linux-kernel - Re: [PATCH 9/9] cgroup: use cgroup->self.refcnt for cgroup refcnting

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Date:	Fri, 16 May 2014 10:37:18 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	ARM kernel mailing list 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 9/9] cgroup: use cgroup->self.refcnt for cgroup refcnting

Hello, Stephen.

On Thu, May 15, 2014 at 03:56:51PM -0600, Stephen Warren wrote:
> On 05/15/2014 01:07 PM, Stephen Warren wrote:
> > On 05/09/2014 03:13 PM, Tejun Heo wrote:
> >> Currently cgroup implements refcnting separately using atomic_t
> >> cgroup->refcnt.  The destruction paths of cgroup and css are rather
> >> complex and bear a lot of similiarities including the use of RCU and
> >> bouncing to a work item.
> >>
> >> This patch makes cgroup use the refcnt of self css for refcnting
> >> instead of using its own.  This makes cgroup refcnting use css's
> >> percpu refcnt and share the destruction mechanism.
> >> ...
> > 
> > In next-20150515, this patch causes silent boot failures on NVIDIA Tegra
> > ARM systems. There isn't even any earlyprintk. Reverting this one patch
> > solves this.
> > 
> > I was a bit surprised that cgroups would cause such an issue, but
> > looking at a normal kernel boot, I guess cgroup does get initialized
> > very early:
> > 
> > Uncompressing Linux... done, booting the kernel.
> > [    0.000000] Booting Linux on physical CPU 0x0
> > [    0.000000] Initializing cgroup subsys cpu
> > [    0.000000] Initializing cgroup subsys cpuacct
> > 
> > Anyway, I'll try to track down what the problem is, but if someone
> > familiar with this patch could give it a look, that'd be very useful,
> > since I'm not at all familiar with this code or feature.
> 
> It looks like this has something to do with the new code using percpu_
> functions from within cgroup_init_early, yet that function being called
> before init/main.c calls setup_per_cpu_areas() etc.? Unfortunately I
> have to run to some meetings now, so won't look at this until tomorrow.

Yeap, I know where the issue is.  Will post a fix soon.

Thanks.

-- 
tejun
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