On Thu, Feb 02, 2012 at 12:43:00PM +0100, Igor Mammedov wrote: > While playing with kvm cpu hot-plug, I've probably stumbled on general > kernel bug. So I'm lookng for advice on approach to fix it. > > When kvm guest uses kvmclock, it may hang on cpu hot-plug at > > BSP: > smp_apic_timer_interrupt > ... > -> do_timer > -> update_wall_time > > and a being on-lined CPU at waiting on sync with BSP at: > start_secondary: > while (!cpumask_test_cpu(smp_processor_id(), cpu_active_mask)) > cpu_relax(); > > because of unusually big value returned by clock->read(clock) in > update_wall_time. > This happens due to overflow in pvclock_get_nsec_offset > > u64 delta = tsc - shadow->tsc_timestamp; > > when shadow->tsc_timestamp is bigger than tsc. > And since pvclock_clocksource_read remembers and returns largest > value of any clock that ever was returned, clock for affected guest > virtually freezes. > > Overflow happens due to reading undefined values from uninitialized > per_cpu variable hv_clock. In case of cpu hot-plug, clock is read at > least once on being on-lined cpu before it is initialized for this > cpu: > > start_secondary > |-> smp_callin > | -> smp_store_cpu_info > | -> identify_secondary_cpu > | -> mtrr_ap_init > | -> mtrr_restore > | -> stop_machine_from_inactive_cpu > | -> queue_stop_cpus_work > | ... > | -> kvm_clock_read > |... > |x86_cpuinit.setup_percpu_clockev > > full call chain is below: > [ 0.002999] [<ffffffff8102fc74>] pvclock_clocksource_read+0x9f/0x275 > [ 0.002999] [<ffffffff81071f9e>] ? check_preempt_wakeup+0x11c/0x1c2 > [ 0.002999] [<ffffffff8102f05d>] kvm_clock_read+0x21/0xd3 > [ 0.002999] [<ffffffff810154b1>] sched_clock+0x9/0xd > [ 0.002999] [<ffffffff81070509>] sched_clock_local+0x12/0x75 > [ 0.002999] [<ffffffff8107065e>] sched_clock_cpu+0x84/0xc6 > [ 0.002999] [<ffffffff8106bfd3>] update_rq_clock+0x28/0x108 > [ 0.002999] [<ffffffff8106c15e>] enqueue_task+0x1d/0x64 > [ 0.002999] [<ffffffff8106c500>] activate_task+0x22/0x24 > [ 0.002999] [<ffffffff8106c90c>] ttwu_do_activate.constprop.39+0x32/0x61 > [ 0.002999] [<ffffffff8106cd0c>] try_to_wake_up+0x17e/0x1e1 > [ 0.002999] [<ffffffff8106cd98>] wake_up_process+0x15/0x17 > [ 0.002999] [<ffffffff8109ebbf>] cpu_stop_queue_work+0x3d/0x5f > [ 0.002999] [<ffffffff8109ec6c>] queue_stop_cpus_work+0x8b/0xb6 > [ 0.002999] [<ffffffff8109f0e3>] stop_machine_from_inactive_cpu+0xb4/0xed > [ 0.002999] [<ffffffff81023896>] ? mtrr_restore+0x4a/0x4a > [ 0.002999] [<ffffffff81023eb3>] mtrr_ap_init+0x5a/0x5c > [ 0.002999] [<ffffffff814b95f7>] identify_secondary_cpu+0x19/0x1b > [ 0.002999] [<ffffffff814bc0d3>] smp_store_cpu_info+0x3c/0x3e > [ 0.002999] [<ffffffff814bc4ed>] start_secondary+0x122/0x263 > > Looking at native_smp_prepare_cpus in arch/x86/kernel/smpboot.c > I see that it unconditionally calls set_mtrr_aps_delayed_init which > in turn effectively converts calls to mtrr_ap_init to nop ops. > And later call to native_smp_cpus_done -> mtrr_aps_init completes > mtrr initialization. > > The same pattern might be noticed in suspend/hibernate handlers, that > call enable_nonboot_cpus. > > enable_nonboot_cpus > -> arch_enable_nonboot_cpus_begin > -> set_mtrr_aps_delayed_init > > -> boot secondary cpus > > -> arch_enable_nonboot_cpus_end > -> mtrr_aps_init > > So question is if the calling mtrr_ap_init so early in > identify_secondary_cpu is really necessary? > From current code it looks like it is never called at normal smp > boot/resume time. And only path that triggers it is cpu hot-plug one. > > I see following possible solutions: > > 1. Could the call to mtrr_ap_init be just moved from identify_secondary_cpu > to start_secondary right after x86_cpuinit.setup_percpu_clockev call? > > This will prevent an access to uninitialized per_cpu clock. Or move > x86_cpuinit.setup_percpu_clockev before smp_callin? > > 2. Another way to prevent access to uninitialized per_cpu clock is to > introduce hook.early_percpu_clock_init that will be called before > smp_callin or right before mtrr_ap_init. > > I hope to see opinions about this matter from a more experienced people than me. It would be more robust to introduce x86_cpuinit.early_percpu_clock_init and use it for KVM, replacing setup_percpu_clockev hook. setup_percpu_clockev initializes the APIC timer, but clearly sched_clock should be functional before that, for example for ftrace (in addition to sched_clock from scheduler as in the trace above). -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
