Re: [PATCH] perf stat: Support old kernels for bperf cgroup counting

[Jiri Olsa]

On Wed, Sep 21, 2022 at 09:14:35PM -0700, Namhyung Kim wrote:
> The recent change in the cgroup will break the backward compatiblity in
> the BPF program.  It should support both old and new kernels using BPF
> CO-RE technique.
> 
> Like the task_struct->__state handling in the offcpu analysis, we can
> check the field name in the cgroup struct.
> 
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>

lgtm

Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>

jirka

> ---
> Arnaldo, I think this should go through the cgroup tree since it depends
> on the earlier change there.  I don't think it'd conflict with other
> perf changes but please let me know if you see any trouble, thanks!
> 
>  tools/perf/util/bpf_skel/bperf_cgroup.bpf.c | 29 ++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> index 488bd398f01d..4fe61043de04 100644
> --- a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> +++ b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> @@ -43,12 +43,39 @@ struct {
>  	__uint(value_size, sizeof(struct bpf_perf_event_value));
>  } cgrp_readings SEC(".maps");
>  
> +/* new kernel cgroup definition */
> +struct cgroup___new {
> +	int level;
> +	struct cgroup *ancestors[];
> +} __attribute__((preserve_access_index));
> +
> +/* old kernel cgroup definition */
> +struct cgroup___old {
> +	int level;
> +	u64 ancestor_ids[];
> +} __attribute__((preserve_access_index));
> +
>  const volatile __u32 num_events = 1;
>  const volatile __u32 num_cpus = 1;
>  
>  int enabled = 0;
>  int use_cgroup_v2 = 0;
>  
> +static inline __u64 get_cgroup_v1_ancestor_id(struct cgroup *cgrp, int level)
> +{
> +	/* recast pointer to capture new type for compiler */
> +	struct cgroup___new *cgrp_new = (void *)cgrp;
> +
> +	if (bpf_core_field_exists(cgrp_new->ancestors)) {
> +		return BPF_CORE_READ(cgrp_new, ancestors[level], kn, id);
> +	} else {
> +		/* recast pointer to capture old type for compiler */
> +		struct cgroup___old *cgrp_old = (void *)cgrp;
> +
> +		return BPF_CORE_READ(cgrp_old, ancestor_ids[level]);
> +	}
> +}
> +
>  static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
>  {
>  	struct task_struct *p = (void *)bpf_get_current_task();
> @@ -70,7 +97,7 @@ static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
>  			break;
>  
>  		// convert cgroup-id to a map index
> -		cgrp_id = BPF_CORE_READ(cgrp, ancestors[i], kn, id);
> +		cgrp_id = get_cgroup_v1_ancestor_id(cgrp, i);
>  		elem = bpf_map_lookup_elem(&cgrp_idx, &cgrp_id);
>  		if (!elem)
>  			continue;
> -- 
> 2.37.3.968.ga6b4b080e4-goog
>