On Wed, 9 May 2018 06:24:16 +0000 Dexuan Cui <de...@microsoft.com> wrote:
> In include/linux/cpumask.h, for_each_cpu is defined like this for UP kernel > (CONFIG_NR_CPUS=1): > > #define for_each_cpu(cpu, mask) \ > for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask) > > Here 'mask' is ignored, but what if 'mask' contains 0 CPU? -- in this case, > the for loop should not > run at all, but with the current code, we run the loop once with cpu==0. > > I think I'm seeing a bug in my UP kernel that is caused by the buggy > for_each_cpu(): > > in kernel/time/tick-broadcast.c: tick_handle_oneshot_broadcast(), > tick_broadcast_oneshot_mask > contains 0 CPU, but due to the buggy for_each_cpu(), the variable > 'next_event' is changed from > its default value KTIME_MAX to "next_event = td->evtdev->next_event"; as a > result, > tick_handle_oneshot_broadcast () -> tick_broadcast_set_event() -> > clockevents_program_event() > -> pit_next_event() is programming the PIT timer by accident, causing an > interrupt storm of PIT > interrupts in some way: I'm seeing that the kernel is receiving ~8000 PIT > interrupts per second for > 1~5 minutes when the UP kernel boots, and it looks the kernel hangs, but in > 1~5 minutes, finally > somehow the kernel can recover and boot up fine. But, occasionally, the > kernel just hangs there > forever, receiving ~8000 PIT timers per second. > > With the below change in kernel/time/tick-broadcast.c, the interrupt storm > will go away: > > +#undef for_each_cpu > +#define for_each_cpu(cpu, mask) \ > + for ((cpu) = 0; (((cpu) < 1) && ((mask)[0].bits[0] & 1)); (cpu)++, > (void)mask) > > Should we fix the for_each_cpu() in include/linux/cpumask.h for UP? I think so, yes. That might reveal new peculiarities, but such is life. I guess we should use bitmap_empty() rather than open-coding it.
