Re: [PATCH 10/48] perf tools: Introduce bpf-output event

[Wangnan (F)]

On 2016/2/24 12:03, Wangnan (F) wrote:
> On 2016/2/24 10:04, Wangnan (F) wrote:
> > On 2016/2/24 9:58, Wangnan (F) wrote:
> > >

[SNIP]

> > > So, there is something strange here:
> > > >          if (unlikely(event->oncpu != smp_processor_id()))
> > > >                  return -EOPNOTSUPP;
> >
> > All failures have 'event->oncpu == -1' here. I guess we should 
> > suppress warning in
> > this case. But why event->oncpu becomes -1?
>
> For this specific test it is not surprising to see these error 
> messages. In this test
> we create bpf-output channel on 'ls' process only, but the BPF script 
> is triggered
> on all procs (BPF triggering is not related to perf event scheduling). 
> Trying to
> output data through 'ls' specific bpf-output channel should fail if 
> this 'sys_write'
> is not issued by 'ls' or its children. So it is a correct behavior.
>
> However, I also see them in system wide channel:
>
>  # echo "" > /sys/kernel/debug/tracing/trace
>  # ./perf record -a -e bpf-output/no-inherit,name=evt/ \
>                     -e ./test_bpf_output.c/map:channel.event=evt/
>                     -a
>  ^C[ perf record: Woken up 0 times to write data ]
>  [ perf record: Captured and wrote 17.534 MB perf.data (264326 samples) ]
>  # cat /sys/kernel/debug/tracing/trace | tail
>    rs:main Q:Reg-582   [000] d..2  4858.711225: : Error: failed to 
> output: -95
>    rs:main Q:Reg-582   [000] d..2  4858.711241: : Error: failed to 
> output: -95
>            gmain-1858  [003] d..2  4858.711436: : Error: failed to 
> output: -95
>            gmain-1858  [003] d..2  4858.711441: : Error: failed to 
> output: -95
>            gmain-1858  [003] d..2  4858.711473: : Error: failed to 
> output: -95
>    rs:main Q:Reg-582   [002] d..2  4858.712215: : Error: failed to 
> output: -95
>    rs:main Q:Reg-582   [002] d..2  4858.712224: : Error: failed to 
> output: -95
>            gmain-1858  [003] d..2  4858.712230: : Error: failed to 
> output: -95
>    rs:main Q:Reg-582   [002] d..2  4858.712235: : Error: failed to 
> output: -95
>    rs:main Q:Reg-582   [002] d..2  4858.712239: : Error: failed to 
> output: -95
>
> System wide events can also be scheduled in and out. If the bpf-output 
> events
> are scheduled out, trying to output data through it causes the above 
> failure.
> I don't think it is a problem.
>
> Peter, Could you please give some infomation? In which case a system wide
> bpf output channel would be scheduled out?

Sorry, I think my brain is not quite well. Actually this is a easy question:
all ENOTSUPP results are generated before PERF_EVENT_IOC_ENABLE or after
PERF_EVENT_IOC_DISABLE. You saw so many failure messages because it is 
sys_write,
and perf itself needs it.

So you can simply ignore these messages.

Thank you.