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

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Date:	Fri, 17 Jul 2015 20:04:04 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 2015/7/17 20:01, 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?
>

Sorry. Should be: "return an error if it doesn't get the lock and the 
caller is a BPF program."

> Thank you.


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