Re: [PATCH 2/2 v3] perf top: populate PMU capabilities data in perf_env

[Namhyung Kim <namhyung@kernel.org>]

On Wed, Jun 11, 2025 at 07:00:04PM +0000, Falcon, Thomas wrote:
> On Tue, 2025-06-10 at 17:25 -0300, Arnaldo Carvalho de Melo wrote:
> > On Mon, Jun 09, 2025 at 04:21:39PM +0000, Falcon, Thomas wrote:
> > > Ping?
> > > 
> > > Thanks,
> > > Tom
> > > 
> > > On Tue, 2025-05-13 at 18:18 -0500, Thomas Falcon wrote:
> > > > Calling perf top with branch filters enabled on Intel CPU's
> > > > with branch counters logging (A.K.A LBR event logging [1]) support
> > > > results in a segfault.
> > > > 
> > > > Thread 27 "perf" received signal SIGSEGV, Segmentation fault.
> > > > [Switching to Thread 0x7fffafff76c0 (LWP 949003)]
> > > > perf_env__find_br_cntr_info (env=0xf66dc0 <perf_env>, nr=0x0, width=0x7fffafff62c0) at util/env.c:653
> > > > 653			*width = env->cpu_pmu_caps ? env->br_cntr_width :
> > > > (gdb) bt
> > > >  #0  perf_env__find_br_cntr_info (env=0xf66dc0 <perf_env>, nr=0x0, width=0x7fffafff62c0) at util/env.c:653
> > > >  #1  0x00000000005b1599 in symbol__account_br_cntr (branch=0x7fffcc3db580, evsel=0xfea2d0, offset=12, br_cntr=8) at util/annotate.c:345
> > > >  #2  0x00000000005b17fb in symbol__account_cycles (addr=5658172, start=5658160, sym=0x7fffcc0ee420, cycles=539, evsel=0xfea2d0, br_cntr=8) at util/annotate.c:389
> > > >  #3  0x00000000005b1976 in addr_map_symbol__account_cycles (ams=0x7fffcd7b01d0, start=0x7fffcd7b02b0, cycles=539, evsel=0xfea2d0, br_cntr=8) at util/annotate.c:422
> > > >  #4  0x000000000068d57f in hist__account_cycles (bs=0x110d288, al=0x7fffafff6540, sample=0x7fffafff6760, nonany_branch_mode=false, total_cycles=0x0, evsel=0xfea2d0) at util/hist.c:2850
> > > >  #5  0x0000000000446216 in hist_iter__top_callback (iter=0x7fffafff6590, al=0x7fffafff6540, single=true, arg=0x7fffffff9e00) at builtin-top.c:737
> > > >  #6  0x0000000000689787 in hist_entry_iter__add (iter=0x7fffafff6590, al=0x7fffafff6540, max_stack_depth=127, arg=0x7fffffff9e00) at util/hist.c:1359
> > > >  #7  0x0000000000446710 in perf_event__process_sample (tool=0x7fffffff9e00, event=0x110d250, evsel=0xfea2d0, sample=0x7fffafff6760, machine=0x108c968) at builtin-top.c:845
> > > >  #8  0x0000000000447735 in deliver_event (qe=0x7fffffffa120, qevent=0x10fc200) at builtin-top.c:1211
> > > >  #9  0x000000000064ccae in do_flush (oe=0x7fffffffa120, show_progress=false) at util/ordered-events.c:245
> > > >  #10 0x000000000064d005 in __ordered_events__flush (oe=0x7fffffffa120, how=OE_FLUSH__TOP, timestamp=0) at util/ordered-events.c:324
> > > >  #11 0x000000000064d0ef in ordered_events__flush (oe=0x7fffffffa120, how=OE_FLUSH__TOP) at util/ordered-events.c:342
> > > >  #12 0x00000000004472a9 in process_thread (arg=0x7fffffff9e00) at builtin-top.c:1120
> > > >  #13 0x00007ffff6e7dba8 in start_thread (arg=<optimized out>) at pthread_create.c:448
> > > >  #14 0x00007ffff6f01b8c in __GI___clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78
> > > > 
> > > > The cause is that perf_env__find_br_cntr_info tries to access a
