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




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