Re: [PATCH v12 04/10] perf record --off-cpu: Dump off-cpu samples in BPF

[Howard Chu]

Hello Namhyung,

On Thu, Dec 12, 2024 at 1:03 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Wed, Dec 11, 2024 at 06:24:14PM -0800, Howard Chu wrote:
> > Collect tid, period, callchain, and cgroup id and dump them when off-cpu
> > time threshold is reached.
> >
> > We don't collect the off-cpu time twice (the delta), it's either in
> > direct samples, or accumulated samples that are dumped at the end of
> > perf.data.
> >
> > Suggested-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > Signed-off-by: Howard Chu <howardchu95@xxxxxxxxx>
> > Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> > Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> > Cc: Ian Rogers <irogers@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: James Clark <james.clark@xxxxxxxxxx>
> > Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> > Cc: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20241108204137.2444151-6-howardchu95@xxxxxxxxx
> > Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > ---
> >  tools/perf/util/bpf_skel/off_cpu.bpf.c | 86 ++++++++++++++++++++++++--
> >  1 file changed, 81 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > index c87132e01eb3..aae63d999abb 100644
> > --- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > +++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > @@ -19,11 +19,17 @@
> >  #define MAX_ENTRIES  102400
> >
> >  #define MAX_CPUS  4096
> > +#define MAX_OFFCPU_LEN 37
> > +
> > +struct stack {
> > +     u64 array[MAX_STACKS];
> > +};
> >
> >  struct tstamp_data {
> >       __u32 stack_id;
> >       __u32 state;
> >       __u64 timestamp;
> > +     struct stack stack;
> >  };
> >
> >  struct offcpu_key {
> > @@ -41,6 +47,10 @@ struct {
> >       __uint(max_entries, MAX_ENTRIES);
> >  } stacks SEC(".maps");
> >
> > +struct offcpu_data {
> > +     u64 array[MAX_OFFCPU_LEN];
> > +};
> > +
> >  struct {
> >       __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
> >       __uint(key_size, sizeof(__u32));
> > @@ -48,6 +58,13 @@ struct {
> >       __uint(max_entries, MAX_CPUS);
> >  } offcpu_output SEC(".maps");
> >
> > +struct {
> > +     __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> > +     __uint(key_size, sizeof(__u32));
> > +     __uint(value_size, sizeof(struct offcpu_data));
> > +     __uint(max_entries, 1);
> > +} offcpu_payload SEC(".maps");
> > +
> >  struct {
> >       __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
> >       __uint(map_flags, BPF_F_NO_PREALLOC);
> > @@ -194,6 +211,47 @@ static inline int can_record(struct task_struct *t, int state)
> >       return 1;
> >  }
> >
> > +static inline int copy_stack(struct stack *from, struct offcpu_data *to, int n)
> > +{
> > +     int len = 0;
> > +
> > +     for (int i = 0; i < MAX_STACKS && from->array[i]; ++i, ++len)
> > +             to->array[n + 2 + i] = from->array[i];
> > +
> > +     return len;
> > +}
> > +
> > +/**
> > + * off_cpu_dump - dump off-cpu samples to ring buffer
> > + * @data: payload for dumping off-cpu samples
> > + * @key: off-cpu data
> > + * @stack: stack trace of the task before being scheduled out
> > + *
> > + * If the threshold of off-cpu time is reached, acquire tid, period, callchain, and cgroup id
> > + * information of the task, and dump it as a raw sample to perf ring buffer
> > + */
> > +static int off_cpu_dump(void *ctx, struct offcpu_data *data, struct offcpu_key *key,
> > +                     struct stack *stack, __u64 delta)
> > +{
> > +     int n = 0, len = 0;
> > +
> > +     data->array[n++] = (u64)key->tgid << 32 | key->pid;
> > +     data->array[n++] = delta;
> > +
> > +     /* data->array[n] is callchain->nr (updated later) */
> > +     data->array[n + 1] = PERF_CONTEXT_USER;
> > +     data->array[n + 2] = 0;
> > +     len = copy_stack(stack, data, n);
> > +
> > +     /* update length of callchain */
> > +     data->array[n] = len + 1;
> > +     n += len + 2;
> > +
> > +     data->array[n++] = key->cgroup_id;
> > +
> > +     return bpf_perf_event_output(ctx, &offcpu_output, BPF_F_CURRENT_CPU, data, n * sizeof(u64));
> > +}
> > +
> >  static int off_cpu_stat(u64 *ctx, struct task_struct *prev,
> >                       struct task_struct *next, int state)
> >  {
> > @@ -218,6 +276,16 @@ static int off_cpu_stat(u64 *ctx, struct task_struct *prev,
> >       pelem->state = state;
> >       pelem->stack_id = stack_id;
> >
> > +     /*
> > +      * If stacks are successfully collected by bpf_get_stackid(), collect them once more
> > +      * in task_storage for direct off-cpu sample dumping
> > +      */
> > +     if (stack_id > 0 && bpf_get_stack(ctx, &pelem->stack, MAX_STACKS * sizeof(u64), BPF_F_USER_STACK)) {
> > +             /*
> > +              * This empty if block is used to avoid 'result unused warning' from bpf_get_stack().
> > +              * If the collection fails, continue with the logic for the next task.
> > +              */
> > +     }
> >  next:
> >       pelem = bpf_task_storage_get(&tstamp, next, NULL, 0);
> >
> > @@ -232,11 +300,19 @@ static int off_cpu_stat(u64 *ctx, struct task_struct *prev,
> >               __u64 delta = ts - pelem->timestamp;
> >               __u64 *total;
> >
> > -             total = bpf_map_lookup_elem(&off_cpu, &key);
> > -             if (total)
> > -                     *total += delta;
> > -             else
> > -                     bpf_map_update_elem(&off_cpu, &key, &delta, BPF_ANY);
> > +             if (delta >= offcpu_thresh_ns) {
>
> It seems offcpu_thresh_ns is not defined at this moment.  You can use
> a hard-coded value with a comment for now.

My bad, I wonder how it builds, turns out I put the definition in the
previous patch... (link:
https://lore.kernel.org/linux-perf-users/20241212022420.1035999-4-howardchu95@xxxxxxxxx/T/#u)
I will move it to this patch, thanks a lot!

Thanks,
Howard