On Mon, Jul 09, 2018 at 12:12:15PM +0100, David Woodhouse wrote: > On Mon, 2018-07-09 at 13:06 +0200, Peter Zijlstra wrote: > > 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. > > I don't know that it is; I'm merely going by the empirical observation > that with a check for rcu_urgent_qs in the vcpu_run() loop, KVM is no > longer screwing over synchronize_sched() for 10 seconds at a time. Or > even 1 second at a time. It would be good to know what exactly sets that variable in your case. > I'm all for considering a CPU in guest mode to be quiescent, and not > waiting for it at all. But we don't do that without full NOHZ even for > CPUs in userspace. Doing it for guests should be easier than for userspace, since vmenter/vmexit are (afaik) _much_ more expensive than sysenter/sysexit.