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

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Message-ID: <cb3b054b-3c21-4941-800c-4519cae9ce31@linux.dev>
Date: Tue, 14 Oct 2025 13:36:44 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: Yonghong Song <yonghong.song@...ux.dev>, Jiri Olsa <olsajiri@...il.com>
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

在 2025/10/14 05:37, Yonghong Song 写道:
> 
> 
> 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@...l.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@...ux.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
>>>
> 

My mistake. I’ll update the code and resend it.

-- 
Best Regards
Tao Chen