for_each_cpu() unintuitively reports CPU0 as set independent of the actual cpumask content on UP kernels. This causes an unexpected PIT interrupt storm on a UP kernel running in an SMP virtual machine on Hyper-V, and as a result, the virtual machine can suffer from a strange random delay of 1~20 minutes during boot-up, and sometimes it can hang forever.
Link: https://lkml.org/lkml/2018/5/9/63 Link: https://lkml.org/lkml/2018/5/15/747 Signed-off-by: Dexuan Cui <[email protected]> Cc: [email protected] --- Some part of the changelog and comments are copied from Thomas Gleixner's 115ef3b7e61a ("watchdog/hardlockup/perf: Cure UP damage") :-) kernel/time/tick-broadcast.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c index b398c2e..2fc59ad 100644 --- a/kernel/time/tick-broadcast.c +++ b/kernel/time/tick-broadcast.c @@ -612,6 +612,14 @@ static void tick_handle_oneshot_broadcast(struct clock_event_device *dev) now = ktime_get(); /* Find all expired events */ for_each_cpu(cpu, tick_broadcast_oneshot_mask) { +#ifndef CONFIG_SMP + /* + * Required because for_each_cpu() reports unconditionally + * CPU0 as set on UP kernels. + */ + if (cpumask_empty(tick_broadcast_oneshot_mask)) + break; +#endif td = &per_cpu(tick_cpu_device, cpu); if (td->evtdev->next_event <= now) { cpumask_set_cpu(cpu, tmpmask); -- 2.7.4
