Subject: Re: [Next] CPU Hotplug test failures on powerpc

From: Xiaotian Feng <[email-masked]>
Date: Wed, 16 Dec 2009 15:57:56 +0800

On Wed, Dec 16, 2009 at 3:18 PM, Peter Zijlstra <[email-masked]> wrote:
> On Wed, 2009-12-16 at 12:24 +0530, Sachin Sant wrote:
>> Xiaotian Feng wrote:
>> > On Wed, Dec 16, 2009 at 2:41 PM, Sachin Sant <[email-masked]> wrote:
>> >
>> >> Xiaotian Feng wrote:
>> >>
>> >>> Does this testcase hotplug cpu 0 off?
>> >>>
>> >>>
>> >> No, i don't think so. It skips cpu0 during online/offline
>> >> process.
>> >>
>> >
>> > Then how could this happen ? Looks like cpu 0 is offline ....
>> > 0:mon> <4>IRQ 17 affinity broken off cpu 0
>> > <4>IRQ 18 affinity broken off cpu 0
>> > <4>IRQ 19 affinity broken off cpu 0
>> > <4>IRQ 264 affinity broken off cpu 0
>> > <4>cpu 0 (hwid 0) Ready to die...
>> > <7>clockevent: decrementer mult[83126e97] shift[32] cpu[0]
>> >
>> Sorry i was looking at only one script. Looking more closely
>> at the test there are 6 different sub tests. The rest of the
>> tests do seem to hotplug CPU 0.
>
> Ooh, cute, so you can actually hotplug cpu 0.. no wonder that didn't get
> exposed on x86.
>
> Still, the only time cpu_active_mask should not be equal to
> cpu_online_mask is when we're in the middle of a hotplug, we clear
> active early and set it late, but its all done under the hotplug mutex,
> so we can at most have 1 cpu differences with online mask.
>

Could follow be possible?  We know there's cpu 0 and cpu 1,

offline cpu1 > done
offline cpu0 > false

consider this in cpu_down code,

int __ref cpu_down(unsigned int cpu)
{
<snip>
        set_cpu_active(cpu, false); // here, we set cpu 0 to inactive

        synchronize_sched();

        err = _cpu_down(cpu, 0);
out:
<snip>
}

Then in _cpu_down code:

static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
<snip>
        if (num_online_cpus() == 1)        // if we're trying to
offline cpu0, num_online_cpus will be 1
                return -EBUSY;                    // after return back
to cpu_down, we didn't change cpu 0 back to active

        if (!cpu_online(cpu))
                return -EINVAL;

        if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
                return -ENOMEM;
<snip>
}

Then cpu 0 is not active, but online, then we try to offline cpu1, .......
This can not be exposed because x86 does not have
/sys/devices/system/cpu0/online.
I guess following patch fixes this bug.

---
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 291ac58..21ddace 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -199,14 +199,18 @@ static int __ref _cpu_down(unsigned int cpu, int
tasks_frozen)
                .hcpu = hcpu,
        };

-       if (num_online_cpus() == 1)
+       if (num_online_cpus() == 1) {
+               set_cpu_active(cpu, true);
                return -EBUSY;
+       }

        if (!cpu_online(cpu))
                return -EINVAL;

-       if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
+       if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL)) {
+               set_cpu_active(cpu, true);
                return -ENOMEM;
+       }

        cpu_hotplug_begin();
        err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,

> Unless of course, I messed up, which appears to be rather likely given
> these problems ;-)
>
>