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

[Tao Chen]

在 2025/10/22 01:28, Alexei Starovoitov 写道:
> 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.

Ok, let's base our discussion on v3, sorry for the overly rapid version 
iterations impacted the maintainers' review process, i will respin 
slowly, thanks.

--
Best Regards
Tao Chen