Hi Hengqi,
On Wed, Apr 26, 2023 at 9:20 PM Hengqi Chen <hengqi.chen@xxxxxxxxx> wrote:
>
> Hi, Namhyung
>
> On 2023/4/27 10:21, Namhyung Kim wrote:
> > Hello Andrii,
> >
> > On Wed, Apr 26, 2023 at 6:19 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> >>
> >> On Wed, Apr 26, 2023 at 5:14 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >>>
> >>> Hello,
> >>>
> >>> I'm having a problem of loading perf lock contention BPF program [1]
> >>> on old kernels. It has collect_lock_syms() to get the address of each
> >>> CPU's run-queue lock. The kernel 5.14 changed the name of the field
> >>> so there's bpf_core_field_exists to check the name like below.
> >>>
> >>> if (bpf_core_field_exists(rq_new->__lock))
> >>> lock_addr = (__u64)&rq_new->__lock;
> >>> else
> >>> lock_addr = (__u64)&rq_old->lock;
> >>
> >> I suspect compiler rewrites it to something like
> >>
> >> lock_addr = (__u64)&rq_old->lock;
> >> if (bpf_core_field_exists(rq_new->__lock))
> >> lock_addr = (__u64)&rq_new->__lock;
> >>
> >> so rq_old relocation always happens and ends up being not guarded
> >> properly. You can try adding barrier_var(rq_new) and
> >> barrier_var(rq_old) around if and inside branches, that should
> >> pessimize compiler
> >>
> >> alternatively if you do
> >>
> >> if (bpf_core_field_exists(rq_new->__lock))
> >> lock_addr = (__u64)&rq_new->__lock;
> >> else if (bpf_core_field_exists(rq_old->lock))
> >> lock_addr = (__u64)&rq_old->lock;
> >> else
> >> lock_addr = 0; /* or signal error somehow */
> >>
> >> It might work as well.
> >
> > Thanks a lot for your comment!
> >
> > I've tried the below code but no luck. :(
> >
> > barrier_var(rq_old);
> > barrier_var(rq_new);
> >
> > if (bpf_core_field_exists(rq_old->lock)) {
> > barrier_var(rq_old);
> > lock_addr = (__u64)&rq_old->lock;
>
> Have you tried `BPF_CORE_READ(rq_old, lock)` ?
No, but I think it'd dereference the lock, right?
I just want to get the address of the lock field.
Thanks,
Namhyung
>
> > } else if (bpf_core_field_exists(rq_new->__lock)) {
> > barrier_var(rq_new);
> > lock_addr = (__u64)&rq_new->__lock;
> > } else
> > lock_addr = 0;
> >
> >
> > ; int BPF_PROG(collect_lock_syms)
> > 0: (b7) r8 = 0 ; R8_w=0
> > 1: (b7) r7 = 1 ; R7_w=1
> > 2: <invalid CO-RE relocation>
> > failed to resolve CO-RE relocation <byte_off> [381] struct
> > rq___old.lock (0:0 @ offset 0)
> > processed 3 insns (limit 1000000) max_states_per_insn 0 total_states 0
> > peak_states 0 mark_read
> >
> > Thanks,
> > Namhyung
>
> Cheers,
> Hengqi
