Re: [patch] Performance Counters for Linux, v3

["stephane eranian" <eranian@googlemail.com>]

On Mon, Dec 15, 2008 at 11:32 PM, Chris Friesen <cfriesen@nortel.com> wrote:
> stephane eranian wrote:
>>
>> On Fri, Dec 12, 2008 at 8:45 PM, Chris Friesen <cfriesen@nortel.com>
>> wrote:
>>
>>> stephane eranian wrote:
>>>
>>>
>>>> What happens in the following test case:
>>>>
>>>>  - 2-way system (cpu0, cpu1)
>>>>
>>>>  - on cpu0, two processes P1, P2, each self-monitoring and counting
>>>> event
>>>> E1.
>>>>   Event E1 can only be measured on counter C1.
>>>>
>>>>  - on cpu1, there is a cpu-wide session, monitoring event E1, thus using
>>>> C1
>>>>
>>>>  - the scheduler decides to migrate P1 onto CPU1. You now have a
>>>> conflict on C1.
>>>>
>>>> How is this managed?
>>>
>>> Prevent the load balancer from moving P1 onto cpu1?
>>>
>>
>> You don't want to do that.
>>
>> There was a reason why the scheduler decided to move the task.
>> Now, because of monitoring you would change the behavior of the task
>> and scheduler.
>> Monitoring should be unintrusive. You want the task/scheduler to
>> behave as if no monitoring
>> was present otherwise what is it you are actually measuring?
>
> In a scenario where the system physically cannot gather the desired data
> without influencing the behaviour of the program, I see two options:
>
> 1) limit the behaviour of the system to ensure that we can gather the
> performance monitoring data as specified
>
> 2) limit the performance monitoring to minimize any influence on the
> program, and report the fact that performance monitoring was limited.
>
> You've indicated that you don't want option 1, so I assume that you prefer
> option 2.  In the above scenario, how would _you_ handle it?
>
That's right, you have to fail monitoring.

In this particular example, it is okay for per-thread sessions to each use C1.
Any cpu-wide session trying to access C1 should fail. Vice versa if a
cpu-wide session is using C1, then no per-thread session can be accessing it.

Things can get even more complicated than that even for per-thread sessions.
Some PMU registers may be shared per core, e.g, Nehalem or Pentium 4. Thus
if HT is enabled, you also have to fail per-thread sessions, as only
one can grab
the resource globally.