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Date: Wed, 16 Sep 2015 07:03:51 -0400
From: Tejun Heo <tj@kernel.org>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
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	<linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace
 signal_struct->group_rwsem with a global percpu_rwsem) causes regression for
 libvirt/kvm
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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?

Thanks.

-- 
tejun