On Wed, 9 May 2018, Andrew Morton wrote: > On Wed, 9 May 2018 06:24:16 +0000 Dexuan Cui <[email protected]> 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.
Agreed. FWIW, this had been discussed before, but there was no real conclusion: https://lkml.kernel.org/r/alpine.DEB.2.20.1709161850010.2105@nanos Thanks, tglx
