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: >> 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?
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. https://lkml.org/lkml/2015/2/16/213. 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. Regards Preeti U Murthy > > cheers > > > _______________________________________________ > Linuxppc-dev mailing list > linuxppc-...@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev > -- 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/