Re: [PATCH bpf-next v8 2/3] perf: Refactor get_perf_callchain

[Tao Chen]

在 2026/2/4 09:08, Andrii Nakryiko 写道:
> On Sun, Jan 25, 2026 at 11:45 PM Tao Chen <chen.dylane@xxxxxxxxx> wrote:
> >  From BPF stack map, we want to ensure that the callchain buffer
> > will not be overwritten by other preemptive tasks and we also aim
> > to reduce the preempt disable interval, Based on the suggestions from Peter
> > and Andrrii, export new API __get_perf_callchain and the usage scenarios
> > are as follows from BPF side:
> >
> > preempt_disable()
> > entry = get_callchain_entry()
> > preempt_enable()
> > __get_perf_callchain(entry)
> > put_callchain_entry(entry)
> >
> > Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> > ---
> >   include/linux/perf_event.h |  5 +++++
> >   kernel/events/callchain.c  | 34 ++++++++++++++++++++++------------
> >   2 files changed, 27 insertions(+), 12 deletions(-)
>
> Looking at the whole __bpf_get_stack() logic again, why didn't we just
> do something like this:
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index da3d328f5c15..80364561611c 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -460,8 +460,8 @@ static long __bpf_get_stack(struct pt_regs *regs,
> struct task_struct *task,
>
>          max_depth = stack_map_calculate_max_depth(size, elem_size, flags);
>
> -       if (may_fault)
> -               rcu_read_lock(); /* need RCU for perf's callchain below */
> +       if (!trace_in)
> +               preempt_disable();
>
>          if (trace_in) {
>                  trace = trace_in;
> @@ -474,8 +474,8 @@ static long __bpf_get_stack(struct pt_regs *regs,
> struct task_struct *task,
>          }
>
>          if (unlikely(!trace) || trace->nr < skip) {
> -               if (may_fault)
> -                       rcu_read_unlock();
> +               if (!trace_in)
> +                       preempt_enable();
>                  goto err_fault;
>          }
>
> @@ -494,8 +494,8 @@ static long __bpf_get_stack(struct pt_regs *regs,
> struct task_struct *task,
>          }
>
>          /* trace/ips should not be dereferenced after this point */
> -       if (may_fault)
> -               rcu_read_unlock();
> +       if (!trace_in)
> +               preempt_enable();
>
>          if (user_build_id)
>                  stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
>
>
> ?
>
> Build ID parsing is happening after we copied data from perf's
> callchain_entry into user-provided memory, so raw callchain retrieval
> can be done with preemption disabled, as it's supposed to be brief.
> Build ID parsing part which indeed might fault and be much slower will
> be done well after that (we even have a comment there saying that
> trace/ips should not be touched).
>
> Am I missing something?

Yes it looks good for bpf_get_stack, and I also proposed a similar 
change previously. But Alexei suggested that we should reduce the 
preemption-disabled section protected in bpf_get_stackid if we do like 
bpf_get_stack. Maybe we can change it first for bpf_get_stack?

https://lore.kernel.org/bpf/20250922075333.1452803-1-chen.dylane@xxxxxxxxx/

> [...]


--
Best Regards
Tao Chen