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 13:03 schrieb Tejun Heo:
> Hello,
> 
> On Wed, Sep 16, 2015 at 12:58:00PM +0200, Christian Borntraeger wrote:
>> 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.
> 
> Booting would usually be the hottest operation for that and it's still
> *relatively* cold path compared to the reader side which is task
> fork/exit paths.  The whole point is shift overhead from hotter reader
> side.  Can you see problems with percpu_rwsem rework?

As I said, it seems the rcu tree with that change seems to work fine on my
system. This needs a more testing on other machines, though. I guess a 
revert plus a re-add in the 4.4 merge window should give us enough test
coverage.

Christian