On 21/01/21 15:18, Peter Zijlstra wrote: > On Thu, Jan 21, 2021 at 02:01:03PM +0000, Valentin Schneider wrote: >> 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. > > Right, specifically RCU needs a little more help there. > >> 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. > > I'm not sure I'm brave enough to add that just now :/
I get you; I couldn't come up with a better scheme that would give us a bit more info than the sched_cpu_dying() splat :/
