linux-kernel - Re: [PATCH bpf-next v8 2/3] perf: Refactor get_perf

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Message-ID: <3b84f222-8d4c-4bb6-b224-946cd934ba56@linux.dev>
Date: Wed, 28 Jan 2026 10:42:53 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: 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, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v8 2/3] perf: Refactor get_perf_callchain

在 2026/1/28 05:07, Andrii Nakryiko 写道:
> On Sun, Jan 25, 2026 at 11:45 PM Tao Chen <chen.dylane@...ux.dev> wrote:
>>
>>  From BPF stack map, we want to ensure that the callchain buffer
>> will not be overwritten by other preemptive tasks and we also aim
>> to reduce the preempt disable interval, Based on the suggestions from Peter
>> and Andrrii, export new API __get_perf_callchain and the usage scenarios
>> are as follows from BPF side:
>>
>> preempt_disable()
>> entry = get_callchain_entry()
>> preempt_enable()
>> __get_perf_callchain(entry)
>> put_callchain_entry(entry)
>>
>> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
>> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
>> ---
>>   include/linux/perf_event.h |  5 +++++
>>   kernel/events/callchain.c  | 34 ++++++++++++++++++++++------------
>>   2 files changed, 27 insertions(+), 12 deletions(-)
> 
> [...]
> 
>> +struct perf_callchain_entry *
>> +get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
>> +                  u32 max_stack, bool crosstask, bool add_mark, u64 defer_cookie)
>> +{
>> +       struct perf_callchain_entry *entry;
>> +       int ret;
>> +
>> +       entry = get_callchain_entry();
>> +       if (!entry)
>> +               return NULL;
>>
>> -exit_put:
>> +       ret = __get_perf_callchain(entry, regs, kernel, user, max_stack, crosstask, add_mark,
>> +                                  defer_cookie);
>>          put_callchain_entry(entry);
>> +       if (ret)
>> +               entry = NULL;
>>
> 
> purely stylistical nit, so this can be ignored if you disagree, but I
> find code that modifies some variable before returning it slightly
> less preferable to more explicit:
> 
> 
> if (__get_perf_callchain(...)) {
>      put_callchain_entry(entry);
>      return NULL;
> }
> 
> return entry;
> 
>>          return entry;
>>   }
> 

agree, will change it in v9, thanks.

>> --
>> 2.48.1
>>


-- 
Best Regards
Tao Chen