linux-kernel - Re: RCU related performance regression in 3.3

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Date:	Wed, 18 Apr 2012 17:00:14 +0200
From:	Pascal Chapperon <pascal.chapperon@...adoo.fr>
To:	paulmck@...ux.vnet.ibm.com
CC:	Josh Boyer <jwboyer@...hat.com>, linux-kernel@...r.kernel.org,
	kernel-team@...oraproject.org
Subject: Re: RCU related performance regression in 3.3

Le 18/04/2012 16:01, Paul E. McKenney a écrit :
> On Wed, Apr 18, 2012 at 11:37:28AM +0200, Pascal Chapperon wrote:
>> Mount and umount operations are not slower with RCU_FAST_NO_HZ during
>> runtime; systemctl start and stop operations are also not slower. In
>> fact, i couldn't find a single operation slower during runtime with
>> RCU_FAST_NO_HZ.
>
> Your boot-time setup is such that all CPUs are online before the
> boot-time mount operations take place, right?
Yes :
[    0.242697] Brought up 8 CPUs
[    0.242699] Total of 8 processors activated (35118.33 BogoMIPS).

>  Struggling to understand
> how RCU can tell the difference between post-CPU-bringup boot time
> and run time...
>
systemd is controlling the whole boot process including mount
operation (apart root filesystem) and as I can see, uses heavily
sockets to do it (not to mention cpu-affinity). It controls also the
major part of umount operations. Is it possible that your patch hits
a systemd bug ?
What I don't understand is that systemd coexists well with
RCU_FAST_NO_HZ on a smaller laptop with older and much less powerful
cpu.
I'll do further tests on another machine.

Pascal

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