Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem) causes regression for libvirt/kvm

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/09/2015 14:22, Oleg Nesterov wrote:
> >  The issue is that rcu_sync doesn't eliminate synchronize_sched,
> 
> Yes, but it eliminates _expedited(). This is good, but otoh this means
> that (say) individual __cgroup_procs_write() can take much more time.
> However, it won't block the readers and/or disturb the whole system.

According to Christian, removing the _expedited() "makes things worse"
in that the system takes ages to boot up and systemd timeouts.  So I'm
still a bit wary about anything that uses RCU for the cgroups write side.

However, rcu_sync is okay with him, so perhaps it is really really
effective.  Christian, can you instrument how many synchronize_sched
(out of the 6479 cgroup_procs_write calls) are actually executed at boot
with the rcu rework?

Paolo

> And percpu_up_write() doesn't do synchronize_sched() at all.