Re: [patch 6/6] mm: memcg: do not trap chargers with full callstackon OOM

[Johannes Weiner]

On Tue, Jul 30, 2013 at 04:09:13PM +0200, Michal Hocko wrote:
> On Fri 26-07-13 17:28:09, Johannes Weiner wrote:
> [...]
> > From: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Subject: [patch] mm: memcg: rework and document OOM serialization
> > 
> > 1. Remove the return value of mem_cgroup_oom_unlock().
> > 
> > 2. Rename mem_cgroup_oom_lock() to mem_cgroup_oom_trylock().
> > 
> > 3. Pull the prepare_to_wait() out of the memcg_oom_lock scope.  This
> >    makes it more obvious that the task has to be on the waitqueue
> >    before attempting to OOM-trylock the hierarchy, to not miss any
> >    wakeups before going to sleep.  It just didn't matter until now
> >    because it was all lumped together into the global memcg_oom_lock
> >    spinlock section.
> > 
> > 4. Pull the mem_cgroup_oom_notify() out of the memcg_oom_lock scope.
> >    It is proctected by the hierarchical OOM-lock.
> > 
> > 5. The memcg_oom_lock spinlock is only required to propagate the OOM
> >    lock in any given hierarchy atomically.  Restrict its scope to
> >    mem_cgroup_oom_(trylock|unlock).
> > 
> > 6. Do not wake up the waitqueue unconditionally at the end of the
> >    function.  Only the lockholder has to wake up the next in line
> >    after releasing the lock.
> > 
> >    Note that the lockholder kicks off the OOM-killer, which in turn
> >    leads to wakeups from the uncharges of the exiting task.  But any
> >    contender is not guaranteed to see them if it enters the OOM path
> >    after the OOM kills but before the lockholder releases the lock.
> >    Thus the wakeup has to be explicitely after releasing the lock.
> > 
> > 7. Put the OOM task on the waitqueue before marking the hierarchy as
> >    under OOM as that is the point where we start to receive wakeups.
> >    No point in listening before being on the waitqueue.
> > 
> > 8. Likewise, unmark the hierarchy before finishing the sleep, for
> >    symmetry.
> > 
> 
> OK, this looks better than what we have today, but still could be done
> better IMO ;)
> 
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > ---
> >  mm/memcontrol.c | 85 +++++++++++++++++++++++++++++++--------------------------
> >  1 file changed, 47 insertions(+), 38 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 30ae46a..0d923df 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2076,15 +2076,18 @@ static int mem_cgroup_soft_reclaim(struct mem_cgroup *root_memcg,
> >  	return total;
> >  }
> >  
> 
> /* Protects oom_lock hierarchy consistent state and oom_notify chain */

See 4. :-)

> > +static DEFINE_SPINLOCK(memcg_oom_lock);
> > +
> >  /*
> >   * Check OOM-Killer is already running under our hierarchy.
> >   * If someone is running, return false.
> > - * Has to be called with memcg_oom_lock
> >   */
> [...]
> > @@ -2195,45 +2197,52 @@ static bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask,
> >  				  int order)
> >  {
> >  	struct oom_wait_info owait;
> > -	bool locked, need_to_kill;
> > +	bool locked;
> >  
> >  	owait.memcg = memcg;
> >  	owait.wait.flags = 0;
> >  	owait.wait.func = memcg_oom_wake_function;
> >  	owait.wait.private = current;
> >  	INIT_LIST_HEAD(&owait.wait.task_list);
> > -	need_to_kill = true;
> > -	mem_cgroup_mark_under_oom(memcg);
> >  
> > -	/* At first, try to OOM lock hierarchy under memcg.*/
> > -	spin_lock(&memcg_oom_lock);
> > -	locked = mem_cgroup_oom_lock(memcg);
> >  	/*
> > +	 * As with any blocking lock, a contender needs to start
> > +	 * listening for wakeups before attempting the trylock,
> > +	 * otherwise it can miss the wakeup from the unlock and sleep
> > +	 * indefinitely.  This is just open-coded because our locking
> > +	 * is so particular to memcg hierarchies.
> > +	 *
> >  	 * Even if signal_pending(), we can't quit charge() loop without
> >  	 * accounting. So, UNINTERRUPTIBLE is appropriate. But SIGKILL
> >  	 * under OOM is always welcomed, use TASK_KILLABLE here.
> 
> Could you take care of this paragraph as well, while you are at it,
> please? I've always found it it confusing. I would remove it completely
> I would remove it completely.

It's removed completely in the same series, that's why I didn't
bother.

> >  	 */
> >  	prepare_to_wait(&memcg_oom_waitq, &owait.wait, TASK_KILLABLE);
> > -	if (!locked || memcg->oom_kill_disable)
> > -		need_to_kill = false;
> > +	mem_cgroup_mark_under_oom(memcg);
> > +
> > +	locked = mem_cgroup_oom_trylock(memcg);
> > +
> >  	if (locked)
> >  		mem_cgroup_oom_notify(memcg);
> > -	spin_unlock(&memcg_oom_lock);
> >  
> > -	if (need_to_kill) {
> > +	if (locked && !memcg->oom_kill_disable) {
> > +		mem_cgroup_unmark_under_oom(memcg);
> >  		finish_wait(&memcg_oom_waitq, &owait.wait);
> >  		mem_cgroup_out_of_memory(memcg, mask, order);
> 
> Killing under hierarchy which is not under_oom sounds strange to me.
> Cannot we just move finish_wait & unmark down after unlock? It would
> also take care about incorrect memcg_oom_recover you have in oom_unlock
> path. The ordering would also be more natural
> 	prepare_wait
> 	mark_under_oom
> 	trylock
> 	unlock
> 	unmark_under_oom
> 	finish_wait

That makes no sense.  The waitqueue is for when you failed the trylock
and need to sleep until the holder unlocks.  If you acquire the lock,
what's the point in staying on the waitqueue?

Look at __mutex_lock_common() in kernel/mutex.c.

There is also no point in leaving the under_oom set during the kill,
this flag is so transient if the in-kernel OOM killer is enabled.
Whether we leave it set during mem_cgroup_out_of_memory() or not, how
would you possibly read it and base any meaningful decisions on it
from userspace?

> >  	} else {
> >  		schedule();
> > +		mem_cgroup_unmark_under_oom(memcg);
> >  		finish_wait(&memcg_oom_waitq, &owait.wait);
> >  	}
> > -	spin_lock(&memcg_oom_lock);
> > -	if (locked)
> > -		mem_cgroup_oom_unlock(memcg);
> > -	memcg_wakeup_oom(memcg);
> > -	spin_unlock(&memcg_oom_lock);
> >  
> > -	mem_cgroup_unmark_under_oom(memcg);
> > +	if (locked) {
> > +		mem_cgroup_oom_unlock(memcg);
> > +		/*
> > +		 * There is no guarantee that a OOM-lock contender
> > +		 * sees the wakeups triggered by the OOM kill
> > +		 * uncharges.  Wake any sleepers explicitely.
> > +		 */
> > +		memcg_oom_recover(memcg);
> 
> This will be a noop because memcg is no longer under_oom (you wanted
> memcg_wakeup_oom here I guess). Moreover, even the killed wouldn't wake
> up anybody for the same reason.

Anybody entering this path will increase the under_oom counter.  The
killer decreases it again, but everybody who is sleeping because they
failed the trylock still hasn't unmarked the hierarchy (they call
schedule() before unmark_under_oom()).  So we issue wakeups when there
is somebody waiting for the lock.
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