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

From: Peter Zijlstra <peterz@infradead.org>
To: "Wangnan (F)" <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>,
	Kaixu Xia <xiakaixu@huawei.com>,
	davem@davemloft.net, acme@kernel.org, mingo@redhat.com,
	masami.hiramatsu.pt@hitachi.com, jolsa@kernel.org,
	daniel@iogearbox.net, linux-kernel@vger.kernel.org,
	pi3orama@163.com, hekuang@huawei.com, netdev@vger.kernel.org
Subject: Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY maps trace data output when perf sampling
Date: Wed, 21 Oct 2015 13:56:30 +0200	[thread overview]
Message-ID: <20151021115630.GO17308@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <56277844.9090201@huawei.com>

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.