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

[Raghavendra Rao Ananta]

On 02/13/2018 08:08 AM, Jiri Olsa wrote:
> On Mon, Feb 12, 2018 at 02:22:30PM -0800, Raghavendra Rao Ananta wrote:
> > 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);
>
> could you add check in ioctl call that set the inactive state
> on the dormant event.. that would start it once the cpu is
> online.. as requested

I am a little confused. When you say "check", do you mean a new ioctl 
command?

So the flow (from the user-space perspective) would go something like this?

1. // CPU offline

2. perf_event_open(); // event started as disabled --> added to dormant 
list in the kernel

3. ioctl(SET_INACTIVE); // change the state of the event to inactive

4. // CPU woken up!

5. // schedule the (inactive) event by traversing the dormant list

Is this what you were trying to mention, or am I missing something?

Raghavendra
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