netdev - Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY maps trace data output when perf sampling

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Date:	Wed, 21 Oct 2015 14:17:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	xiakaixu <xiakaixu@...wei.com>,
	Alexei Starovoitov <ast@...mgrid.com>, davem@...emloft.net,
	acme@...nel.org, mingo@...hat.com, masami.hiramatsu.pt@...achi.com,
	jolsa@...nel.org, daniel@...earbox.net,
	linux-kernel@...r.kernel.org, pi3orama@....com, hekuang@...wei.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY
 maps trace data output when perf sampling

On Wed, Oct 21, 2015 at 07:49:34PM +0800, Wangnan (F) wrote:
> If our task is sampling cycle events during a function is running,
> and if two cores start that function overlap:
> 
> Time:   ...................A
> Core 0: sys_write----\
>                       \
>                        \
> Core 1:             sys_write%return
> Core 2: ................sys_write
> 
> Then without counter at time A it is highly possible that
> BPF program on core 1 and core 2 get conflict with each other.
> The final result is we make some of those events be turned on
> and others turned off. Using atomic counter can avoid this
> problem.

But but, how and why can an eBPF program access a !local event? I
thought we had hard restrictions on that.

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