Re: [PATCH v1 3/4] perf off-cpu: Fix cgroup_filter map sizing

[James Clark <james.clark@linaro.org>]

On 27/10/2026 17:04, Ian Rogers wrote:
> off_cpu_prepare() assumed that if evlist__first(evlist)->cgrp is set,
> the number of cgroups is evlist__nr_entries(evlist) - 1 (excluding the
> dummy bpf-output event). This can under-size cgroup_filter when
> non-cgroup events precede cgroup events or when no dummy bpf-output
> event is added.
> 
> Count the number of evsels in evlist that have a cgroup set and use that
> count to size and populate cgroup_filter.
> 
> Fixes: 685439a7a037 ("perf record: Add cgroup support for off-cpu profiling")
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>   tools/perf/util/bpf_off_cpu.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
> index 16948761cc45..71933a71c8d1 100644
> --- a/tools/perf/util/bpf_off_cpu.c
> +++ b/tools/perf/util/bpf_off_cpu.c
> @@ -151,9 +151,10 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target,
>   		    struct record_opts *opts)
>   {
>   	int err, fd, i;
> -	int ncpus = 1, ntasks = 1, ncgrps = 1;
> +	int ncpus = 1, ntasks = 1, ncgrps = 0;
>   	struct strlist *pid_slist = NULL;
>   	struct str_node *pos;
> +	struct evsel *evsel;
>   
>   	if (off_cpu_config(evlist) < 0) {
>   		pr_err("Failed to config off-cpu BPF event\n");
> @@ -208,8 +209,12 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target,
>   		skel->rodata->uses_tgid = 1;
>   	}
>   
> -	if (evlist__first(evlist)->cgrp) {
> -		ncgrps = evlist__nr_entries(evlist) - 1; /* excluding a dummy */
> +	evlist__for_each_entry(evlist, evsel) {
> +		if (evsel->cgrp)
> +			ncgrps++;
> +	}
> +
> +	if (ncgrps > 0) {
>   		bpf_map__set_max_entries(skel->maps.cgroup_filter, ncgrps);
>   
>   		if (!cgroup_is_v2("perf_event"))
> @@ -274,8 +279,7 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target,
>   		}
>   	}
>   
> -	if (evlist__first(evlist)->cgrp) {
> -		struct evsel *evsel;
> +	if (ncgrps > 0) {
>   		u8 val = 1;
>   
>   		fd = bpf_map__fd(skel->maps.cgroup_filter);

Reviewed-by: James Clark <james.clark@linaro.org>