linux-kernel - [PATCH bpf-next v4 0/2] Pass external callchain entry to get_perf

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Message-ID: <20251028162502.3418817-1-chen.dylane@linux.dev>
Date: Wed, 29 Oct 2025 00:25:00 +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,
	jolsa@...nel.org,
	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 v4 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
====
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

 - v3 -> v4:
   From Peter
   - refactor get_perf_callchain and add three new APIs to use perf
     callchain easily.
   From Andrii
   - reuse the perf callchain management.

   - rename patch1 and patch2.
 - v3: https://lore.kernel.org/bpf/20251019170118.2955346-1-chen.dylane@linux.dev

Tao Chen (2):
  perf: Refactor get_perf_callchain
  bpf: Hold the perf callchain entry until used completely

 include/linux/perf_event.h | 11 +++++-
 kernel/bpf/stackmap.c      | 61 ++++++++++++++++++++++++-------
 kernel/events/callchain.c  | 75 ++++++++++++++++++++++++--------------
 kernel/events/core.c       |  2 +-
 4 files changed, 107 insertions(+), 42 deletions(-)

-- 
2.48.1