Re: [PATCH v3 2/4] perf lock contention: Run BPF slab cache iterator

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Sat, Dec 21, 2024 at 03:55:32PM -0800, Namhyung Kim wrote:
> Hi Alexei,
> 
> On Fri, Dec 20, 2024 at 03:52:36PM -0800, Alexei Starovoitov wrote:
> > On Thu, Dec 19, 2024 at 10:01 PM Namhyung Kim <namhyung@kernel.org> wrote:
> > > +struct bpf_iter__kmem_cache___new {
> > > +       struct kmem_cache *s;
> > > +} __attribute__((preserve_access_index));
> > > +
> > > +SEC("iter/kmem_cache")
> > > +int slab_cache_iter(void *ctx)
> > > +{
> > > +       struct kmem_cache *s = NULL;
> > > +       struct slab_cache_data d;
> > > +       const char *nameptr;
> > > +
> > > +       if (bpf_core_type_exists(struct bpf_iter__kmem_cache)) {
> > > +               struct bpf_iter__kmem_cache___new *iter = ctx;
> > > +
> > > +               s = BPF_CORE_READ(iter, s);
> > > +       }
> > > +
> > > +       if (s == NULL)
> > > +               return 0;
> > > +
> > > +       nameptr = BPF_CORE_READ(s, name);
> > 
> > since the feature depends on the latest kernel please use
> > direct access. There is no need to use BPF_CORE_READ() to
> > be compatible with old kernels.
> > Just iter->s and s->name will work and will be much faster.
> > Underneath these loads will be marked with PROBE_MEM flag and
> > will be equivalent to probe_read_kernel calls, but faster
> > since the whole thing will be inlined by JITs.
> 
> Oh, thanks for your review.  I thought it was requried, but it'd
> be definitely better if we can access them directly.  I'll fold
> the below to v4, unless Arnaldo does it first. :)

I'll check and adjust, thanks everybody :-)

- Arnaldo
 
> Thanks,
> Namhyung
> 
> 
> ---8<---
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index 6c771ef751d83b43..6533ea9b044c71d1 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -635,13 +635,13 @@ int slab_cache_iter(void *ctx)
>         if (bpf_core_type_exists(struct bpf_iter__kmem_cache)) {
>                 struct bpf_iter__kmem_cache___new *iter = ctx;
>  
> -               s = BPF_CORE_READ(iter, s);
> +               s = iter->s;
>         }
>  
>         if (s == NULL)
>                 return 0;
>  
> -       nameptr = BPF_CORE_READ(s, name);
> +       nameptr = s->name;
>         bpf_probe_read_kernel_str(d.name, sizeof(d.name), nameptr);
>  
>         d.id = ++slab_cache_id << LCB_F_SLAB_ID_SHIFT;