linux-kernel - Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get the selected hardware PMU conuter

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Message-ID: <20150717121836.GH19282@twins.programming.kicks-ass.net>
Date:	Fri, 17 Jul 2015 14:18:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	kaixu xia <xiakaixu@...wei.com>, ast@...mgrid.com,
	davem@...emloft.net, acme@...nel.org, mingo@...hat.com,
	masami.hiramatsu.pt@...achi.com, jolsa@...nel.org,
	linux-kernel@...r.kernel.org, pi3orama@....com, hekuang@...wei.com
Subject: Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get
 the selected hardware PMU conuter

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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