> don't think ARM or SH actually need wrappers (the only extra thing that
> ARM does is locking which SH should probably do too) but I assumed there
> was a reason that these functions pointers were exposed originally. I
> haven't look at what other architectures would do. I'll take a look at
> that.

I am not sure if we need such wrappers, and if so we could implement
it anyway, e.g.:

 oprofile_perf_init(perf_ops, name, num_events);

 op_sh_setup():

	/* setup something */
	...

	perf_ops->setup();

	/* setup more */
	...

But I don't think we need this. And the above makes the interface much
cleaner.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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