Re: [PATCH v2] cgroup, blkcg: prevent dirty inodes to pin dying memory cgroups

[Roman Gushchin]

On Tue, Oct 15, 2019 at 11:09:33AM +0200, Jan Kara wrote:
> On Thu 10-10-19 16:40:36, Roman Gushchin wrote:
> > We've noticed that the number of dying cgroups on our production hosts
> > tends to grow with the uptime. This time it's caused by the writeback
> > code.
> > 
> > An inode which is getting dirty for the first time is associated
> > with the wb structure (look at __inode_attach_wb()). It can later
> > be switched to another wb under some conditions (e.g. some other
> > cgroup is writing a lot of data to the same inode), but generally
> > stays associated up to the end of life of the inode structure.
> > 
> > The problem is that the wb structure holds a reference to the original
> > memory cgroup. So if an inode has been dirty once, it has a good chance
> > to pin down the original memory cgroup.
> > 
> > An example from the real life: some service runs periodically and
> > updates rpm packages. Each time in a new memory cgroup. Installed
> > .so files are heavily used by other cgroups, so corresponding inodes
> > tend to stay alive for a long. So do pinned memory cgroups.
> > In production I've seen many hosts with 1-2 thousands of dying
> > cgroups.
> > 
> > This is not the first problem with the dying memory cgroups. As
> > always, the problem is with their relative size: memory cgroups
> > are large objects, easily 100x-1000x larger that inodes. So keeping
> > a couple of thousands of dying cgroups in memory without a good reason
> > (what we easily do with inodes) is quite costly (and is measured
> > in tens and hundreds of Mb).
> > 
> > To solve this problem let's perform a periodic scan of inodes
> > attached to the dying wbs, and detach those of them, which are clean
> > and don't have an active io operation.
> > That will eventually release the wb structure and corresponding
> > memory cgroup.
> > 
> > To make this scanning effective, let's keep a list of attached
> > inodes. inode->i_io_list can be reused for this purpose.
> > 
> > The scan is performed from the cgroup offlining path. Dying wbs
> > are placed on the global list. On each cgroup removal we traverse
> > the whole list ignoring wbs with active io operations. That will
> > allow the majority of io operations to be finished after the
> > removal of the cgroup.
> > 
> > Big thanks to Jan Kara and Dennis Zhou for their ideas and
> > contribution to this patch.
> > 
> > Signed-off-by: Roman Gushchin <guro@xxxxxx>
> > ---
> >  fs/fs-writeback.c                | 52 +++++++++++++++++++++++---
> >  include/linux/backing-dev-defs.h |  2 +
> >  include/linux/writeback.h        |  1 +
> >  mm/backing-dev.c                 | 63 ++++++++++++++++++++++++++++++--
> >  4 files changed, 108 insertions(+), 10 deletions(-)
> > 
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index e88421d9a48d..c792db951274 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -136,16 +136,21 @@ static bool inode_io_list_move_locked(struct inode *inode,
> >   * inode_io_list_del_locked - remove an inode from its bdi_writeback IO list
> >   * @inode: inode to be removed
> >   * @wb: bdi_writeback @inode is being removed from
> > + * @keep_attached: keep the inode on the list of inodes attached to wb
> >   *
> >   * Remove @inode which may be on one of @wb->b_{dirty|io|more_io} lists and
> >   * clear %WB_has_dirty_io if all are empty afterwards.
> >   */
> >  static void inode_io_list_del_locked(struct inode *inode,
> > -				     struct bdi_writeback *wb)
> > +				     struct bdi_writeback *wb,
> > +				     bool keep_attached)
> >  {
> >  	assert_spin_locked(&wb->list_lock);
> >  
> > -	list_del_init(&inode->i_io_list);
> > +	if (keep_attached)
> > +		list_move(&inode->i_io_list, &wb->b_attached);
> > +	else
> > +		list_del_init(&inode->i_io_list);
> >  	wb_io_lists_depopulated(wb);
> >  }
> 
> Rather than adding this (somewhat ugly) bool argument to
> inode_io_list_del_locked() I'd teach inode_io_list_move_locked() about the
> new b_attached list and use that function where needed...

Ok, will do in v3.

