Messages in this thread Patch in this message | | | From | Michael Ellerman <> | Subject | Re: Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot) | Date | Tue, 11 Oct 2016 22:22:13 +1100 |
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Tejun Heo <tj@kernel.org> writes:
> Hello, Michael. > > On Mon, Oct 10, 2016 at 09:22:55PM +1100, Michael Ellerman wrote: >> This patch seems to be causing one of my Power8 boxes not to boot. >> >> Specifically commit 3347fa092821 ("workqueue: make workqueue available >> early during boot") in linux-next. >> >> If I revert this on top of next-20161005 then the machine boots again. >> >> I've attached the oops below. It looks like the cfs_rq of p->se is NULL? > > Hah, weird that it's arch dependent, or maybe it's just different > config options. Most likely, it's caused by workqueue_init() call > being moved too early. Can you please try the following patch and see > whether the problem goes away?
No that doesn't help.
What does is this:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 94732d1ab00a..4e79549d242f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1614,7 +1614,8 @@ int select_task_rq(struct task_struct *p, int cpu, int sd_flags, int wake_flags) * [ this allows ->select_task() to simply return task_cpu(p) and * not worry about this generic constraint ] */ - if (unlikely(!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)) || + if (unlikely(cpu >= nr_cpu_ids || + !cpumask_test_cpu(cpu, tsk_cpus_allowed(p)) || !cpu_online(cpu))) cpu = select_fallback_rq(task_cpu(p), p); The oops happens because we're in enqueue_task_fair() and p->se->cfs_rq is NULL.
The cfs_rq is NULL because we did set_task_rq(p, 2048), where 2048 is NR_CPUS. That causes us to index past the end of the tg->cfs_rq array in set_task_rq() and happen to get NULL.
We never should have done set_task_rq(p, 2048), because 2048 is >= nr_cpu_ids, which means it's not a valid CPU number, and set_task_rq() doesn't cope with that.
The reason we're calling set_task_rq() with CPU 2048 is because in select_task_rq() we had tsk_nr_cpus_allowed() = 0, because tsk_cpus_allowed(p) is an empty cpu mask.
That means we do in select_task_rq(): cpu = cpumask_any(tsk_cpus_allowed(p)); And when tsk_cpus_allowed(p) is empty cpumask_any() returns nr_cpu_ids, causing cpu to be set to 2048 in my case.
select_task_rq() then does the check to see if it should use a fallback rq:
if (unlikely(!cpumask_test_cpu(cpu, tsk_cpus_allowed(p)) || !cpu_online(cpu))) cpu = select_fallback_rq(task_cpu(p), p);
But in both those checks we end up indexing off the end of the cpu mask, because cpu is >= nr_cpu_ids. At least on my system they both return true and so we return cpu == 2048.
The patch above is pretty clearly not the right fix, though maybe it's a good safety measure.
Presumably we shouldn't be ending up with tsk_cpus_allowed() being empty, but I haven't had time to track down why that's happening.
cheers
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