Date: Fri, 17 Jul 2015 14:18:36 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Wangnan (F)" <wangnan0@huawei.com>
Cc: kaixu xia <xiakaixu@huawei.com>, ast@plumgrid.com, davem@davemloft.net,
        acme@kernel.org, mingo@redhat.com, masami.hiramatsu.pt@hitachi.com,
        jolsa@kernel.org, linux-kernel@vger.kernel.org, pi3orama@163.com,
        hekuang@huawei.com
Subject: Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get
 the selected hardware PMU conuter
Message-ID: <20150717121836.GH19282@twins.programming.kicks-ass.net>
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 <20150717110541.GJ12596@twins.programming.kicks-ass.net>
 <55A8E703.70306@huawei.com>
 <20150717113924.GD19282@twins.programming.kicks-ass.net>
 <55A8EABE.1060308@huawei.com>
 <20150717115505.GF19282@twins.programming.kicks-ass.net>
 <20150717115615.GA18673@twins.programming.kicks-ass.net>
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On Fri, Jul 17, 2015 at 08:01:07PM +0800, Wangnan (F) wrote:
> 
> 
> On 2015/7/17 19:56, Peter Zijlstra wrote:
> >On Fri, Jul 17, 2015 at 01:55:05PM +0200, Peter Zijlstra wrote:
> >>On Fri, Jul 17, 2015 at 07:45:02PM +0800, Wangnan (F) wrote:
> >>
> >>>>Depends on what all you need, if you need full perf events to work then
> >>>>yes perf_event_read_value() is your only option.
> >>>>
> >>>>But note that that requires scheduling, so you cannot actually use it
> >>>>for tracing purposes etc..
> >>>What you mean "full perf events"? Even with your code some event still not
> >>>work?
> >>The code I posted only works for events that do not have inherit set.
> >>And only works from IRQ/NMI context for events that monitor the current
> >>task or the current CPU (although that needs a little extra code still).
> >>
> >>Anything else and it does not work (correctly).
> >Scratch that from NMI, for that to work we need more magic still.
> The scheduling you said is caused by
> 
> mutex_lock(&event->child_mutex)
> 
> right?
> 
> What about replacing it to mutex_trylock() and simply return an error
> if it read from a BPF program?

That is vile and unreliable.

I think you really want to put very strict limits on what kind of events
you accept, or create the events yourself.

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