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

[Viktor Malik <vmalik@redhat.com>]

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.

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.

Viktor