Re: [PATCH] perf: Add support for creating offline events

[Raghavendra Rao Ananta]

On 02/12/2018 01:21 PM, Jiri Olsa wrote:
> On Mon, Feb 12, 2018 at 10:04:42PM +0100, Jiri Olsa wrote:
> > On Mon, Feb 12, 2018 at 09:42:05AM -0800, Raghavendra Rao Ananta wrote:
> > > Hi Jiri,
> > >
> > > Thank you for the response.
> > >
> > > Does perf tool has its own check to see if the CPU was offline during the
> > > lifetime of an event? If so, it might ignore these type of events.
> >
> > nope, we don't check on that
> >
> > > Initially, I tested the same using perf tool and found similar results.
> > > Then I debugged further and found that the perf core was actually sending
> > > data to the userspace (copy_to_user()) and the corresponding count for the
> > > data. Hence, I tested this further by writing my own userspace application,
> > > and I was able to read the count through this,
> > > even when the CPU was made offline and back online.
> > >
> > > Do you think we also have to modify the perf tool accordingly?
> >
> > hum, I wonder what's wrong.. will check
>
> I think the user space needs to enable the event once the
> cpu gets online.. which we dont do and your app does..?
>
> maybe we could add perf_event_attr::enable_on_online ;-)
>
> I'll check what we can do in user space, I guess we can
> monitor the cpu state and enable event accordingly
>
> jirka
Yes, probably that's the reason.

In order for an event to get scheduled-in, it expects the event to be at 
least in PERF_EVENT_STATE_INACTIVE state. If you notice, in my patch,
when the cpu wakes up, we are initializing the state of the event 
(perf_event__state_init()) and then trying to schedule-in. Since the 
event was created with a disabled state, it seems that the same this is 
followed and the state gets initialized to PERF_EVENT_STATE_OFF. 
Unfortunately, events in this state could not be scheduled.

One way for things to get working is, instead of calling 
perf_event__state_init() before the event is scheduled-in (when the cpu 
wakes up), we can do something like:
perf_event_set_state(event, PERF_EVENT_STATE_INACTIVE);

I made this change and ran the same test as yours, and I see things 
working out for us:

# ./perf stat -C 1 -e sched:sched_switch -v -I 1000
failed to read counter sched:sched_switch
#           time             counts unit events
     1.000115547      <not counted>      sched:sched_switch 

failed to read counter sched:sched_switch
     2.000265492      <not counted>      sched:sched_switch 

failed to read counter sched:sched_switch
     3.000379462      <not counted>      sched:sched_switch 

failed to read counter sched:sched_switch
     4.000523872      <not counted>      sched:sched_switch 

failed to read counter sched:sched_switch
     5.000614808      <not counted>      sched:sched_switch

/* CPU bought ONLINE here */

sched:sched_switch: 541 284808940 284808940
     6.000767761                541      sched:sched_switch 

sched:sched_switch: 180 1000119686 1000119686
     7.000907234                180      sched:sched_switch 

sched:sched_switch: 248 1000129929 1000129929
     8.001026518                248      sched:sched_switch 

sched:sched_switch: 253 1000173050 1000173050
     9.001203689                253      sched:sched_switch 

sched:sched_switch: 620 1000113378 1000113378
    10.001323334                620      sched:sched_switch 

sched:sched_switch: 366 1000121839 1000121839
    11.001448354                366      sched:sched_switch 

sched:sched_switch: 327 1000147664 1000147664
    12.001591432                327      sched:sched_switch 

^Csched:sched_switch: 272 488810681 488810681
    12.490414290                272      sched:sched_switch 

sched:sched_switch: 6 75893 75893

Yes, so as you mentioned adding something like 
perf_event_attr::enable_on_online gives us a control as to put the event 
in INACTIVE state.
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