Re: [RFC PATCH 2/2] bpf: Implement bpf_perf_event_sample_enable/disable() helpers

[He Kuang]

hi, Alexei

> > What about using similar
> > implementation
> > like PERF_EVENT_IOC_SET_OUTPUT, creating a new ioctl like
> > PERF_EVENT_IOC_SET_ENABLER,
> > then let perf to select an event as 'enabler', then BPF can still
> > control one atomic
> > variable to enable/disable a set of events.
>
> you lost me on that last sentence. How this 'enabler' will work?
> Also I'm still missing what's wrong with perf doing ioctl() on
> events on all cpus manually when bpf program tells it to do so.
> Is it speed you concerned about or extra work in perf ?

For not having too much wakeups, perf ringbuffer has a watermark
limit to cache events and reduce the wakeups, which causes perf
userspace tool can not receive perf events immediately.

Here's a simple demo expamle to prove it, 'sleep_exec' does some
writes and prints a timestamp every second, and an lable is
printed when perf poll gets events.

  $ perf record -m 2 -e syscalls:sys_enter_write sleep_exec 1000
  userspace sleep time: 0 seconds
  userspace sleep time: 1 seconds
  userspace sleep time: 2 seconds
  userspace sleep time: 3 seconds
  perf record wakeup onetime 0
  userspace sleep time: 4 seconds
  userspace sleep time: 5 seconds
  userspace sleep time: 6 seconds
  userspace sleep time: 7 seconds
  perf record wakeup onetime 1
  userspace sleep time: 8 seconds
  perf record wakeup onetime 2
  ..

  $ perf record -m 1 -e syscalls:sys_enter_write sleep_exec 1000
  userspace sleep time: 0 seconds
  userspace sleep time: 1 seconds
  perf record wakeup onetime 0
  userspace sleep time: 2 seconds
  userspace sleep time: 3 seconds
  perf record wakeup onetime 1
  userspace sleep time: 4 seconds
  userspace sleep time: 5 seconds
  ..

By default, if no mmap_pages is specified, perf tools wakeup only
when the target executalbe finished:

  $ perf record -e syscalls:sys_enter_write sleep_exec 5
  userspace sleep time: 0 seconds
  userspace sleep time: 1 seconds
  userspace sleep time: 2 seconds
  userspace sleep time: 3 seconds
  userspace sleep time: 4 seconds
  perf record wakeup onetime 0
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.006 MB perf.data (54 samples) ]

If we want perf to reflect as soon as our sample event be generated,
--no-buffering should be used, but this option has a greater
impact on performance.

  $ perf record --no-buffering -e syscalls:sys_enter_write sleep_exec 1000
  userspace sleep time: 0 seconds
  perf record wakeup onetime 0
  perf record wakeup onetime 1
  perf record wakeup onetime 2
  perf record wakeup onetime 3
  perf record wakeup onetime 4
  perf record wakeup onetime 5
  perf record wakeup onetime 6
  ..

Thank you

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