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

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Message-ID: <4FAA10FB.1030008@linux.vnet.ibm.com>
Date:	Wed, 09 May 2012 12:08:51 +0530
From:	Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.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 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 ?

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