> 
> > @@ -426,7 +431,7 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
> >  	if (!list_empty(&inode->i_io_list)) {
> >  		struct inode *pos;
> >  
> > -		inode_io_list_del_locked(inode, old_wb);
> > +		inode_io_list_del_locked(inode, old_wb, false);
> >  		inode->i_wb = new_wb;
> >  		list_for_each_entry(pos, &new_wb->b_dirty, i_io_list)
> >  			if (time_after_eq(inode->dirtied_when,
> 
> This bit looks wrong. Not the change you made as such but the fact that you
> can now move inode from b_attached list of old wb to the dirty list of new
> wb.

Hm, can you, please, elaborate a bit more why it's wrong?
The reference to the old_wb will be dropped by the switching code.

> 
> > @@ -544,6 +549,41 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
> >  	kfree(isw);
> >  }
> >  
> > +/**
> > + * cleanup_offline_wb - detach attached clean inodes
> > + * @wb: target wb
> > + *
> > + * Clear the ->i_wb pointer of the attached inodes and drop
> > + * the corresponding wb reference. Skip inodes which are dirty,
> > + * freeing, switching or in the active writeback process.
> > + */
> > +void cleanup_offline_wb(struct bdi_writeback *wb)
> > +{
> > +	struct inode *inode, *tmp;
> > +	bool ret = true;
> > +
> > +	spin_lock(&wb->list_lock);
> > +	if (list_empty(&wb->b_attached))
> > +		goto unlock;
> 
> What's the point of this check? list_for_each_entry_safe() below will just
> do the same...

Right, will remove.

> 
> > +
> > +	list_for_each_entry_safe(inode, tmp, &wb->b_attached, i_io_list) {
> > +		if (!spin_trylock(&inode->i_lock))
> > +			continue;
> > +		xa_lock_irq(&inode->i_mapping->i_pages);
> > +		if (!(inode->i_state &
> > +		      (I_FREEING | I_CLEAR | I_SYNC | I_DIRTY | I_WB_SWITCH))) {
> > +			WARN_ON_ONCE(inode->i_wb != wb);
> > +			inode->i_wb = NULL;
> > +			wb_put(wb);
> 
> Hum, currently the code assumes that once i_wb is set, it never becomes
> NULL again. In particular the inode e.g. in
> fs/fs-writeback.c:inode_congested() or generally unlocked_inode_to_wb_begin()
> users could get broken by this. The i_wb switching code is so complex
> exactly because of these interactions.
> 
> Maybe you thought through the interactions and things are actually fine but
> if nothing else you'd need a big fat comment here explaining why this is
> fine and update inode_congested() comments etc.

Yeah, I thought that once inode is clean and not switching it's safe to clear
the i_wb pointer, but seems that it's not completely true.

One idea I have is to always release wbs using rcu delayed work, so that
it will be save to dereference i_wb pointer under rcu, if only it's not NULL
(the check has to be added). I'll try to implement this scheme, but if you
know in advance that it's not gonna work, please, let me know.

> 
> > +			list_del_init(&inode->i_io_list);
> > +		}
> > +		xa_unlock_irq(&inode->i_mapping->i_pages);
> > +		spin_unlock(&inode->i_lock);
> > +	}
> > +unlock:
> > +	spin_unlock(&wb->list_lock);
> > +}
> > +
> 
> ...
> 
> > diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> > index 4fc87dee005a..68b167fda259 100644
> > --- a/include/linux/backing-dev-defs.h
> > +++ b/include/linux/backing-dev-defs.h
> > @@ -137,6 +137,7 @@ struct bdi_writeback {
> >  	struct list_head b_io;		/* parked for writeback */
> >  	struct list_head b_more_io;	/* parked for more writeback */
> >  	struct list_head b_dirty_time;	/* time stamps are dirty */
> > +	struct list_head b_attached;	/* attached inodes */
> 
> Maybe
> 	/* clean inodes pointing to this wb through inode->i_wb */
> would be more explanatory?

Sure, np.

> 
> >  	spinlock_t list_lock;		/* protects the b_* lists */
> >  
> >  	struct percpu_counter stat[NR_WB_STAT_ITEMS];
> 
> 									Honza
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

Thank you for looking into it!