Subject: Re: [next-20170609] Oops while running CPU off-on
 (cpuset.c/cpuset_can_attach)
From: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
To: Tejun Heo <tj@kernel.org>
Cc: sachinp <sachinp@linux.vnet.ibm.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>, ego <ego@linux.vnet.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Ingo Molnar <mingo@kernel.org>, mpe <mpe@ellerman.id.au>
Date: Fri, 07 Jul 2017 12:09:34 +0530
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         <20170627153608.GD2289@htj.duckdns.org>
         <1499092582.10651.15.camel@abdul.in.ibm.com>
         <20170705152855.GD19330@htj.duckdns.org>
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On Wed, 2017-07-05 at 11:28 -0400, Tejun Heo wrote:
> Hello, Abdul.
> 
> Thanks for the debug info.  Can you please see whether the following
> patch fixes the issue?  

It is my pleasure and yes the patch fixes the problem.

> If the problem is too difficult to reproduce

The problem was reproducible all the time. 

With the patch fix, I tried multiple times and long runs of cpu off-on
cycles but no Oops is seen.

Thank you for spending your valuable time on fixing this issue.

Reported-and-tested-by : Abdul Haleem <abdhalee@linux.vnet.ibm.com>

> to confirm the fix by seeing whether it no longer triggers, please let
> me know.  We can instead apply a patch which triggers WARN on the
> failing condition to confirm the diagnosis.
> 
> Thanks.
> 
> diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
> index 793565c05742..8b4c3c2f2509 100644
> --- a/kernel/cgroup/cgroup-internal.h
> +++ b/kernel/cgroup/cgroup-internal.h
> @@ -33,6 +33,9 @@ struct cgroup_taskset {
>  	struct list_head	src_csets;
>  	struct list_head	dst_csets;
> 
> +	/* the number of tasks in the set */
> +	int			nr_tasks;
> +
>  	/* the subsys currently being processed */
>  	int			ssid;
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index dbfd7028b1c6..e3c4152741a3 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -1954,6 +1954,8 @@ static void cgroup_migrate_add_task(struct task_struct *task,
>  	if (!cset->mg_src_cgrp)
>  		return;
> 
> +	mgctx->tset.nr_tasks++;
> +
>  	list_move_tail(&task->cg_list, &cset->mg_tasks);
>  	if (list_empty(&cset->mg_node))
>  		list_add_tail(&cset->mg_node,
> @@ -2047,16 +2049,18 @@ static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
>  		return 0;
> 
>  	/* check that we can legitimately attach to the cgroup */
> -	do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
> -		if (ss->can_attach) {
> -			tset->ssid = ssid;
> -			ret = ss->can_attach(tset);
> -			if (ret) {
> -				failed_ssid = ssid;
> -				goto out_cancel_attach;
> +	if (tset->nr_tasks) {
> +		do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
> +			if (ss->can_attach) {
> +				tset->ssid = ssid;
> +				ret = ss->can_attach(tset);
> +				if (ret) {
> +					failed_ssid = ssid;
> +					goto out_cancel_attach;
> +				}
>  			}
> -		}
> -	} while_each_subsys_mask();
> +		} while_each_subsys_mask();
> +	}
> 
>  	/*
>  	 * Now that we're guaranteed success, proceed to move all tasks to
> @@ -2085,25 +2089,29 @@ static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
>  	 */
>  	tset->csets = &tset->dst_csets;
> 
> -	do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
> -		if (ss->attach) {
> -			tset->ssid = ssid;
> -			ss->attach(tset);
> -		}
> -	} while_each_subsys_mask();
> +	if (tset->nr_tasks) {
> +		do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
> +			if (ss->attach) {
> +				tset->ssid = ssid;
> +				ss->attach(tset);
> +			}
> +		} while_each_subsys_mask();
> +	}
> 
>  	ret = 0;
>  	goto out_release_tset;
> 
>  out_cancel_attach:
> -	do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
> -		if (ssid == failed_ssid)
> -			break;
> -		if (ss->cancel_attach) {
> -			tset->ssid = ssid;
> -			ss->cancel_attach(tset);
> -		}
> -	} while_each_subsys_mask();
> +	if (tset->nr_tasks) {
> +		do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
> +			if (ssid == failed_ssid)
> +				break;
> +			if (ss->cancel_attach) {
> +				tset->ssid = ssid;
> +				ss->cancel_attach(tset);
> +			}
> +		} while_each_subsys_mask();
> +	}
>  out_release_tset:
>  	spin_lock_irq(&css_set_lock);
>  	list_splice_init(&tset->dst_csets, &tset->src_csets);
> 


-- 
Regard's

Abdul Haleem
IBM Linux Technology Centre