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SubjectRe: [PATCH 2/3] perf/bpf: Always use perf callchains if exist
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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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