linux-kernel - Re: [PATCH RESEND v1 02/11] perf mem: Introduce weak function perf_mem_events_

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Date:   Mon, 31 Aug 2020 10:52:30 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     James Clark <james.clark@....com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Ian Rogers <irogers@...gle.com>,
        Kemeng Shi <shikemeng@...wei.com>,
        Wei Li <liwei391@...wei.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Al Grant <Al.Grant@....com>, linux-kernel@...r.kernel.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mike Leach <mike.leach@...aro.org>, nd <nd@....com>
Subject: Re: [PATCH RESEND v1 02/11] perf mem: Introduce weak function
 perf_mem_events__ptr()

On Fri, Aug 28, 2020 at 04:40:29PM +0100, James Clark wrote:
> Hi Leo,
> 
> On 06/08/2020 04:07, Leo Yan wrote:
> >  
> >  	for (j = 0; j < PERF_MEM_EVENTS__MAX; j++) {
> > -		if (!perf_mem_events[j].record)
> > +		e = perf_mem_events__ptr(j);
> > +		if (!e->record)
> >  			continue;
> >  
> > -		if (!perf_mem_events[j].supported) {
> > +		if (!e->supported) {
> >  			pr_err("failed: event '%s' not supported\n",
> > -			       perf_mem_events[j].name);
> > +			       perf_mem_events__name(j));
> >  			free(rec_argv);
> >  			return -1;
> 
> Does it make sense to do something like:
> 
>    for(j = 0; e = perf_mem_events__ptr(j); j++) {
>        ...
>    }
> 
> now that it's a weak function that returns NULL when the argument out of range. That way the caller
> doesn't need to know about PERF_MEM_EVENTS__MAX as well and it could potentially be a different
> value. I don't know if it would ever make sense to have a different number of events on different platforms?

Thanks for reviewing, James.

If you look into the later patch "perf mem: Support new memory event
PERF_MEM_EVENTS__LOAD_STORE", you could find it introduces a new event
which will be only used for Arm SPE but will not be used by other
archs.

Your suggestion is good to encapsulate the macro PERF_MEM_EVENTS__MAX
into perf_mem_events__ptr(), I will try it in next spin.

Thanks,
Leo