> > > > null pointer pmu_caps in the perf_env struct. A similar issue exists
> > > > for homogeneous core systems which use the cpu_pmu_caps structure.
> > > > 
> > > > Fix this by populating cpu_pmu_caps and pmu_caps structures with
> > > > values from sysfs when calling perf top with branch stack sampling
> > > > enabled.
> > > > 
> > > > [1], LBR event logging introduced here:
> > > > https://lore.kernel.org/all/20231025201626.3000228-5-kan.liang@linux.intel.com/
> > > > 
> > > > Signed-off-by: Thomas Falcon <thomas.falcon@intel.com>
> > > > ---
> > > > v3: constify struct perf_pmu *pmu in __perf_env__read_core_pmu_caps()
> > > >     use perf_pmus__find_core_pmu() instead of perf_pmus__scan_core(NULL)
> > > > 
> > > > v2: update commit message with more meaningful stack trace from
> > > >     gdb and indicate that affected systems are limited to CPU's
> > > >     with LBR event logging support and that both hybrid and
> > > >     non-hybrid core systems are affected.
> > > > ---
> > > >  tools/perf/builtin-top.c |   8 +++
> > > >  tools/perf/util/env.c    | 114 +++++++++++++++++++++++++++++++++++++++
> > > >  tools/perf/util/env.h    |   1 +
> > > >  3 files changed, 123 insertions(+)
> > > > 
> > > > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> > > > index f9f31391bddb..c9d679410591 100644
> > > > --- a/tools/perf/builtin-top.c
> > > > +++ b/tools/perf/builtin-top.c
> > > > @@ -1729,6 +1729,14 @@ int cmd_top(int argc, const char **argv)
> > > >  	if (opts->branch_stack && callchain_param.enabled)
> > > >  		symbol_conf.show_branchflag_count = true;
> > > >  
> > > > +	if (opts->branch_stack) {
> > > > +		status = perf_env__read_core_pmu_caps(&perf_env);
> > > > +		if (status) {
> > > > +			pr_err("PMU capability data is not available\n");
> > > > +			goto out_delete_evlist;
> > > > +		}
> > > > +	}
> > > > +
> > > >  	sort__mode = SORT_MODE__TOP;
> > > >  	/* display thread wants entries to be collapsed in a different tree */
> > > >  	perf_hpp_list.need_collapse = 1;
> > > > diff --git a/tools/perf/util/env.c b/tools/perf/util/env.c
> > > > index 36411749e007..6735786a1d22 100644
> > > > --- a/tools/perf/util/env.c
> > > > +++ b/tools/perf/util/env.c
> > > > @@ -416,6 +416,120 @@ static int perf_env__read_nr_cpus_avail(struct perf_env *env)
> > > >  	return env->nr_cpus_avail ? 0 : -ENOENT;
> > > >  }
> > > >  
> > > > +static int __perf_env__read_core_pmu_caps(const struct perf_pmu *pmu,
> > > > +					  int *nr_caps, char ***caps,
> > > > +					  unsigned int *max_branches,
> > > > +					  unsigned int *br_cntr_nr,
> > > > +					  unsigned int *br_cntr_width)
> > > > +{
> > > > +	struct perf_pmu_caps *pcaps = NULL;
> > > > +	char *ptr, **tmp;
> > > > +	int ret = 0;
> > > > +
> > > > +	*nr_caps = 0;
> > > > +	*caps = NULL;
> > > > +
> > > > +	if (!pmu->nr_caps)
> > > > +		return 0;
> > > > +
> > > > +	*caps = zalloc(sizeof(char *) * pmu->nr_caps);
> > 
> > calloc?
> 
> Thanks for reviewing. Is there a reason not to use zalloc here or is this related to using free
> instead of zfree later?

