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    SubjectRe: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS back to DEFAULT
    On 08/14/2014 01:00 PM, Peter Zijlstra wrote:
    > On Thu, Aug 14, 2014 at 12:29:32PM +0200, Daniel Lezcano wrote:
    >> Hi Chuansheng,
    >>
    >> On 14 August 2014 04:11, Chuansheng Liu <chuansheng.liu@intel.com> wrote:
    >>
    >>> We found sometimes even after we let PM_QOS back to DEFAULT,
    >>> the CPU still stuck at C0 for 2-3s, don't do the new suitable C-state
    >>> selection immediately after received the IPI interrupt.
    >>>
    >>> The code model is simply like below:
    >>> {
    >>> pm_qos_update_request(&pm_qos, C1 - 1);
    >>> < == Here keep all cores at C0
    >>> ...;
    >>> pm_qos_update_request(&pm_qos, PM_QOS_DEFAULT_VALUE);
    >>> < == Here some cores still stuck at C0 for 2-3s
    >>> }
    >>>
    >>> The reason is when pm_qos come back to DEFAULT, there is IPI interrupt to
    >>> wake up the core, but when core is in poll idle state, the IPI interrupt
    >>> can not break the polling loop.
    >
    > So seeing how you're from @intel.com I'm assuming you're using x86 here.
    >
    > I'm not seeing how this can be possible, MWAIT is interrupted by IPIs
    > just fine, which means we'll fall out of the cpuidle_enter(), which
    > means we'll cpuidle_reflect(), and then leave cpuidle_idle_call().
    >
    > It will indeed not leave the cpu_idle_loop() function and go right back
    > into cpuidle_idle_call(), but that will then call cpuidle_select() which
    > should pick a new C state.
    >
    > So the interrupt _should_ work. If it doesn't you need to explain why.

    I think the issue is related to the poll_idle state, in
    drivers/cpuidle/driver.c. This state is x86 specific and inserted in the
    cpuidle table as the state 0 (POLL). There is no mwait for this state.
    It is a bit confusing because this state is not listed in the acpi /
    intel idle driver but inserted implicitly at the beginning of the idle
    table by the cpuidle framework when the driver is registered.

    static int poll_idle(struct cpuidle_device *dev,
    struct cpuidle_driver *drv, int index)
    {
    local_irq_enable();
    if (!current_set_polling_and_test()) {
    while (!need_resched())
    cpu_relax();
    }
    current_clr_polling();

    return index;
    }


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