Re: [PATCH 2/2] perf tools: Improve IBS error handling

[Kim Phillips]

On 10/7/21 12:28 PM, Jiri Olsa wrote:
> On Mon, Oct 04, 2021 at 04:41:14PM -0500, Kim Phillips wrote:
> > ---
> >   tools/perf/util/evsel.c | 24 ++++++++++++++++++++++++
> >   1 file changed, 24 insertions(+)
> >
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index b915840690d4..f8a9cbd99314 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -2743,9 +2743,22 @@ static bool find_process(const char *name)
> >   	return ret ? false : true;
> >   }
> >   
> > +static bool is_amd(const char *arch, const char *cpuid)
> > +{
> > +	return arch && !strcmp("x86", arch) && cpuid && strstarts(cpuid, "AuthenticAMD");
> > +}
> > +
> > +static bool is_amd_ibs(struct evsel *evsel)
> > +{
> > +	return evsel->core.attr.precise_ip || !strncmp(evsel->pmu_name, "ibs", 3);
> > +}
> > +
> >   int evsel__open_strerror(struct evsel *evsel, struct target *target,
> >   			 int err, char *msg, size_t size)
> >   {
> > +	struct perf_env *env = evsel__env(evsel);
> > +	const char *arch = perf_env__arch(env);
> > +	const char *cpuid = perf_env__cpuid(env);
> >   	char sbuf[STRERR_BUFSIZE];
> >   	int printed = 0, enforced = 0;
> >   
> > @@ -2841,6 +2854,17 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
> >   			return scnprintf(msg, size, "wrong clockid (%d).", clockid);
> >   		if (perf_missing_features.aux_output)
> >   			return scnprintf(msg, size, "The 'aux_output' feature is not supported, update the kernel.");
> > +		if (is_amd(arch, cpuid)) {
> > +			if (is_amd_ibs(evsel)) {
>
> would single 'is_amd_ibs' call be better? checking on both amd and ibs

Good suggestion. If you look at the later patch in the
BRS series, I have rewritten it to add the new
AMD PMU like so:

 if (is_amd()) {
 	if (is_amd_ibs()) {
 		if (evsel->this)
 			return
 		if (evsel->that)
 			return
 	}
+	if (is_amd_brs()) {
+		if (evsel->this)
+			return
+		if (evsel->that)
+			return
+	}
 }

Below is the full proposed replacement patch for patch 12
of 13 of the BRS series.

Another option is to have the is_amd_{ibs,brs} functions
call is_amd() themselves, so the if (evsel->) code could be
unindented by one tab, would that be better?

Thanks,

Kim

From a4cbab762719b30bddec2e278cf8b8eb82e83865 Mon Sep 17 00:00:00 2001
From: Stephane Eranian <eranian@xxxxxxxxxx>
Date: Thu, 9 Sep 2021 00:56:59 -0700
Subject: [PATCH] perf tools: Improve error handling of AMD Branch Sampling

This patch improves the error message printed by perf when
perf_event_open() fails on AMD Zen3 when using the branch sampling
feature. In the case of EINVAL, there are two main reasons: frequency
mode or period is smaller than the depth of the branch sampling
buffer (16). The patch checks the parameters of the call and tries
to print a relevant message to explain the error:

$ perf record -b -e cpu/branch-brs/ -c 10 ls
Error:
AMD Branch Sampling does not support sampling period smaller than what is reported in /sys/devices/cpu/caps/branches.

$ perf record -b -e cpu/branch-brs/ ls
Error:
AMD Branch Sampling does not support frequency mode sampling, must pass a fixed sampling period via -c option or cpu/branch-brs,period=xxxx/.

Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
[Rebased on commit 9fe8895a27a84 ("perf env: Add perf_env__cpuid, perf_env__{nr_}pmu_mappings")]
Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
---
 tools/perf/util/evsel.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index f8a9cbd99314..e1f5eff07355 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2753,6 +2753,12 @@ static bool is_amd_ibs(struct evsel *evsel)
 	return evsel->core.attr.precise_ip || !strncmp(evsel->pmu_name, "ibs", 3);
 }

+static bool is_amd_brs(struct evsel *evsel)
+{
+	return ((evsel->core.attr.config & 0xff) == 0xc4) &&
+	       (evsel->core.attr.sample_type & PERF_SAMPLE_BRANCH_STACK);
+}
+
 int evsel__open_strerror(struct evsel *evsel, struct target *target,
 			 int err, char *msg, size_t size)
 {
@@ -2863,6 +2869,14 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
 					return scnprintf(msg, size,
 	"AMD IBS may only be available in system-wide/per-cpu mode.  Try using -a, or -C and workload affinity");
 			}
+			if (is_amd_brs(evsel)) {
+				if (evsel->core.attr.freq)
+					return scnprintf(msg, size,
+	"AMD Branch Sampling does not support frequency mode sampling, must pass a fixed sampling period via -c option or cpu/branch-brs,period=xxxx/.");
+				/* another reason is that the period is too small */
+				return scnprintf(msg, size,
+	"AMD Branch Sampling does not support sampling period smaller than what is reported in /sys/devices/cpu/caps/branches.");
+			}
 		}

 		break;
--
2.31.1