Conceptually, zmalloc() = malloc() + memset() for a single entry.
calloc() would be more appropriate if you allocate multiple.

> 
> > 
> > > > +	if (!*caps)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	tmp = *caps;
> > > > +	list_for_each_entry(pcaps, &pmu->caps, list) {
> > > > +
> > 
> > Needless blank line
> > 
> > > > +		if (asprintf(&ptr, "%s=%s", pcaps->name, pcaps->value) < 0) {
> > > > +			ret = -ENOMEM;
> > > > +			goto error;
> > > > +		}
> > > > +
> > > > +		*tmp++ = ptr;
> > > > +
> > > > +		if (!strcmp(pcaps->name, "branches"))
> > > > +			*max_branches = atoi(pcaps->value);
> > > > +
> > > > +		if (!strcmp(pcaps->name, "branch_counter_nr"))
> > > > +			*br_cntr_nr = atoi(pcaps->value);
> > > > +
> > > > +		if (!strcmp(pcaps->name, "branch_counter_width"))
> > > > +			*br_cntr_width = atoi(pcaps->value);
> > 
> > else if?
> > 
> > I.e. why test it repeatedly when it can't be the three of them?
> 
> I was borrowing from a similar implementation here,
> 
> https://github.com/torvalds/linux/blob/aef17cb3d3c43854002956f24c24ec8e1a0e3546/tools/perf/util/header.c#L3283
> 
> but I see what you mean. That may explain why I used free instead zfree as well.
> 
> 
> > 
> > What if it is not one of these three? Free and error out?
> > 
> 
> In that case, the capability data should still be written to the caps array in struct pmu_caps.
> These members seem to be added to pmu_caps for convenience. 
> 
> > > > +	}
> > > > +	*nr_caps = pmu->nr_caps;
> > > > +	return 0;
> > > > +error:
> > > > +	while (tmp-- != *caps)
> > > > +		free(*tmp);
> > 
> > zfree(tmp)
> > 
> > > > +	free(*caps);
> > > > +	*caps = NULL;
> > 
> > zfree(caps)
> > 
> > > > +	*nr_caps = 0;
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +int perf_env__read_core_pmu_caps(struct perf_env *env)
> > > > +{
> > > > +	struct perf_pmu *pmu = NULL;
> > 
> > why init it to NULL if it will be initialized to something else later on
> > before being used?
> 
> I wanted to insure it was NULL before passing to perf_pmus__scan_core, just being paranoid I guess.
> 
> > 
> > > > +	struct pmu_caps *pmu_caps;
> > > > +	int nr_pmu = 0, i = 0, j;
> > > > +	int ret;
> > > > +
> > > > +	nr_pmu = perf_pmus__num_core_pmus();
> > 
> > nr_pmu = 0 followed by this call?
> > 
> > > > +
> > > > +	if (!nr_pmu)
> > > > +		return -ENODEV;
> > > > +
> > > > +	if (nr_pmu == 1) {
> > > > +		pmu = perf_pmus__find_core_pmu();
> > > > +		if (!pmu)
> > > > +			return -ENODEV;
> > > > +		ret = perf_pmu__caps_parse(pmu);
> > > > +		if (ret < 0)
> > > > +			return ret;
> > > > +		return __perf_env__read_core_pmu_caps(pmu, &env->nr_cpu_pmu_caps,
> > > > +						      &env->cpu_pmu_caps,
> > > > +						      &env->max_branches,
> > > > +						      &env->br_cntr_nr,
> > > > +						      &env->br_cntr_width);
> > > > +	}
> > > > +
> > > > +	pmu_caps = zalloc(sizeof(*pmu_caps) * nr_pmu);
> > > > +	if (!pmu_caps)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	while ((pmu = perf_pmus__scan_core(pmu)) != NULL) {
> > > > +		if (perf_pmu__caps_parse(pmu) <= 0)
> > > > +			continue;
> > > > +		ret = __perf_env__read_core_pmu_caps(pmu, &pmu_caps[i].nr_caps,
> > > > +						     &pmu_caps[i].caps,
> > > > +						     &pmu_caps[i].max_branches,
> > > > +						     &pmu_caps[i].br_cntr_nr,
> > > > +						     &pmu_caps[i].br_cntr_width);
> > > > +		if (ret)
> > > > +			goto error;
> > > > +
> > > > +		pmu_caps[i].pmu_name = strdup(pmu->name);
> > > > +		if (!pmu_caps[i].pmu_name) {
> > > > +			ret = -ENOMEM;
> > > > +			goto error;
> > > > +		}
> > > > +		i++;
> > > > +	}
> > > > +
> > > > +	env->nr_pmus_with_caps = nr_pmu;
> > > > +	env->pmu_caps = pmu_caps;
> > > > +
> > > > +	return 0;
> > > > +error:
> > > > +	for (i = 0; i < nr_pmu; i++) {
> > > > +		for (j = 0; j < pmu_caps[i].nr_caps; j++)
> > > > +			free(pmu_caps[i].caps[j]);
> > > > +		free(pmu_caps[i].caps);
> > > > +		free(pmu_caps[i].pmu_name);
> > 
> > zfree in all the frees above?
> 
> Thanks again, I can use zfree here and address the rest of the comments in a new version if this
> hasn't been applied already?

It's not, please update. :)

Thanks,
Namhyung
 
> > 
> > > > +	}
> > > > +	free(pmu_caps);
> > > > +	return ret;
> > > > +}
> > > > +
> > > >  const char *perf_env__raw_arch(struct perf_env *env)
> > > >  {
> > > >  	return env && !perf_env__read_arch(env) ? env->arch : "unknown";
> > > > diff --git a/tools/perf/util/env.h b/tools/perf/util/env.h
> > > > index d90e343cf1fa..135a1f714905 100644
> > > > --- a/tools/perf/util/env.h
> > > > +++ b/tools/perf/util/env.h
> > > > @@ -152,6 +152,7 @@ struct btf_node;
> > > >  
> > > >  extern struct perf_env perf_env;
> > > >  
> > > > +int perf_env__read_core_pmu_caps(struct perf_env *env);
> > > >  void perf_env__exit(struct perf_env *env);
> > > >  
> > > >  int perf_env__kernel_is_64_bit(struct perf_env *env);
> > > 
> > 
>