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

[Viktor Malik <vmalik@redhat.com>]

On 6/30/26 07:42, Viktor Malik wrote:
> 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.

Btw, I just noticed that util/bpf_skel/lock_contention.bpf.c already
uses bpf_loop without any fallback so newer perf (at least `perf lock`)
won't be usable on kernels without bpf_loop anyways.