linux-kernel - Re: [PATCH v7 04/10] perf record --off-cpu: Preparation of off-cpu BPF program

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Message-ID: <CAH0uvogqT9w7nY7gCyD_agXzTvfbGr1ywrBf-U2OPB2ZN9OqTQ@mail.gmail.com>
Date: Tue, 12 Nov 2024 12:52:29 -0800
From: Howard Chu <howardchu95@...il.com>
To: Ian Rogers <irogers@...gle.com>
Cc: acme@...nel.org, peterz@...radead.org, namhyung@...nel.org, 
	mingo@...hat.com, mark.rutland@....com, james.clark@...aro.org, 
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org, adrian.hunter@...el.com, 
	kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 04/10] perf record --off-cpu: Preparation of off-cpu
 BPF program

Hi Ian,


On Mon, Nov 11, 2024 at 9:47 AM Ian Rogers <irogers@...gle.com> wrote:
>
> On Fri, Nov 8, 2024 at 12:41 PM Howard Chu <howardchu95@...il.com> wrote:
> >
> > Set the perf_event map in BPF for dumping off-cpu samples.
> >
> > Set the offcpu_thresh to specify the threshold.
> >
> > Signed-off-by: Howard Chu <howardchu95@...il.com>
> > ---
> >  tools/perf/util/bpf_off_cpu.c          | 20 ++++++++++++++++++++
> >  tools/perf/util/bpf_skel/off_cpu.bpf.c |  2 ++
> >  2 files changed, 22 insertions(+)
> >
> > diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
> > index 558c5e5c2dc3..cfe5b17393e9 100644
> > --- a/tools/perf/util/bpf_off_cpu.c
> > +++ b/tools/perf/util/bpf_off_cpu.c
> > @@ -13,6 +13,7 @@
> >  #include "util/cgroup.h"
> >  #include "util/strlist.h"
> >  #include <bpf/bpf.h>
> > +#include <internal/xyarray.h>
> >
> >  #include "bpf_skel/off_cpu.skel.h"
> >
> > @@ -73,6 +74,23 @@ static void off_cpu_start(void *arg)
> >                 bpf_map_update_elem(fd, &pid, &val, BPF_ANY);
> >         }
> >
> > +       /* update BPF perf_event map */
> > +       evsel = evlist__find_evsel_by_str(evlist, OFFCPU_EVENT);
> > +       if (evsel == NULL) {
> > +               pr_err("%s evsel not found\n", OFFCPU_EVENT);
> > +               return;
> > +       }
> > +
> > +       perf_cpu_map__for_each_cpu(pcpu, i, evsel->core.cpus) {
> > +               err = bpf_map__update_elem(skel->maps.offcpu_output, &pcpu.cpu, sizeof(__u32),
> > +                                          xyarray__entry(evsel->core.fd, i, 0),
> > +                                          sizeof(__u32), BPF_ANY);
> > +               if (err) {
> > +                       pr_err("Failed to update perf event map for direct off-cpu dumping\n");
> > +                       return;
> > +               }
> > +       }
> > +
> >         skel->bss->enabled = 1;
> >  }
> >
> > @@ -261,6 +279,8 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target,
> >                 }
> >         }
> >
> > +       skel->bss->offcpu_thresh = opts->off_cpu_thresh * 1000;
> > +
> >         err = off_cpu_bpf__attach(skel);
> >         if (err) {
> >                 pr_err("Failed to attach off-cpu BPF skeleton\n");
> > diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > index c152116df72f..dc6acafb9353 100644
> > --- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > +++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > @@ -97,6 +97,8 @@ const volatile bool uses_cgroup_v1 = false;
> >
> >  int perf_subsys_id = -1;
> >
> > +__u64 offcpu_thresh;
>
> nit: would be nice to include the unit in the variable name, ie
> offcpu_thresh_us.

I didn't see it yesterday sorry, but in BPF the unit should be
nanosecond, so I'll do offcpu_thresh_ns.

Thanks,
Howard

>
> Reviewed-by: Ian Rogers <irogers@...gle.com>
>
> Thanks,
> Ian
>
> > +
> >  /*
> >   * Old kernel used to call it task_struct->state and now it's '__state'.
> >   * Use BPF CO-RE "ignored suffix rule" to deal with it like below:
> > --
> > 2.43.0
> >