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 <pre...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
