Re: [PATCH v3 7/7] cpuset: fix to migrate mm correctly in a corner case

[Tejun Heo <tj@kernel.org>]

On Sun, Jun 09, 2013 at 05:17:03PM +0800, Li Zefan wrote:
> Before moving tasks out of empty cpusets, update_tasks_nodemask()
> is called, which calls do_migrate_pages(xx, from, to). Then those
> tasks are moved to an ancestor, and do_migrate_pages() is called
> again.
> 
> The first time: from = node_to_be_offlined, to = empty.
> The second time: from = empty, to = ancestor's nodemask.
> 
> so looks like no pages will be migrated.
> 
> Fix this by:
> 
> - Don't call update_tasks_nodemask() on empty cpusets.
> - Pass cs->old_mems_allowed to do_migrate_pages().
> 
> Signed-off-by: Li Zefan <lizefan@huawei.com>
> ---
>  kernel/cpuset.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index d1d6782..3cd68b8 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -1563,9 +1563,16 @@ static void cpuset_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
>  		struct cpuset *mems_oldcs = effective_nodemask_cpuset(oldcs);
>  
>  		mpol_rebind_mm(mm, &cpuset_attach_nodemask_to);
> -		if (is_memory_migrate(cs))
> -			cpuset_migrate_mm(mm, &mems_oldcs->mems_allowed,
> +		if (is_memory_migrate(cs)) {
> +			/*
> +			 * old_mems_allowed is the same with mems_allowed here,
> +			 * except if this task is being moved automatically due
> +			 * to hotplug, and in this case mems_allowed is empty
> +			 * and old_mems_allowed is the offlined node.
> +			 */

Wouldn't it be better if the above explain why rather than what?

> +			cpuset_migrate_mm(mm, &mems_oldcs->old_mems_allowed,
>  					  &cpuset_attach_nodemask_to);
> +		}
>  		mmput(mm);
>  	}
>  
> @@ -2155,7 +2162,7 @@ retry:
>  	 * for empty cpuset to take on ancestor's cpumask.
>  	 */
>  	if ((sane && cpumask_empty(cs->cpus_allowed)) ||
> -	    !cpumask_empty(&off_cpus))
> +	    (!cpumask_empty(&off_cpus) && !cpumask_empty(cs->cpus_allowed)))
>  		update_tasks_cpumask(cs, NULL);

No biggie but maybe we want briefly explain the conditions being tested?

Thanks.

-- 
tejun