linux-kernel - Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu

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Message-ID: <55F9621A.7070803@redhat.com>
Date:	Wed, 16 Sep 2015 14:35:38 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace
 signal_struct->group_rwsem with a global percpu_rwsem) causes regression for
 libvirt/kvm



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.
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