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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2008-12-10 17:49:06]:

> 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()

You mean for_each_subsys() right?

> 				=> 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 ?
>

for_each_subsys is hierarchy aware, so if we try to add the same task
to different hierachies, it should not be a problem right?
 
> Sorry if I misunderstand.

I hope I understood your scenario correctly.

-- 
	Balbir

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