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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Message-ID: <20151021115630.GO17308@twins.programming.kicks-ass.net>
Date:	Wed, 21 Oct 2015 13:56:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	Kaixu Xia <xiakaixu@...wei.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 07:34:28PM +0800, Wangnan (F) wrote:
> >If you want to actually disable the event: pmu->stop() will make it
> >stop, and you can restart using pmu->start().xiezuo
> 
> I also prefer totally disabling event because our goal is to reduce
> sampling overhead as mush as possible. However, events in perf is
> CPU bounded, one event in perf cmdline becomes multiple 'perf_event'
> in kernel in multi-core system. Disabling/enabling events on all CPUs
> by a BPF program a hard task due to racing, NMI, ...

But eBPF perf events must already be local afaik. Look at the
constraints perf_event_read_local() places on the events.
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