linux-kernel - Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu

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Message-ID: <55F95784.8070608@de.ibm.com>
Date:	Wed, 16 Sep 2015 13:50:28 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Tejun Heo <tj@...nel.org>
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

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

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