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

Tao Chen
2025-10-13T17:47:57Z

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.

Tao Chen (2):
perf: Use extern perf_callchain_entry for get_perf_callchain
bpf: Pass external callchain entry to get_perf_callchain

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(-)

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2.48.1