linux-kernel - Re: [PATCH 1/2] sched/cgroup: move sched_online_group() back into css

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Date:   Thu, 26 Jan 2017 13:27:21 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/cgroup: move sched_online_group() back into
 css_online()



On 26.01.2017 13:17, Peter Zijlstra wrote:
> On Thu, Jan 26, 2017 at 12:41:41PM +0300, Konstantin Khlebnikov wrote:
>> Commit 2f5177f0fd7e ("sched/cgroup: Fix/cleanup cgroup teardown/init") moved
>> sched_online_group() from css_online() to css_alloc(). It exposes half-baked
>> task group into global lists before initializing generic cgroup stuff.
>>
>> LTP testcase (third in cgroup_regression_test) written for testing
>> similar race in kernels 2.6.26-2.6.28 easily triggers this oops:
>>
>
> So nobody's run LTP against the kernel for almost a year?

Yep. Nobody runs LTP =)

CONFIG_RT_GROUP_SCHED must be y (which almost impossible to use IRL, I have some patches for it out of tree)

Also systemd by default binds cgroups cpu and cpuacct together - this breaks testcase.

>
>>
>> Here task group already linked into global RCU-protected list task_groups
>> but pointer css->cgroup is still NULL.
>>
>> This patch reverts this chunk and moves online back to css_online().
>
> Maybe put a comment with it that explains why this is needed?
>
> Something along the lines of this perhaps?
>

Actually online is called before complete initialization. See second patch. =)
I don't know what to do with this. cgroups are messy as always.

> /*
>  * Don't expose the cgroup until initialization of it is complete in the
>  * cgroup core. Otherwise things like cgroup_path() will return NULL
>  * pointers and the like.
>  */
>
>> +static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
>> +{
>> +	struct task_group *tg = css_tg(css);
>> +	struct task_group *parent = css_tg(css->parent);
>> +
>> +	if (parent)
>> +		sched_online_group(tg, parent);
>> +	return 0;
>> +}