Re: [PATCH bpf-next v8 2/3] perf: Refactor get_perf_callchain

[Tao Chen <chen.dylane@gmail.com>]

在 2026/2/6 01:34, Andrii Nakryiko 写道:
> On Wed, Feb 4, 2026 at 10:16 PM Tao Chen <chen.dylane@linux.dev> wrote:
>>
>> 在 2026/2/4 09:08, Andrii Nakryiko 写道:
>>> On Sun, Jan 25, 2026 at 11:45 PM Tao Chen <chen.dylane@linux.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@kernel.org>
>>>> Signed-off-by: Tao Chen <chen.dylane@linux.dev>
>>>> ---
>>>>    include/linux/perf_event.h |  5 +++++
>>>>    kernel/events/callchain.c  | 34 ++++++++++++++++++++++------------
>>>>    2 files changed, 27 insertions(+), 12 deletions(-)
>>>>
>>>
>>> Looking at the whole __bpf_get_stack() logic again, why didn't we just
>>> do something like this:
>>>
>>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>>> index da3d328f5c15..80364561611c 100644
>>> --- a/kernel/bpf/stackmap.c
>>> +++ b/kernel/bpf/stackmap.c
>>> @@ -460,8 +460,8 @@ static long __bpf_get_stack(struct pt_regs *regs,
>>> struct task_struct *task,
>>>
>>>           max_depth = stack_map_calculate_max_depth(size, elem_size, flags);
>>>
>>> -       if (may_fault)
>>> -               rcu_read_lock(); /* need RCU for perf's callchain below */
>>> +       if (!trace_in)
>>> +               preempt_disable();
>>>
>>>           if (trace_in) {
>>>                   trace = trace_in;
>>> @@ -474,8 +474,8 @@ static long __bpf_get_stack(struct pt_regs *regs,
>>> struct task_struct *task,
>>>           }
>>>
>>>           if (unlikely(!trace) || trace->nr < skip) {
>>> -               if (may_fault)
>>> -                       rcu_read_unlock();
>>> +               if (!trace_in)
>>> +                       preempt_enable();
>>>                   goto err_fault;
>>>           }
>>>
>>> @@ -494,8 +494,8 @@ static long __bpf_get_stack(struct pt_regs *regs,
>>> struct task_struct *task,
>>>           }
>>>
>>>           /* trace/ips should not be dereferenced after this point */
>>> -       if (may_fault)
>>> -               rcu_read_unlock();
>>> +       if (!trace_in)
>>> +               preempt_enable();
>>>
>>>           if (user_build_id)
>>>                   stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
>>>
>>>
>>> ?
>>>
>>> Build ID parsing is happening after we copied data from perf's
>>> callchain_entry into user-provided memory, so raw callchain retrieval
>>> can be done with preemption disabled, as it's supposed to be brief.
>>> Build ID parsing part which indeed might fault and be much slower will
>>> be done well after that (we even have a comment there saying that
>>> trace/ips should not be touched).
>>>

Hi Andrii,

Building upon your work, I have also added preempt_disable() to 
bpf_get_stackid, and try to reduce the length of preempt section.
Please review, thanks.

https://lore.kernel.org/bpf/20260206090653.1336687-1-chen.dylane@linux.dev

>>> Am I missing something?
>>
>> Yes it looks good for bpf_get_stack, and I also proposed a similar
>> change previously. But Alexei suggested that we should reduce the
>> preemption-disabled section protected in bpf_get_stackid if we do like
>> bpf_get_stack. Maybe we can change it first for bpf_get_stack?
>>
>> https://lore.kernel.org/bpf/20250922075333.1452803-1-chen.dylane@linux.dev/
> 
> This is broken because we are still using trace after you re-enabled
> preemption. We need to keep preemption disabled until we copy captured
> stack trace data from ips into our own memory.
> 
>>
>>>
>>> [...]
>>
>>
>> --
>> Best Regards
>> Tao Chen
> 


-- 
Best Regards
Tao Chen