Re: [PATCH V2 2/2] bpf: control a set of perf events by creating a new ioctl PERF_EVENT_IOC_SET_ENABLER

Alexei Starovoitov
2015-10-14T21:28:55Z

On 10/14/15 5:37 AM, Kaixu Xia wrote:
> + event->p_sample_disable = &enabler_event->sample_disable;

I don't like it as a concept and it's buggy implementation.
What happens here when enabler is alive, but other event is destroyed?

> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -221,9 +221,12 @@ static u64 bpf_perf_event_sample_control(u64 r1, u64 index, u64 flag, u64 r4, u6
> struct bpf_array *array = container_of(map, struct bpf_array, map);
> struct perf_event *event;
>
> - if (unlikely(index >= array->map.max_entries))
> + if (unlikely(index > array->map.max_entries))
> return -E2BIG;
>
> + if (index == array->map.max_entries)
> + index = 0;

what is this hack for ?

Either use notification and user space disable or
call bpf_perf_event_sample_control() manually for each cpu.