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

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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