Hello, Peter!
TL;DR: If a normal !PF_NO_SETAFFINITY kthread invokes sched_setaffinity(),
and sched_setaffinity() returns 0, is it expected behavior for that
kthread to be running on some CPU other than one of the ones specified by
the in_mask argument? All CPUs are online, and there is no CPU-hotplug
activity taking place.
Thanx, Paul
Details:
I have long showed the following "toy" synchronize_rcu() implementation:
void synchronize_rcu(void)
{
int cpu;
for_each_online_cpu(cpu)
run_on(cpu);
}
I decided that if I was going to show it, I should test it. And it
occurred to me that run_on() can be a call to sched_setaffinity():
void synchronize_rcu(void)
{
int cpu;
for_each_online_cpu(cpu)
sched_setaffinity(current->pid, cpumask_of(cpu));
}
This actually passes rcutorture. But, as Andrea noted, not klitmus.
After some investigation, it turned out that klitmus was creating kthreads
with PF_NO_SETAFFINITY, hence the failures. But that prompted me to
put checks into my code: After all, rcutorture can be fooled.
void synchronize_rcu(void)
{
int cpu;
for_each_online_cpu(cpu) {
sched_setaffinity(current->pid, cpumask_of(cpu));
WARN_ON_ONCE(raw_smp_processor_id() != cpu);
}
}
This triggers fairly quickly, usually in less than a minute of rcutorture
testing. And further investigation shows that sched_setaffinity()
always returned 0. So I tried this hack:
void synchronize_rcu(void)
{
int cpu;
for_each_online_cpu(cpu) {
while (raw_smp_processor_id() != cpu)
sched_setaffinity(current->pid,
cpumask_of(cpu));
WARN_ON_ONCE(raw_smp_processor_id() != cpu);
}
}
This never triggers, and rcutorture's grace-period rate is not significantly
affected.
Is this expected behavior? Is there some configuration or setup that I
might be missing?
