Re: [PATCH v1] perf header: Fix missing PMU caps

[Arnaldo Carvalho de Melo]

Em Sat, Aug 19, 2023 at 12:16:09PM +0800, liwei (GF) escreveu:
> Hi Ian:
> 
> On 2023/8/19 1:19, Ian Rogers wrote:
> > PMU caps are written as HEADER_PMU_CAPS or for the special case of the
> > PMU "cpu" as HEADER_CPU_PMU_CAPS. As the PMU "cpu" is special, and not
> > any "core" PMU, the logic had become broken and core PMUs not called
> > "cpu" were not having their caps written. This affects ARM and s390
> > non-hybrid PMUs.
> > 
> > Simplify the PMU caps writing logic to scan one fewer time and to be
> > more explicit in its behavior.
> > 
> > Reported-by: Wei Li <liwei391@xxxxxxxxxx>
> > Fixes: 178ddf3bad98 ("perf header: Avoid hybrid PMU list in write_pmu_caps")
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/perf/util/header.c | 31 ++++++++++++++++---------------
> >  1 file changed, 16 insertions(+), 15 deletions(-)
> > 
> > diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> > index 52fbf526fe74..13c71d28e0eb 100644
> > --- a/tools/perf/util/header.c
> > +++ b/tools/perf/util/header.c
> > @@ -1605,8 +1605,15 @@ static int write_pmu_caps(struct feat_fd *ff,
> >  	int ret;
> >  
> >  	while ((pmu = perf_pmus__scan(pmu))) {
> > -		if (!pmu->name || !strcmp(pmu->name, "cpu") ||
> > -		    perf_pmu__caps_parse(pmu) <= 0)
> > +		if (!strcmp(pmu->name, "cpu")) {
> 
> So you removed the check of 'pmu->name', does this check really redundant? since
> we can find such checks in many places in the perf code. If not, i think it is
> necessary for strcmp().

Indeed, when sorting in tools/perf/util/pmus.c in cmp_sevent() we have:

        /* Order by PMU name. */
        if (as->pmu != bs->pmu) {
                a_pmu_name = a_pmu_name ?: (as->pmu->name ?: "");
                b_pmu_name = b_pmu_name ?: (bs->pmu->name ?: "");
                ret = strcmp(a_pmu_name, b_pmu_name);
                if (ret)
                        return ret;
        }


And even if in this specific case, for some reason, we could guarantee
that pmu->name isn't NULL, then removing that check should be best left
for a separate patch with an explanation as to why that is safe.

Having it as:

 	while ((pmu = perf_pmus__scan(pmu))) {
-		if (!pmu->name || !strcmp(pmu->name, "cpu") ||
-		    perf_pmu__caps_parse(pmu) <= 0)
+		if (!pmu->name || !strcmp(pmu->name, "cpu")) {

even eases a bit reviewing, as we see we're just removing that
perf_pmu__caps_parse(pmu) line.

Ian?

- Arnaldo

 
> > +			/*
> > +			 * The "cpu" PMU is special and covered by
> > +			 * HEADER_CPU_PMU_CAPS. Note, core PMUs are
> > +			 * counted/written here for ARM, s390 and Intel hybrid.
> > +			 */
> > +			continue;
> > +		}
> > +		if (perf_pmu__caps_parse(pmu) <= 0)
> >  			continue;
> >  		nr_pmu++;
> >  	}
> > @@ -1619,23 +1626,17 @@ static int write_pmu_caps(struct feat_fd *ff,
> >  		return 0;
> >  
> >  	/*
> > -	 * Write hybrid pmu caps first to maintain compatibility with
> > -	 * older perf tool.
> > +	 * Note older perf tools assume core PMUs come first, this is a property
> > +	 * of perf_pmus__scan.
> >  	 */
> > -	if (perf_pmus__num_core_pmus() > 1) {
> > -		pmu = NULL;
> > -		while ((pmu = perf_pmus__scan_core(pmu))) {
> > -			ret = __write_pmu_caps(ff, pmu, true);
> > -			if (ret < 0)
> > -				return ret;
> > -		}
> > -	}
> > -
> >  	pmu = NULL;
> >  	while ((pmu = perf_pmus__scan(pmu))) {
> > -		if (pmu->is_core || !pmu->nr_caps)
> > +		if (!strcmp(pmu->name, "cpu")) {
> 
> same here
> 
> Thanks,
> Wei
> 
> > +			/* Skip as above. */
> > +			continue;
> > +		}
> > +		if (perf_pmu__caps_parse(pmu) <= 0)
> >  			continue;
> > -
> >  		ret = __write_pmu_caps(ff, pmu, true);
> >  		if (ret < 0)
> >  			return ret;

-- 

- Arnaldo