Messages in this thread Patch in this message | | | Date | Wed, 10 Apr 2024 15:55:19 +0200 | From | Oleg Nesterov <> | Subject | Re: Nohz_full on boot CPU is broken (was: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work) |
| |
Hi Nicholas,
On 04/10, Nicholas Piggin wrote: > > Thanks for this. Taking a while to page this back in, the intention is > for housekeeping to be done by boot CPU until house keeper is awake, so > returning smp_processor_id() seems like the right thing to do here for > ephemeral jobs like timers and work, provided that CPU / mask is not > stored somewhere long term by the caller. > > For things that set an affinity like kthread, sched, maybe managed > irqs, and such. > > There are not many callers of housekeeping_any_cpu() so that's easy > enough to verify. But similar like housekeeping_cpumask() and others > could be an issue or at least a foot-gun, I'm not sure how well I > convinced myself of those. > > Could you test like this? > > WARN_ON_ONCE(system_state == SYSTEM_RUNNING || > type != HK_TYPE_TIMER); > > With a comment to say other ephemeral mask types could be exempted if > needed.
Sorry, I don't understand... Let me repeat, I know absolutely nothing about nonhz/etc. I didn't even try to really fix the problem(s), I am only trying to find a minimal/simple workaround to fix the problem we hit in Red Hat.
This is what I was going to send:
--- a/kernel/sched/isolation.c +++ b/kernel/sched/isolation.c @@ -46,7 +46,15 @@ int housekeeping_any_cpu(enum hk_type type) if (cpu < nr_cpu_ids) return cpu; - return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); + cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); + if (likely(cpu < nr_cpu_ids)) + return cpu; + /* + * Unless we have another problem this can only happen + * at boot time before start_secondary() brings the 1st + * housekeeping CPU up. + */ + WARN_ON_ONCE(system_state == SYSTEM_RUNNING); } } return smp_processor_id();
Yes, this fixes the symptom, not the problem. And yes, "another problem" mentioned in the comment is very possible, say "maxcpus" kernel-parameter can be less than the first housekeeping cpu. But in this case the user should blame himself (and I am not sure the kernel will boot).
I don't understand why do you suggest to add "|| type != HK_TYPE_TIMER", currently all the callers of housekeeping_any_cpu() use type == HK_TIMER. But OK, I can add this check. I guess for the case it finds another user with type != HK_TYPE_TIMER which can't use smp_processor_id() even at boot time or stores the returned CPU for the long term.
Will you agree with the change above or what do you suggest instead as a simple workaround?
> It would also be nice to warn for cases that would be bugs if the boot > CPU was not in the HK mask. Could that be done by having a > housekeepers_online() call after smp_init() (maybe at the start of > sched_init_smp()) that could verify there is at least one online, and > set a flag that could be used to create warnings.
Again, I am not sure I understand, but I too thought that something like
housekeeping_check(void) { for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX) { if (!cpumask_intersects(cpu_online, housekeeping.cpumasks[type])) panic(); } after bringup_nonboot_cpus(setup_max_cpus).
But I am not sure this is correct and this is another (although related) issue.
Oleg.
| |