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 10:48:12 +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().
> 
>> +static void perf_event_check_dump_flag(struct perf_event *event)
>> +{
>> +    if (event->attr.dump_enable == 1)it 
>> +        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.

It is really hard that adding a test to samples/bpf/. We need to implement most of
'perf record/report' commands from tools/perf/, like mmap(), dump trace, etc. Only
the perf_event_open syscall is really not enough.

Actually, this patch set is only the kernel space side, and it still needs the perf
user space side, you can find the necessary patches in Wang Nan's git tree[1].
Based on Wang Nan's git tree, we can config BPF maps through perf cmdline.
We also need to confing attr->soft_disable in perf user side based on tree[1]. so
it was not included in this patchset. I will send out the perf userspace part after
this patch set is applied.

[1] git://git.kernel.org/pub/scm/linux/kernel/git/pi3orama/linux.git perf/ebpf
> .