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Date: Fri, 10 May 2024 10:54:06 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Ian Rogers <irogers@google.com>, Kan Liang <kan.liang@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	linux-perf-users@vger.kernel.org
Subject: Re: [PATCH] perf tools: Ignore deleted cgroups
Message-ID: <Zj4m_v5uEjmSLMb0@x1>
References: <20240509182235.2319599-1-namhyung@kernel.org>
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In-Reply-To: <20240509182235.2319599-1-namhyung@kernel.org>

On Thu, May 09, 2024 at 11:22:35AM -0700, Namhyung Kim wrote:
> 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.

Thanks, applied to perf-tools-next,

- Arnaldo
 
> Signed-off-by: Namhyung Kim <namhyung@kernel.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