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

Peter Zijlstra
2015-07-17T12:18:50Z

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.