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

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Message-ID: <4FAB1548.6050806@intel.com>
Date:	Thu, 10 May 2012 09:09:28 +0800
From:	"Yan, Zheng" <zheng.z.yan@...el.com>
To:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
CC:	a.p.zijlstra@...llo.nl, mingo@...e.hu, andi@...stfloor.org,
	eranian@...gle.com, jolsa@...hat.com, ming.m.lin@...el.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] perf: Allow pmu to choose cpu on which to install
 event

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@...el.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@...el.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.

Regards
Yan, Zheng
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