linux-kernel - [PATCH bpf-next RFC 0/2] Pass external callchain entry to get_perf

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Message-ID: <20251013174721.2681091-1-chen.dylane@linux.dev>
Date: Tue, 14 Oct 2025 01:47:19 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: peterz@...radead.org,
	mingo@...hat.com,
	acme@...nel.org,
	namhyung@...nel.org,
	mark.rutland@....com,
	alexander.shishkin@...ux.intel.com,
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	irogers@...gle.com,
	adrian.hunter@...el.com,
	kan.liang@...ux.intel.com,
	song@...nel.org,
	ast@...nel.org,
	daniel@...earbox.net,
	andrii@...nel.org,
	martin.lau@...ux.dev,
	eddyz87@...il.com,
	yonghong.song@...ux.dev,
	john.fastabend@...il.com,
	kpsingh@...nel.org,
	sdf@...ichev.me,
	haoluo@...gle.com
Cc: linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org,
	Tao Chen <chen.dylane@...ux.dev>
Subject: [PATCH bpf-next RFC 0/2] Pass external callchain entry to get_perf_callchain

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(-)

-- 
2.48.1