Re: [PREEMPT-RT] [PATCH] s390/cpum_sf: Remove superfluous SMP function call

Heiko Carstens
2016-04-05T11:36:55Z

On Tue, Apr 05, 2016 at 01:23:36PM +0200, Heiko Carstens wrote:
> On Tue, Apr 05, 2016 at 01:13:06PM +0200, Sebastian Andrzej Siewior wrote:
> > On 04/05/2016 12:49 PM, Heiko Carstens wrote:
> > >> --- a/arch/s390/kernel/perf_cpum_sf.c
> > >> +++ b/arch/s390/kernel/perf_cpum_sf.c
> > >> @@ -1510,7 +1510,6 @@ static void cpumf_measurement_alert(stru
> > >> static int cpumf_pmu_notifier(struct notifier_block *self,
> > >> unsigned long action, void *hcpu)
> > >> {
> > >> - unsigned int cpu = (long) hcpu;
> > >> int flags;
> > >>
> > >> /* Ignore the notification if no events are scheduled on the PMU.
> > >> @@ -1523,11 +1522,15 @@ static int cpumf_pmu_notifier(struct not
> > >> case CPU_ONLINE:
> > >> case CPU_DOWN_FAILED:
> > >> flags = PMC_INIT;
> > >> - smp_call_function_single(cpu, setup_pmc_cpu, &flags, 1);
> > >> + local_irq_disable();
> > >> + setup_pmc_cpu(&flags);
> > >> + local_irq_enable();
> > >> break;
> > >
> > > ...but at least the CPU_DOWN_FAILED callback will not necessarily be called
> > > on the cpu that couldn't be brought offline.
> >
> > I don't follow.
>
> I was trying to say that if bringing a cpu down fails, then the cpu hotplug
> notifier with CPU_DOWN_FAILED might be called on a cpu that is _not_ the
> same cpu that was supposed to be brought offline.
>
> Subsequently, in this case, the setup_pmc_cpu() call will be executed on
> the wrong cpu.

.. or to illustrate this behaviour: the following patch (white space
damaged due to copy-paste) results in the following:

# chcpu -d 2
[console] failed cpu: 2 - this cpu: 1

diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 40a6b4f9c36c..48d417abfdae 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -852,6 +852,7 @@ int __cpu_disable(void)
{
unsigned long cregs[16];

+ return -EBUSY;
/* Handle possible pending IPIs */
smp_handle_ext_call();
set_cpu_online(smp_processor_id(), false);
@@ -1065,6 +1066,10 @@ static int smp_cpu_notify(struct notifier_block *self, unsigned long action,
int err = 0;

switch (action & ~CPU_TASKS_FROZEN) {
+ case CPU_DOWN_FAILED:
+ printk("failed cpu: %d - this cpu: %d\n", cpu, get_cpu());
+ put_cpu();
+ break;
case CPU_ONLINE:
err = sysfs_create_group(&s->kobj, &cpu_online_attr_group);
break;