Messages in this thread | | | Date | Fri, 9 Sep 2022 14:55:28 -0700 | Subject | Re: [PATCH 2/3] perf/bpf: Always use perf callchains if exist | From | sdf@google ... |
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On 09/08, Namhyung Kim wrote: > If the perf_event has PERF_SAMPLE_CALLCHAIN, BPF can use it for stack > trace. > The problematic cases like PEBS and IBS already handled in the PMU driver > and > they filled the callchain info in the sample data. For others, we can > call > perf_callchain() before the BPF handler.
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
At least from the description it make sense. We're filling a callchain when it's been requested by the event, but it's missing on the sample data (aka, software fallback?). perf_callchain also seems to always fallback to &__empty_callchain in case of an error, so seems safe.
> --- > kernel/bpf/stackmap.c | 4 ++-- > kernel/events/core.c | 12 ++++++++++-- > 2 files changed, 12 insertions(+), 4 deletions(-)
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c > index 1adbe67cdb95..aecea7451b61 100644 > --- a/kernel/bpf/stackmap.c > +++ b/kernel/bpf/stackmap.c > @@ -338,7 +338,7 @@ BPF_CALL_3(bpf_get_stackid_pe, struct > bpf_perf_event_data_kern *, ctx, > int ret;
> /* perf_sample_data doesn't have callchain, use bpf_get_stackid */ > - if (!(event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY)) > + if (!(event->attr.sample_type & PERF_SAMPLE_CALLCHAIN)) > return bpf_get_stackid((unsigned long)(ctx->regs), > (unsigned long) map, flags, 0, 0);
> @@ -506,7 +506,7 @@ BPF_CALL_4(bpf_get_stack_pe, struct > bpf_perf_event_data_kern *, ctx, > int err = -EINVAL; > __u64 nr_kernel;
> - if (!(event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY)) > + if (!(event->attr.sample_type & PERF_SAMPLE_CALLCHAIN)) > return __bpf_get_stack(regs, NULL, NULL, buf, size, flags);
> if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK | > diff --git a/kernel/events/core.c b/kernel/events/core.c > index b8af9fdbf26f..2ea93ce75ad4 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -10003,8 +10003,16 @@ static void bpf_overflow_handler(struct > perf_event *event, > goto out; > rcu_read_lock(); > prog = READ_ONCE(event->prog); > - if (prog) > + if (prog) { > + if (prog->call_get_stack && > + (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) && > + !(data->sample_flags & PERF_SAMPLE_CALLCHAIN)) { > + data->callchain = perf_callchain(event, regs); > + data->sample_flags |= PERF_SAMPLE_CALLCHAIN; > + } > + > ret = bpf_prog_run(prog, &ctx); > + } > rcu_read_unlock(); > out: > __this_cpu_dec(bpf_prog_active); > @@ -10030,7 +10038,7 @@ static int perf_event_set_bpf_handler(struct > perf_event *event,
> if (event->attr.precise_ip && > prog->call_get_stack && > - (!(event->attr.sample_type & __PERF_SAMPLE_CALLCHAIN_EARLY) || > + (!(event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) || > event->attr.exclude_callchain_kernel || > event->attr.exclude_callchain_user)) { > /* > -- > 2.37.2.789.g6183377224-goog
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