Messages in this thread | | | Subject | Re: [PATCH V4] tick/hotplug: Handover time related duties before cpu offline | From | Michael Ellerman <> | Date | Tue, 17 Feb 2015 12:58:04 +1100 |
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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: > http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html > > Signed-off-by: Preeti U Murthy <preeti@linux.vnet.ibm.com> > --- > Changes from V3: https://lkml.org/lkml/2015/1/20/236 > 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?
cheers
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