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.

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?

Find an inline comment.

On 02/12/2018 01:43 AM, Jiri Olsa wrote:
On Fri, Feb 09, 2018 at 03:07:00PM -0800, Raghavendra Rao Ananta wrote:
Perf framework doesn't allow creation of hardware events if
the requested CPU is offline. However, creation of an event
is achievable if the event is attached to the PMU as soon
as the CPU is online again.

So, introducing a feature that could allow to create events
even when the CPU is offline and return a success to the caller.
If, during the time of event creation, the CPU is found offline,
the event is moved to a new state (PERF_EVENT_STATE_DORMANT). As
and when the CPU is know to be woken up (through hotplug notifiers),
all the dormant events would be attached to the PMU (by
perf_install_in_context()). If during the life time of the event,
the CPU hasn't come online, the dormant event would just be freed.

Signed-off-by: Raghavendra Rao Ananta <rana...@codeaurora.org>

hum, I tried and for some reason I'm getting zero counts/times
when the cpu 1 is set back online (in 9.second)

[root@ibm-x3650m4-02 perf]# ./perf stat -C 1 -e sched:sched_switch -v -I 1000
failed to read counter sched:sched_switch
#           time             counts unit events
      1.000921624      <not counted>      sched:sched_switch
failed to read counter sched:sched_switch
      2.001725364      <not counted>      sched:sched_switch
failed to read counter sched:sched_switch
      3.002685350      <not counted>      sched:sched_switch
failed to read counter sched:sched_switch
      4.003463851      <not counted>      sched:sched_switch
failed to read counter sched:sched_switch
      5.004651601      <not counted>      sched:sched_switch
failed to read counter sched:sched_switch
      6.005338294      <not counted>      sched:sched_switch
failed to read counter sched:sched_switch
      7.006351155      <not counted>      sched:sched_switch
failed to read counter sched:sched_switch
      8.007239698      <not counted>      sched:sched_switch
sched:sched_switch: 0 0 0
      9.008665621      <not counted>      sched:sched_switch
sched:sched_switch: 0 0 0
     10.009570492      <not counted>      sched:sched_switch
sched:sched_switch: 0 0 0
     11.010811591      <not counted>      sched:sched_switch
sched:sched_switch: 0 0 0
     12.011614182      <not counted>      sched:sched_switch
sched:sched_switch: 0 0 0
     13.012299851      <not counted>      sched:sched_switch

looks like the dormant event wasn't scheduled in properly

also while at it, could we also handle cpu going offline case,
so the event would survive until it's back online
Sure, I have a plan for that one as well, but wondering how this goes first.

jirka


Thank you.

Raghavendra

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