Re: [PATCH v7 05/10] perf record --off-cpu: Dump off-cpu samples in BPF

[Howard Chu]

Hi Ian,

On Mon, Nov 11, 2024 at 9:54 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Fri, Nov 8, 2024 at 12:41 PM Howard Chu <howardchu95@xxxxxxxxx> 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>
> > ---
> >  tools/perf/util/bpf_skel/off_cpu.bpf.c | 83 ++++++++++++++++++++++++--
> >  1 file changed, 78 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 dc6acafb9353..bf652c30b1c9 100644
> > --- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > +++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> > @@ -18,10 +18,18 @@
> >  #define MAX_STACKS   32
> >  #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 {
> > @@ -39,6 +47,24 @@ 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));
> > +       __uint(value_size, sizeof(__u32));
> > +       __uint(max_entries, MAX_CPUS);
> > +} offcpu_output SEC(".maps");
>
> Does patch 4 build without this definition? (we're in patch 5 here). I
> think this should be in patch 4.

Okay sure thanks :)

>
> > +
> > +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);
> > @@ -185,6 +211,39 @@ 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;
> > +}
> > +
>
> Dump is something of a generic name. Could you kernel-doc this
> function to describe the behavior?

Sure.

>
> > +static int off_cpu_dump(void *ctx, struct offcpu_data *data, struct offcpu_key *key,
> > +                       struct stack *stack, __u64 delta, __u64 timestamp)
> > +{
> > +       /* dump tid, period, callchain, and cgroup id */
> > +       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)
> >  {
> > @@ -209,6 +268,12 @@ 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)) {
> > +       }
>
> Why the empty if?

Forgot to say in the commit message, bpf_get_stack() has a return
value and if I don't use it I'll get result unused warning from clang,
so either this or:
    int __attribute__((unused)) len;
    len = bpf_get_stack(ctx, stack_p->array, MAX_STACKS * sizeof(u64),

We don't need error handling, it goes to "next:" naturally, there's no
code in between.

>
> > +
> >  next:
> >         pelem = bpf_task_storage_get(&tstamp, next, NULL, 0);
> >
> > @@ -223,11 +288,19 @@ 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) {
> > +                       int zero = 0;
> > +                       struct offcpu_data *data = bpf_map_lookup_elem(&offcpu_payload, &zero);
> > +
> > +                       if (data)
> > +                               off_cpu_dump(ctx, data, &key, &pelem->stack, delta, pelem->timestamp);
> > +               } else {
> > +                       total = bpf_map_lookup_elem(&off_cpu, &key);
> > +                       if (total)
> > +                               *total += delta;
> > +                       else
> > +                               bpf_map_update_elem(&off_cpu, &key, &delta, BPF_ANY);
> > +               }
>
> Looks good! :-)
>
> Thanks,
> Ian
>
> >
> >                 /* prevent to reuse the timestamp later */
> >                 pelem->timestamp = 0;
> > --
> > 2.43.0
> >

Thanks,
Howard