Subject: Re: [RFC PATCH 1/2] perf: Add the flag sample_disable not to output data on samples

From: xiakaixu <[email-masked]>
Date: Tue, 13 Oct 2015 10:30:06 +0800

于 2015/10/13 3:20, Alexei Starovoitov 写道:
> On 10/12/15 2:02 AM, Kaixu Xia wrote:
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index f57d7fe..25e073d 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -39,6 +39,7 @@ struct bpf_map {
>>       u32 max_entries;
>>       const struct bpf_map_ops *ops;
>>       struct work_struct work;
>> +    atomic_t perf_sample_disable;
>>   };
>>
>>   struct bpf_map_type_list {
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 092a0e8..0606d1d 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -483,6 +483,8 @@ struct perf_event {
>>       perf_overflow_handler_t        overflow_handler;
>>       void                *overflow_handler_context;
>>
>> +    atomic_t            *sample_disable;
> 
> this looks fragile and unnecessary.
> Why add such field to generic bpf_map and carry its pointer into perf_event?
> Single extra field in perf_event would have been enough.
> Even better is to avoid adding any fields.
> There is already event->state why not to use that?
> The proper perf_event_enable/disable are so heavy that another
> mechanism needed? cpu_function_call is probably too much to do
> from bpf program, but that can be simplified?
> Based on the use case from cover letter, sounds like you want
> something like soft_disable?
> Then extending event->state would make the most sense.
> Also consider the case of re-entrant event enable/disable.
> So inc/dec of a flag may be needed?

Thanks for your comments!
I've tried perf_event_enable/disable, but there is a warning caused
by cpu_function_call. The main reason as follows,
 int smp_call_function_single(...)
 {
	...
	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
		     && !oops_in_progress);
	...
}
So I added the extra atomic flag filed in order to avoid this problem.
> .