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

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Date:	Wed, 16 Sep 2015 09:44:27 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Tejun Heo <tj@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List" 
	<linux-kernel@...r.kernel.org>, KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace
 signal_struct->group_rwsem with a global percpu_rwsem) causes regression for
 libvirt/kvm

Am 16.09.2015 um 03:24 schrieb Tejun Heo:
> Hello, Paul.
> 
> On Tue, Sep 15, 2015 at 04:38:18PM -0700, Paul E. McKenney wrote:
>> Well, the decision as to what is too big for -stable is owned by the
>> -stable maintainers, not by me.
> 
> Is it tho?  Usually the subsystem maintainer knows the best and has
> most say in it.  I was mostly curious whether you'd think that the
> changes would be too risky.  If not, great.
> 
>> I am suggesting trying the options and seeing what works best, then
>> working to convince people as needed.
> 
> Yeah, sure thing.  Let's wait for Christian.

Well, I have optimized my testcase now that is puts enough pressure to
the system to  confuses system (the older 209 version, which still has
some event loop issues) that systemd restarts the journal deamon and does
several other recoveries.
To avoid regressions - even for somewhat shaky userspaces - we should
consider a revert for 4.2 stable.
There are several followup patches, which makes the revert non-trivial,
though.

The rework of the percpu rwsem seems to work fine, but we are beyond the
merge window so 4.4 seems better to me. (and consider a revert for 4.3)

Christian

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