linux-kernel - Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample

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Message-ID: <561C7CDA.8050004@plumgrid.com>
Date:	Mon, 12 Oct 2015 20:39:06 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>,
	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:	linux-kernel@...r.kernel.org, pi3orama@....com, hekuang@...wei.com,
	netdev@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] bpf: Implement
 bpf_perf_event_sample_enable/disable() helpers

On 10/12/15 8:27 PM, Wangnan (F) wrote:
> Then how to avoid racing? For example, when one core disabling all events
> in a map, another core is enabling all of them. This racing may causes
> sereval
> perf events in a map dump samples while other events not. To avoid such
> racing
> I think some locking must be introduced, then cost is even higher.
>
> The reason why we introduce an atomic pointer is because each operation
> should
> controls a set of events, not one event, due to the per-cpu manner of
> perf events.

why 'set disable' is needed ?
the example given in cover letter shows the use case where you want
to receive samples only within sys_write() syscall.
The example makes sense, but sys_write() is running on this cpu, so just
disabling it on the current one is enough.

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