Re: [PATCH bpf-next v4 2/2] bpf: Use per-cpu BPF callchain entry to save callchain

[Alexei Starovoitov]

On Tue, Oct 21, 2025 at 9:07 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote:
>
> As Alexei noted, get_perf_callchain() return values may be reused
> if a task is preempted after the BPF program enters migrate disable
> mode. Drawing on the per-cpu design of bpf_bprintf_buffers,
> per-cpu BPF callchain entry is used here.
>
> Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> ---
>  kernel/bpf/stackmap.c | 98 ++++++++++++++++++++++++++++++++-----------
>  1 file changed, 73 insertions(+), 25 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 94e46b7f340..97028d39df1 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -31,6 +31,52 @@ struct bpf_stack_map {
>         struct stack_map_bucket *buckets[] __counted_by(n_buckets);
>  };
>
> +struct bpf_perf_callchain_entry {
> +       u64 nr;
> +       u64 ip[PERF_MAX_STACK_DEPTH];
> +};
> +
> +#define MAX_PERF_CALLCHAIN_PREEMPT 3
> +static DEFINE_PER_CPU(struct bpf_perf_callchain_entry[MAX_PERF_CALLCHAIN_PREEMPT],
> +                     bpf_perf_callchain_entries);
> +static DEFINE_PER_CPU(int, bpf_perf_callchain_preempt_cnt);

This is too much extra memory. Above adds 1k * 3 * num_cpus.
Let's reuse perf callchains.
Especially since they're controlled by perf_event_max_stack sysctl.
See Peter's suggestion in v3.
And for the future don't respin so quickly.