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

[Namhyung Kim]

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?

Thanks,
Namhyung