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    SubjectRe: [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@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.

    Regards
    Yan, Zheng


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