Subject: Re: [PATCH v6 3/4] bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter

From: xiakaixu <[email-masked]>
Date: Wed, 5 Aug 2015 18:31:55 +0800

于 2015/8/5 18:04, Peter Zijlstra 写道:
> On Tue, Aug 04, 2015 at 08:58:15AM +0000, Kaixu Xia wrote:
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 6251b53..726ca1b 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -8599,6 +8599,25 @@ struct perf_event_attr *perf_event_attrs(struct perf_event *event)
>>  	return &event->attr;
>>  }
>>  
>> +u64 perf_event_read_internal(struct perf_event *event)
> 
> Maybe: perf_event_read_local(), as this is this function only works for
> events active on the current CPU.
> 
>> +{
>> +	if (!event)
>> +		return -EINVAL;
>> +
>> +	if (unlikely(event->state != PERF_EVENT_STATE_ACTIVE))
>> +		return -EINVAL;
> 
> You can return perf_event_count() in that case.
> 
>> +
>> +	if (event->oncpu != raw_smp_processor_id() &&
> 
> That _must_ be smp_processor_id(). If that gives a warning (ie.
> preemption is not disabled or we're not affine to this one cpu) then the
> warning is valid.
> 
>> +	    event->ctx->task != current)
> 
> Write it like:
> 
> 	if (event->ctx->task != current &&
> 	    event->oncpu != smp_processor_id())
> 
> That way you'll not evaluate smp_processor_id() for current task events.
> 
>> +		return -EINVAL;
>> +
>> +	if (unlikely(event->attr.inherit))
>> +		return -EINVAL;
> 
> This should be in your accept function, inherited events should never
> get this far.
> 
> You need IRQs disabled while calling __perf_event_read(), did you test
> with lockdep enabled?

Thanks for your review! I've not tested it, will do that from now on.
> 
>> +	__perf_event_read(event);
>> +	return perf_event_count(event);
>> +}
> 
> Also, you probably want a WARN_ON(in_nmi()) there, this function is
> _NOT_ NMI safe.
> 
> 
> 
> .
>