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

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Message-ID: <Z2mR-o9I3CobBoNB@x1>
Date: Mon, 23 Dec 2024 13:38:18 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Ian Rogers <irogers@...gle.com>,
	Kan Liang <kan.liang@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	"linux-perf-use." <linux-perf-users@...r.kernel.org>,
	Andrii Nakryiko <andrii@...nel.org>, Song Liu <song@...nel.org>,
	bpf <bpf@...r.kernel.org>, Stephane Eranian <eranian@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>, Kees Cook <kees@...nel.org>,
	Chun-Tse Shao <ctshao@...gle.com>
Subject: Re: [PATCH v3 2/4] perf lock contention: Run BPF slab cache iterator

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@...nel.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;