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

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Date:	Wed, 14 Oct 2015 14:28:55 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Kaixu Xia <xiakaixu@...wei.com>, davem@...emloft.net,
	acme@...nel.org, mingo@...hat.com, a.p.zijlstra@...llo.nl,
	masami.hiramatsu.pt@...achi.com, jolsa@...nel.org,
	daniel@...earbox.net
Cc:	wangnan0@...wei.com, linux-kernel@...r.kernel.org,
	pi3orama@....com, hekuang@...wei.com, netdev@...r.kernel.org
Subject: Re: [PATCH V2 2/2] bpf: control a set of perf events by creating a
 new ioctl PERF_EVENT_IOC_SET_ENABLER

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.


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