Subject: Re: [PATCH V3 1/2] bpf: control the trace data output on current cpu when perf sampling

From: xiakaixu <[email-masked]>
Date: Mon, 19 Oct 2015 12:03:15 +0800

于 2015/10/17 6:06, Alexei Starovoitov 写道:
> On 10/16/15 12:42 AM, Kaixu Xia wrote:
>> This patch adds the flag dump_enable to control the trace data
>> output process when perf sampling. By setting this flag and
>> integrating with ebpf, we can control the data output process and
>> get the samples we are most interested in.
>>
>> The bpf helper bpf_perf_event_dump_control() can control the
>> perf_event on current cpu.
>>
>> Signed-off-by: Kaixu Xia <[email-masked]>
>> ---
>>   include/linux/perf_event.h      |  1 +
>>   include/uapi/linux/bpf.h        |  5 +++++
>>   include/uapi/linux/perf_event.h |  3 ++-
>>   kernel/bpf/verifier.c           |  3 ++-
>>   kernel/events/core.c            | 13 ++++++++++++
>>   kernel/trace/bpf_trace.c        | 44 +++++++++++++++++++++++++++++++++++++++++
>>   6 files changed, 67 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 092a0e8..2af527e 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -472,6 +472,7 @@ struct perf_event {
>>       struct irq_work            pending;
>>
>>       atomic_t            event_limit;
>> +    atomic_t            dump_enable;
> 
> The naming is the hardest...
> How about calling it 'soft_enable' instead?
> 
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -287,6 +287,11 @@ enum bpf_func_id {
>>        * Return: realm if != 0
>>        */
>>       BPF_FUNC_get_route_realm,
>> +
>> +    /**
>> +     * u64 bpf_perf_event_dump_control(&map, index, flag)
>> +     */
>> +    BPF_FUNC_perf_event_dump_control,
> 
> and this one is too long.
> May be bpf_perf_event_control() ?
> 
> Daniel, any thoughts on naming?
> 
>> --- a/include/uapi/linux/perf_event.h
>> +++ b/include/uapi/linux/perf_event.h
>> @@ -331,7 +331,8 @@ struct perf_event_attr {
>>                   comm_exec      :  1, /* flag comm events that are due to an exec */
>>                   use_clockid    :  1, /* use @clockid for time fields */
>>                   context_switch :  1, /* context switch data */
>> -                __reserved_1   : 37;
>> +                dump_enable    :  1, /* don't output data on samples */
> 
> either comment or name is wrong.
> how about calling this one 'soft_disable',
> since you want zero to be default and the event should be on.
> 
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index b11756f..74a16af 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -6337,6 +6337,9 @@ static int __perf_event_overflow(struct perf_event *event,
>>           irq_work_queue(&event->pending);
>>       }
>>
>> +    if (!atomic_read(&event->dump_enable))
>> +        return ret;
> 
> I'm not an expert in this piece of perf, but should it be 'return 0'
> instead ?
> and may be moved to is_sampling_event() check?
> Also please add unlikely().

The is_sampling_event() is checked in many other kernel places, not only in
perf events interrupt overflow handle. I'm not sure it is fine if we move it
to there. In addition, I think hwc->interrupts++ should be done in
__perf_event_overflow() before event->soft_enable is checked.
> 
>> +static void perf_event_check_dump_flag(struct perf_event *event)
>> +{
>> +    if (event->attr.dump_enable == 1)
>> +        atomic_set(&event->dump_enable, 1);
>> +    else
>> +        atomic_set(&event->dump_enable, 0);
> 
> that looks like it breaks perf, since default for bits is zero
> and all events will be soft-disabled?
> How did you test it?
> Please add a test to samples/bpf/ for this feature.
> 
> 
> .
>