linux-kernel - Re: [PATCH -tip/perf/core] perf, x86: P4 PMU

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Date:	Thu, 21 Jul 2011 12:07:55 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Don Zickus <dzickus@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Lin Ming <ming.m.lin@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH -tip/perf/core] perf, x86: P4 PMU - Introduce event alias
 feature

On Thu, Jul 21, 2011 at 09:52:01AM +0200, Ingo Molnar wrote:
> 
> * Cyrill Gorcunov <gorcunov@...il.com> wrote:
> 
> > > > +static u64 p4_get_alias_event(u64 config)
> > > > +{
> > > > +	u64 config_match;
> > > > +	int i;
> > > > +
> > > > +	/*
> > > > +	 * Probably we're lucky and don't have to do
> > > > +	 * matching over all config bits.
> > > > +	 */
> > > > +	if (!(config & P4_CONFIG_ALIASABLE))
> > > > +		return 0;
> > > 
> > > 'all' config bits? There's a single alias mapping in 
> > > p4_event_aliases[] right now which makes the comment rather 
> > > misleading ...
> > 
> > no, the cycle below does check for all bits in config, and test
> > for single bit might help us to return early.
> 
> The loop below? AFAICS it does:
> 
> > > > +	 */
> > > > +	for (i = 0; i < ARRAY_SIZE(p4_event_aliases); i++) {
> > > > +		if (config_match == p4_event_aliases[i].orig) {
> > > > +			config_match = p4_event_aliases[i].alter;
> > > > +			break;
> > > > +		} else if (config_match == p4_event_aliases[i].alter) {
> > > > +			config_match = p4_event_aliases[i].orig;
> > > > +			break;
> > > > +		}
> > > > +	}
> 
> and p4_event_aliases[] is a single-entry array, so ARRAY_SIZE() is 1 
> and this loop iterates once.

I know, but Ingo, I can't guarantee that we'll never bring some new
alias(es) later ;) And test over 64bit variable is not the same as a test
over a single bit (where gcc knows the only one bit is tested and can
optimize it). I think I simply drop this comment, early return case is
understandable without it at all.

> 
> > > Since this .c file is P4 specific and p4_event_aliases[] is a 
> > > file-scope array, is the p4_ prefix even needed?
> > 
> > I prefer them to be distinguished this way, since the .c file in 
> > real is included into another (perf_event.c) file.
> 
> That should probably be fixed btw. - but yeah, until the PMU drivers 
> are separated out better the prefix is fine.

I thought about adding some additional Kconfig where we would choose which
PMU to compile in, not sure though where is the best place for it (under
which menu section I mean).

> 
> > > > +
> > > > +	if (i >= ARRAY_SIZE(p4_event_aliases))
> > > > +		return 0;
> > > > +
> > > > +	return (config_match |
> > > > +		(config & P4_CONFIG_EVENT_ALIAS_IMMUTABLE_BITS));
> > > 
> > > 'return' is not a function. Also, please don't break the line 
> > > pointlessly.
> > 
> > It is perfectly fine to return args in braces,
> > but I will change it.
> 
> The parantheses are syntactically valid but not 'perfectly fine'. As 
> per kernel coding style we don't use them, to signal that 'return' is 
> not a function.

ok, i'll fix

> 
> > 
> > ...
> > > > +			/*
> > > > +			 * Probably an event alias is still available.
> > > 
> > > s/Probably/Possibly?
> > 
> > Hm? I don't get what is wrong with 'probably'.
> 
> Probably means "most likely, maybe" - and that's not the case here.
> 
> (It's misleading in this context because it declares the probability 
> of this action as always higher than 50% - and that's not always the 
> case here.)
> 
> 'Possibly' leaves the probability more undefined.
> 
> You could also write 'Check whether an event alias is still 
> available'.
> 
> Thanks,
> 
> 	Ingo

ok


	Cyrill
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