Re: [PATCH 2/9] perf: Allow pmu to choose cpu on which to install event

[Anshuman Khandual <khandual@linux.vnet.ibm.com>]

On Thursday 10 May 2012 06:39 AM, Yan, Zheng wrote:

> On 05/09/2012 02:38 PM, Anshuman Khandual wrote:
>> On Wednesday 02 May 2012 07:37 AM, Yan, Zheng wrote:
>>
>>> From: "Yan, Zheng" <zheng.z.yan@intel.com>
>>>
>>> Allow the pmu->event_init callback to change event->cpu, so pmu can
>>> choose cpu on which to install event.
>>>
>>> Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
>>> ---
>>>  kernel/events/core.c |   13 +++++++++----
>>>  1 files changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>> index 32cfc76..84911de 100644
>>> --- a/kernel/events/core.c
>>> +++ b/kernel/events/core.c
>>> @@ -6250,6 +6250,8 @@ SYSCALL_DEFINE5(perf_event_open,
>>>  		}
>>>  	}
>>>
>>> +	get_online_cpus();
>>
>> Why this protection against cpu hotplug operation ? Is this because PMU now can change event->cpu
>> during event initialization (specific to uncore PMU events) or this protection has always been required
>> for normal on-cpu HW PMU events also and we added it right now ?
>>
> I think it's always required. Because when creating a perf event, 'cpu online' is checked by
> find_get_context, the cpu can go offline after find_get_context return.

Agreed. So here the get_online_cpus()/put_online_cpus() pair solves an existing problem. Could you please
put an additional statement explaining this in the patch documentation. Thank you.


> Regards
> Yan, Zheng
>