Re: [PATCH bpf-next RFC 0/2] Pass external callchain entry to get_perf_callchain

[Tao Chen]

在 2025/10/14 05:37, Yonghong Song 写道:
> On 10/13/25 1:41 PM, Jiri Olsa wrote:
> > On Tue, Oct 14, 2025 at 01:47:19AM +0800, Tao Chen wrote:
> > > Background
> > > ==========
> > > Alexei noted we should use preempt_disable to protect get_perf_callchain
> > > in bpf stackmap.
> > > https://lore.kernel.org/bpf/CAADnVQ+s8B7-fvR1TNO- 
> > > bniSyKv57cH_ihRszmZV7pQDyV=VDQ@xxxxxxxxxxxxxx
> > >
> > > A previous patch was submitted to attempt fixing this issue. And Andrii
> > > suggested teach get_perf_callchain to let us pass that buffer 
> > > directly to
> > > avoid that unnecessary copy.
> > > https://lore.kernel.org/bpf/20250926153952.1661146-1- 
> > > chen.dylane@xxxxxxxxx
> > >
> > > Proposed Solution
> > > =================
> > > Add external perf_callchain_entry parameter for get_perf_callchain to
> > > allow us to use external buffer from BPF side. The biggest advantage is
> > > that it can reduce unnecessary copies.
> > >
> > > Todo
> > > ====
> > > If the above changes are reasonable, it seems that 
> > > get_callchain_entry_for_task
> > > could also use an external perf_callchain_entry.
> > >
> > > But I'm not sure if this modification is appropriate. After all, the
> > > implementation of get_callchain_entry in the perf subsystem seems 
> > > much more
> > > complex than directly using an external buffer.
> > >
> > > Comments and suggestions are always welcome.
> > >
> > > Tao Chen (2):
> > >    perf: Use extern perf_callchain_entry for get_perf_callchain
> > >    bpf: Pass external callchain entry to get_perf_callchain
> > hi,
> > I can't get this applied on bpf-next/master, what do I miss?
>
> This path is not based on top of latest bpf/bpf-next tree.
> The current diff:
>
>   struct perf_callchain_entry *
> -get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool 
> user,
> -           u32 max_stack, bool crosstask, bool add_mark)
> +get_perf_callchain(struct pt_regs *regs, struct perf_callchain_entry 
> *external_entry,
> +           u32 init_nr, bool kernel, bool user, u32 max_stack, bool 
> crosstask,
> +           bool add_mark)
>   {
>
> The actual signature in kernel/events/callchain.c
>
> struct perf_callchain_entry *
> get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
>                     u32 max_stack, bool crosstask, bool add_mark)
> {
>
>
> > thanks,
> > jirka
> >
> >
> > >   include/linux/perf_event.h |  5 +++--
> > >   kernel/bpf/stackmap.c      | 19 +++++++++++--------
> > >   kernel/events/callchain.c  | 18 ++++++++++++------
> > >   kernel/events/core.c       |  2 +-
> > >   4 files changed, 27 insertions(+), 17 deletions(-)
> > >
> > > --
> > > 2.48.1

My mistake. I’ll update the code and resend it.

--
Best Regards
Tao Chen