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

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Message-ID: <aO1j747N7pkBTBAb@krava>
Date: Mon, 13 Oct 2025 22:41:19 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
	namhyung@...nel.org, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, 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, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next RFC 0/2] Pass external callchain entry to
 get_perf_callchain

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@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

hi,
I can't get this applied on bpf-next/master, what do I miss?

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
>