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@mail.gmail.com

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@linux.dev

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.

Change list:
 - v1 -> v2
   From Jiri
   - rebase code, fix conflict
 - v1: https://lore.kernel.org/bpf/20251013174721.2681091-1-chen.dylane@linux.dev
 
 - v2 -> v3:
   From Andrii
   - entries per CPU used in a stack-like fashion
 - v2: https://lore.kernel.org/bpf/20251014100128.2721104-1-chen.dylane@linux.dev

Tao Chen (2):
  perf: Use extern perf_callchain_entry for get_perf_callchain
  bpf: Use per-cpu BPF callchain entry to save callchain

 include/linux/perf_event.h |   4 +-
 kernel/bpf/stackmap.c      | 100 ++++++++++++++++++++++++++++---------
 kernel/events/callchain.c  |  13 +++--
 kernel/events/core.c       |   2 +-
 4 files changed, 88 insertions(+), 31 deletions(-)

-- 
2.48.1