linux-kernel - [PATCH] perf tools: Ignore deleted cgroups

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Date: Thu,  9 May 2024 11:22:35 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.com>
Cc: Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org
Subject: [PATCH] perf tools: Ignore deleted cgroups

On a large system, cgroups can be created and deleted often.  That means
there's a race between perf tools and cgroups when it gets the cgroup
name and opens the cgroup.  I got a report that perf stat with many
cgroups failed a quite often due to the missing cgroups on such a large
machine.

I think we can ignore such cgroups when expanding events and use id 0 if
it fails to read the cgroup id.  IIUC 0 is not a vaild cgroup id so it
won't update event counts for the failed cgroups.

Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/util/bpf_counter_cgroup.c | 5 ++---
 tools/perf/util/cgroup.c             | 4 +++-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/bpf_counter_cgroup.c b/tools/perf/util/bpf_counter_cgroup.c
index 1c82377ed78b..ea29c372f339 100644
--- a/tools/perf/util/bpf_counter_cgroup.c
+++ b/tools/perf/util/bpf_counter_cgroup.c
@@ -136,9 +136,8 @@ static int bperf_load_program(struct evlist *evlist)
 		cgrp = evsel->cgrp;
 
 		if (read_cgroup_id(cgrp) < 0) {
-			pr_err("Failed to get cgroup id\n");
-			err = -1;
-			goto out;
+			pr_debug("Failed to get cgroup id for %s\n", cgrp->name);
+			cgrp->id = 0;
 		}
 
 		map_fd = bpf_map__fd(skel->maps.cgrp_idx);
diff --git a/tools/perf/util/cgroup.c b/tools/perf/util/cgroup.c
index fcb509058499..0f759dd96db7 100644
--- a/tools/perf/util/cgroup.c
+++ b/tools/perf/util/cgroup.c
@@ -465,9 +465,11 @@ int evlist__expand_cgroup(struct evlist *evlist, const char *str,
 		name = cn->name + prefix_len;
 		if (name[0] == '/' && name[1])
 			name++;
+
+		/* the cgroup can go away in the meantime */
 		cgrp = cgroup__new(name, open_cgroup);
 		if (cgrp == NULL)
-			goto out_err;
+			continue;
 
 		leader = NULL;
 		evlist__for_each_entry(orig_list, pos) {
-- 
2.45.0.118.g7fe29c98d7-goog