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Date: Wed, 16 Sep 2015 19:00:07 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
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        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>, KVM list <kvm@vger.kernel.org>
Subject: Re: [PATCH cgroup/for-4.3-fixes 1/2] Revert "cgroup: simplify
	threadgroup locking"
Message-ID: <20150916170007.GA10436@redhat.com>
References: <55F8097A.7000206@de.ibm.com> <20150916155545.GB3243@mtj.duckdns.org>
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On 09/16, Tejun Heo wrote:
>
> From f9f9e7b776142fb1c0782cade004cc8e0147a199 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Wed, 16 Sep 2015 11:51:12 -0400
>
> This reverts commit b5ba75b5fc0e8404e2c50cb68f39bb6a53fc916f.
>
> d59cfc09c32a ("sched, cgroup: replace signal_struct->group_rwsem with
> a global percpu_rwsem") and b5ba75b5fc0e ("cgroup: simplify
> threadgroup locking") changed how cgroup synchronizes against task
> fork and exits so that it uses global percpu_rwsem instead of
> per-process rwsem; unfortunately, the write [un]lock paths of
> percpu_rwsem always involve synchronize_rcu_expedited() which turned
> out to be too expensive.
>
> Improvements for percpu_rwsem are scheduled to be merged in the coming
> v4.4-rc1 merge window which alleviates this issue.  For now, revert
> the two commits to restore per-process rwsem.  They will be re-applied
> for the v4.4-rc1 merge window.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
> Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: stable@vger.kernel.org # v4.2+

So just in case, I agree. Perhaps we could merge the percpu_rwsem changes
in v4.3, but these patches look much safer for -stable.

Oleg.