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

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Message-ID: <20150916124348.GA30646@redhat.com>
Date:	Wed, 16 Sep 2015 14:43:48 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Paolo Bonzini <pbonzini@...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 09/16, Paolo Bonzini wrote:
>
>
> 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"

Yes sure, we can not just remove _expedited() from down/up_read().

> in that the system takes ages to boot up and systemd timeouts.

Yes, this is clear

> 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?

Heh, another change I have in mind. It would be nice to add some trace
points. But firstly we should merge the current code.

Oleg.

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