Re: [RFC][RFT] memcg fix cgroup_mutex deadlock when cpuset reclaims memory

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 10 Dec 2008 10:49:47 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> Hi,
> 
> Here is a proposed fix for the memory controller cgroup_mutex deadlock
> reported. It is lightly tested and reviewed. I need help with review
> and test. Is the reported deadlock reproducible after this patch? A
> careful review of the cpuset impact will also be highly appreciated.
> 
> From: Balbir Singh <balbir@linux.vnet.ibm.com>
> 
> cpuset_migrate_mm() holds cgroup_mutex throughout the duration of
> do_migrate_pages(). The issue with that is that
> 
> 1. It can lead to deadlock with memcg, as do_migrate_pages()
>    enters reclaim
> 2. It can lead to long latencies, preventing users from creating/
>    destroying other cgroups anywhere else
> 
> The patch holds callback_mutex through the duration of cpuset_migrate_mm() and
> gives up cgroup_mutex while doing so.
> 
> Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
> ---
> 
>  include/linux/cpuset.h |   13 ++++++++++++-
>  kernel/cpuset.c        |   23 ++++++++++++-----------
>  2 files changed, 24 insertions(+), 12 deletions(-)
> 
> diff -puN kernel/cgroup.c~cpuset-remove-cgroup-mutex-from-update-path kernel/cgroup.c
> diff -puN kernel/cpuset.c~cpuset-remove-cgroup-mutex-from-update-path kernel/cpuset.c
> --- a/kernel/cpuset.c~cpuset-remove-cgroup-mutex-from-update-path
> +++ a/kernel/cpuset.c
> @@ -369,7 +369,7 @@ static void guarantee_online_mems(const 
>   * task has been modifying its cpuset.
>   */
>  
> -void cpuset_update_task_memory_state(void)
> +void __cpuset_update_task_memory_state(bool held)
>  {
>  	int my_cpusets_mem_gen;
>  	struct task_struct *tsk = current;
> @@ -380,7 +380,8 @@ void cpuset_update_task_memory_state(voi
>  	rcu_read_unlock();
>  
>  	if (my_cpusets_mem_gen != tsk->cpuset_mems_generation) {
> -		mutex_lock(&callback_mutex);
> +		if (!held)
> +			mutex_lock(&callback_mutex);
>  		task_lock(tsk);
>  		cs = task_cs(tsk); /* Maybe changed when task not locked */
>  		guarantee_online_mems(cs, &tsk->mems_allowed);
> @@ -394,7 +395,8 @@ void cpuset_update_task_memory_state(voi
>  		else
>  			tsk->flags &= ~PF_SPREAD_SLAB;
>  		task_unlock(tsk);
> -		mutex_unlock(&callback_mutex);
> +		if (!held)
> +			mutex_unlock(&callback_mutex);
>  		mpol_rebind_task(tsk, &tsk->mems_allowed);
>  	}
>  }
> @@ -949,13 +951,15 @@ static int update_cpumask(struct cpuset 
>   *    so that the migration code can allocate pages on these nodes.
>   *
>   *    Call holding cgroup_mutex, so current's cpuset won't change
> - *    during this call, as manage_mutex holds off any cpuset_attach()
> + *    during this call, as callback_mutex holds off any cpuset_attach()
>   *    calls.  Therefore we don't need to take task_lock around the
>   *    call to guarantee_online_mems(), as we know no one is changing
>   *    our task's cpuset.
>   *
>   *    Hold callback_mutex around the two modifications of our tasks
> - *    mems_allowed to synchronize with cpuset_mems_allowed().
> + *    mems_allowed to synchronize with cpuset_mems_allowed(). Give
> + *    up cgroup_mutex to avoid deadlocking with other subsystems
> + *    as we enter reclaim from do_migrate_pages().
>   *
>   *    While the mm_struct we are migrating is typically from some
>   *    other task, the task_struct mems_allowed that we are hacking
> @@ -976,17 +980,14 @@ static void cpuset_migrate_mm(struct mm_
>  {
>  	struct task_struct *tsk = current;
>  
> -	cpuset_update_task_memory_state();
> -
> +	cgroup_unlock();
>  	mutex_lock(&callback_mutex);
> +	cpuset_update_task_memory_state_locked();
>  	tsk->mems_allowed = *to;
> -	mutex_unlock(&callback_mutex);
> -
>  	do_migrate_pages(mm, from, to, MPOL_MF_MOVE_ALL);
> -
> -	mutex_lock(&callback_mutex);
>  	guarantee_online_mems(task_cs(tsk),&tsk->mems_allowed);
>  	mutex_unlock(&callback_mutex);
> +	cgroup_lock();
>  }
>  

Hmm...can't this happen ?

Assume there is a task X and cgroup Z1 and Z2. Z1 and Z2 doesn't need to be in
the same hierarchy.
== 
	CPU A attach task X to cgroup Z1
		cgroup_lock()
			for_each_subsys_state()
				=> attach(X,Z)
					=> migrate_mm()
						=> cgroup_unlock()
							migration

	CPU B attach task X to cgroup Z2 at the same time
		cgroup_lock()
			replace css_set.
==

Works on CPU B can't break for_each_subsys_state() in CPU A ?

Sorry if I misunderstand.

Thanks,
-Kame

		





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