Re: [PATCH V4] tick/hotplug: Handover time related duties before cpu offline

From: Preeti U Murthy
Date: Mon Feb 16 2015 - 23:09:58 EST

On 02/17/2015 07:28 AM, Michael Ellerman wrote:
> On Sat, 2015-01-31 at 09:44 +0530, Preeti U Murthy wrote:
>> These duties include do_timer to update jiffies and broadcast wakeups on those
>> platforms which do not have an external device to handle wakeup of cpus from deep
>> idle states. The handover of these duties is not robust against a cpu offline
>> operation today.
>> The do_timer duty is handed over in the CPU_DYING phase today to one of the online
>> cpus. This relies on the fact that *all* cpus participate in stop_machine phase.
>> But if this design is to change in the future, i.e. if all cpus are not
>> required to participate in stop_machine, the freshly nominated do_timer cpu
>> could be idle at the time of handover. In that case, unless its interrupted,
>> it will not wakeup to update jiffies and timekeeping will hang.
>> With regard to broadcast wakeups, today if the cpu handling broadcast of wakeups
>> goes offline, the job of broadcasting is handed over to another cpu in the CPU_DEAD
>> phase. The CPU_DEAD notifiers are run only after the offline cpu sets its state as
>> CPU_DEAD. Meanwhile, the kthread doing the offline is scheduled out while waiting for
>> this transition by queuing a timer. This is fatal because if the cpu on which
>> this kthread was running has no other work queued on it, it can re-enter deep
>> idle state, since it sees that a broadcast cpu still exists. However the broadcast
>> wakeup will never come since the cpu which was handling it is offline, and the cpu
>> on which the kthread doing the hotplug operation was running never wakes up to see
>> this because its in deep idle state.
>> Fix these issues by handing over the do_timer and broadcast wakeup duties just before
>> the offline cpu kills itself, to the cpu performing the hotplug operation. Since the
>> cpu performing the hotplug operation is up and running, it becomes aware of the handover
>> of do_timer duty and queues the broadcast timer upon itself so as to seamlessly
>> continue both these operations.
>> It fixes the bug reported here:
>> Signed-off-by: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx>
>> ---
>> Changes from V3:
>> 1. Move handover of broadcast duty away from CPU_DYING phase to just before
>> the cpu kills itself.
>> 2. Club the handover of timekeeping duty along with broadcast duty to make
>> timekeeping robust against hotplug.
> Hi Preeti,
> This bug is still causing breakage for people on Power8 machines.
> Are we just waiting for Thomas to take the patch?

Hi mpe,

Thomas has included the patch for fixing this issue in a recent patchset
that he posted for cleaning up tick/clockevents related code. I think it will go into this
merge-window. There are a couple of issues there, once that is fixed I
will remind him to mark it for stable.

Preeti U Murthy

> cheers
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