Jolsa Features Togle Event

Revision as of 18:28, 24 September 2013

Branch

• perf/core_toggle

Description

Adding perf interface that allows to create toggle events, which can enable or disable another event. Whenever the toggle event is triggered (has overflow), it toggles another event state and either starts or stops it.

The goal is to be able to create toggling tracepoint events to enable and disable HW counters, but the interface is generic enough to be used for any kind of event.

Interface

There are changes for both kernel and perf tool part.

Kernel

The interface to create a toggle event is similar as the one for defining event group. Use perf syscall with:

int sys_perf_event_open(struct perf_event_attr *attr,
                        pid_t pid, int cpu, int group_fd,
                        unsigned long flags)

flags    - PERF_FLAG_TOGGLE_ON or PERF_FLAG_TOGGLE_OFF
group_fd - event (or group) fd to be toggled

Created event will toggle ON(start) or OFF(stop) the event specified via group_fd.

Obviously this way it's not possible for toggle event to be part of group other than group leader. This is where you need to use PERF_EVENT_IOC_SET_TOGGLE ioctl.

This ioctl has 2 goals:

• allowing the toggle event being part of the group
• allowing to define toggle setting after event is created

u64 args[2] = { toggled_fd, flag };
err = ioctl(fd, PERF_EVENT_IOC_SET_TOGGLE, args);

Where:

toggled_fd - is file description of the event we want to toggle
flag       - is one of PERF_FLAG_TOGGLE_ON|PERF_FLAG_TOGGLE_OFF
err        -  0 when successful
             -1 otherwise with errno:
                EBUSY  - event has already toggled event defined
                EFAULT - could not copy user data
                EINVAL - wrong data

Perf Tool

The support for toggling events is added into record and stat commands.

The toggling events are defined via on/off terms, assigned with the name of the event they should toggle.

Toggling events are define within -e option using on/off terms, like

-e 'cycles,irq_entry/on=cycles/,irq_exit/off=cycles/'

Meaning:

• irq_entry toggles on (starts) cycles, and irq_exit toggled off (stops) cycles.
• cycles is started as paused

Example - using k(ret)probes

• Define toggle(on/off) events:

# perf probe -a fork_entry=do_fork
# perf probe -a fork_exit=do_fork%return

• Following record session samples only within do_fork function:

# perf record -g -e '{cycles,cache-misses}:k,probe:fork_entry/on=cycles/,probe:fork_exit/off=cycles/' \ 
  perf bench sched messaging

• Following stat session measure cycles within do_fork function:

# perf stat -e '{cycles,cache-misses}:k,probe:fork_entry/on=cycles/,probe:fork_exit/off=cycles/' \ 
  perf bench sched messaging

# Running sched/messaging benchmark...
# 20 sender and receiver processes per group
# 1 groups == 40 processes run

     Total time: 0.073 [sec]

 Performance counter stats for './perf bench sched messaging -g 1':

        20,935,464 cycles                    #    0.000 GHz
            18,897 cache-misses
                40 probe:fork_entry
                40 probe:fork_exit

       0.086319682 seconds time elapsed

Example - using u(ret)probes

• Sample program:

void krava(void)
{
        asm volatile ("nop; nop");
}

int main(void)
{
        krava();
        return 0;
}

• Define toggle(on/off) events:

# perf probe -x ./ex entry=krava
# perf probe -x ./ex exit=krava%return

• Following stat session measure instructions within krava function:

# perf stat -e instructions:u,probe_ex:entry/on=instructions/,probe_ex:exit/off=instructions/ ./ex

    Performance counter stats for './ex':

                 9 instructions:u            #    0.00  insns per cycle 
                 1 probe_ex:entry
                 1 probe_ex:exit

       0.000556743 seconds time elapsed

Following stat session measure cycles, instructions and cache-misses within krava function:

# perf stat -e '{cycles,instructions,cache-misses}:u,probe_ex:entry/on=cycles/,probe_ex:exit/off=cycles/' ./ex

     Performance counter stats for './ex':

           2,068 cycles                    #    0.000 GHz
               9 instructions              #    0.00  insns per cycle 
               0 cache-misses
               1 probe_ex:entry
               1 probe_ex:exit

     0.000557504 seconds time elapsed