Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

Peter Zijlstra
2018-07-09T11:07:05Z

On Mon, Jul 09, 2018 at 11:56:41AM +0100, David Woodhouse wrote:

> > But either proposal is exactly the same in this respect. The whole
> > rcu_urgent_qs thing won't be set any earlier either.
>
> Er.... Marius, our latencies in expand_fdtable() definitely went from
> ~10s to well below one second when we just added the rcu_all_qs() into
> the loop, didn't they? And that does nothing if !rcu_urgent_qs.

Argh I never found that, because obfuscation:

ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
...
smp_store_release(ruqp, true);

I, using git grep "rcu_urgent_qs.*true" only found
rcu_request_urgent_qs_task() and sync_sched_exp_handler().

But how come KVM even triggers that case; rcu_implicit_dynticks_qs() is
for NOHZ and offline CPUs.