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

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Message-ID: <3cf98c31-4475-4e4a-8ce0-bc9c62922313@linux.dev>
Date: Mon, 13 Oct 2025 14:37:54 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Jiri Olsa <olsajiri@...il.com>, 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, 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 10/13/25 1:41 PM, Jiri Olsa wrote:
> 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?

This path is not based on top of latest bpf/bpf-next tree.
The current diff:

  struct perf_callchain_entry *
-get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
-		   u32 max_stack, bool crosstask, bool add_mark)
+get_perf_callchain(struct pt_regs *regs, struct perf_callchain_entry *external_entry,
+		   u32 init_nr, bool kernel, bool user, u32 max_stack, bool crosstask,
+		   bool add_mark)
  {

The actual signature in kernel/events/callchain.c

struct perf_callchain_entry *
get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
                    u32 max_stack, bool crosstask, bool add_mark)
{


>
> 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
>>