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 07:03:51 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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

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
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