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

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 16.09.2015 um 09:44 schrieb Christian Borntraeger:
> 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)

FWIW, I added a printk to percpu_down_write. With KVM and uprobes disabled,
just booting up a fedora20 gives me __6749__ percpu_down_write calls on 4.2.
systemd seems to do that for the processes. 

So a revert is really the right thing to do. In fact, I dont know if the
rcu_sync_enter rework is enough. With systemd setting the cgroup seem to
be NOT a cold/seldom case.

Christian