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

[Namhyung Kim]

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@xxxxxxxxxx> 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. :)

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;