Re: [PATCH] perf trace: Refactor augmented_raw_syscalls using bpf_loop

[Viktor Malik]

On 6/29/26 22:35, Namhyung Kim wrote:
> On Thu, Jun 25, 2026 at 02:05:29PM +0200, Viktor Malik wrote:
>> On 6/24/26 21:24, Namhyung Kim wrote:
>>> On Wed, Jun 24, 2026 at 08:47:38AM +0200, Viktor Malik wrote:
>>>> On 6/23/26 19:10, Namhyung Kim wrote:
>>>>> Hello,
>>>>>
>>>>> On Tue, Jun 23, 2026 at 08:27:39AM -0700, Alexei Starovoitov wrote:
>>>>>> On Tue Jun 23, 2026 at 4:25 AM PDT, Viktor Malik wrote:
>>> [SNIP]
>>>>>>> +	struct args_loop_ctx loop_ctx = {
>>>>>>> +		.args = args,
>>>>>>> +		.beauty_map = beauty_map,
>>>>>>> +		.payload_offset = payload_offset,
>>>>>>> +		.value_size = value_size,
>>>>>>> +		.output = &output,
>>>>>>> +		.do_output = &do_output
>>>>>>> +	};
>>>>>>> +	iters = bpf_loop(6, process_arg_cb, &loop_ctx, 0);
>>>>>>
>>>>>> bpf_loop() is old and generally not recommended.
>>>>>> Please use bpf_for() then the diff will be one line change and
>>>>>> can scale to any number of args. Not just 6.
>>>>
>>>> Thanks Alexei, I didn't know about this preference.
>>>>
>>>>> One thing we should take care is to support old kernels.  The oldest
>>>>> LTS kernel in the kernel.org is 5.10 and bpf_loop() was introduced in
>>>>> 5.17 and bpf_for (bpf_iter_num) was 6.4.
>>>>
>>>> The problematic loop was introduced in 6.12 by a68fd6a6cdd3 ("perf
>>>> trace: Collect augmented data using BPF") so we should be good using
>>>> bpf_for. Or is perf from 7.2 supposed to work on 5.10 LTS kernels?
>>>
>>> Yep, we'd like to support old kernels.
>>
>> How much strict are you on this requirement? IMHO, the very least we
>> need to fix the verifier issue is bpf_loop, so that would still not work
>> on 5.10 and 5.15 LTS kernels.
> 
> I don't think it's an absolute requirement, but I think we don't want to
> break any existing working setup (old kernel + old compiler).
> 
>>
>> We could probably keep the open-coded loop in case bpf_loop is not
>> available but `perf trace` would still fail on kernels without bpf_loop
>> for new perf built with Clang>=22. Also, the code would be a bit ugly
>> and I'm not sure how well the feature check for helpers (bpf_loop) works
>> on old kernels.
>  
> Any chance process_arg_cb() can be called directly in the regular for
> loop on old kernels?

That's my thinking, too. Should be pretty straightforward, I'm going to
give it a try in v2.

Viktor