Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH -v3 8/9] sched: Fix CPU hotplug / tighten is_per_cpu_kthread() | Date | Thu, 21 Jan 2021 14:01:03 +0000 |
| |
On 21/01/21 11:17, Peter Zijlstra wrote: > @@ -7504,6 +7525,9 @@ int sched_cpu_deactivate(unsigned int cp > * preempt-disabled and RCU users of this state to go away such that > * all new such users will observe it. > * > + * Specifically, we rely on ttwu to no longer target this CPU, see > + * ttwu_queue_cond() and is_cpu_allowed(). > + *
So the last time ttwu_queue_wakelist() can append a task onto a dying CPU's wakelist is before sched_cpu_deactivate()'s synchronize_rcu() returns.
As discussed on IRC, paranoia would have us issue a
flush_smp_call_function_from_idle()
upon returning from said sync, but this will require further surgery.
Do we want something like the below in the meantime? Ideally we'd warn on setting rq->ttwu_pending when !cpu_active(), but as per the above this is allowed before the synchronize_rcu() returns.
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index ed6ff94aa68a..4b5b4b02ee64 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7590,6 +7590,7 @@ int sched_cpu_starting(unsigned int cpu) */ int sched_cpu_wait_empty(unsigned int cpu) { + WARN_ON_ONCE(READ_ONCE(cpu_rq(cpu)->ttwu_pending)); balance_hotplug_wait(); return 0; } > * Do sync before park smpboot threads to take care the rcu boost case. > */ > synchronize_rcu();
| |