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Date:   Sat, 5 Jun 2021 21:34:41 +0000
From:   Dennis Zhou <dennis@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Jan Kara <jack@suse.cz>, Tejun Heo <tj@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>,
        Dave Chinner <dchinner@redhat.com>, cgroups@vger.kernel.org
Subject: Re: [PATCH v7 6/6] writeback, cgroup: release dying cgwbs by
 switching attached inodes
Message-ID: <YLvt8f+r7aFkmluG@google.com>
References: <20210604013159.3126180-1-guro@fb.com>
 <20210604013159.3126180-7-guro@fb.com>
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Hello,

On Thu, Jun 03, 2021 at 06:31:59PM -0700, Roman Gushchin wrote:
> Asynchronously try to release dying cgwbs by switching attached inodes
> to the bdi's wb. It helps to get rid of per-cgroup writeback
> structures themselves and of pinned memory and block cgroups, which
> are significantly larger structures (mostly due to large per-cpu
> statistics data). This prevents memory waste and helps to avoid
> different scalability problems caused by large piles of dying cgroups.
> 
> Reuse the existing mechanism of inode switching used for foreign inode
> detection. To speed things up batch up to 115 inode switching in a
> single operation (the maximum number is selected so that the resulting
> struct inode_switch_wbs_context can fit into 1024 bytes). Because
> every switching consists of two steps divided by an RCU grace period,
> it would be too slow without batching. Please note that the whole
> batch counts as a single operation (when increasing/decreasing
> isw_nr_in_flight). This allows to keep umounting working (flush the
> switching queue), however prevents cleanups from consuming the whole
> switching quota and effectively blocking the frn switching.
> 
> A cgwb cleanup operation can fail due to different reasons (e.g. not
> enough memory, the cgwb has an in-flight/pending io, an attached inode
> in a wrong state, etc). In this case the next scheduled cleanup will
> make a new attempt. An attempt is made each time a new cgwb is offlined
> (in other words a memcg and/or a blkcg is deleted by a user). In the
> future an additional attempt scheduled by a timer can be implemented.

I've been thinking about this for a little while and the only thing I'm
not super thrilled by is that the subsequent cleanup work trigger isn't
due to forward progress.

As future work, we could tag the inodes to switch when writeback
completes instead of using a timer. This would be nice because then we
only have to make a single (successful) pass switching the inodes we can
and then mark the others to switch. Once a cgwb is killed no one else
can attach to it so we should be good there.

I don't think this is a blocker or even necessary, I just wanted to put
it out there as possible future direction instead of a timer.

Thanks,
Dennis

> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
>  fs/fs-writeback.c                | 93 ++++++++++++++++++++++++++++----
>  include/linux/backing-dev-defs.h |  1 +
>  include/linux/writeback.h        |  1 +
>  mm/backing-dev.c                 | 67 ++++++++++++++++++++++-
>  4 files changed, 150 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 5f5502238bf0..b63420c9cf41 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -225,6 +225,12 @@ void wb_wait_for_completion(struct wb_completion *done)
>  					/* one round can affect upto 5 slots */
>  #define WB_FRN_MAX_IN_FLIGHT	1024	/* don't queue too many concurrently */
>  
> +/*
> + * Maximum inodes per isw.  A specific value has been chosen to make
> + * struct inode_switch_wbs_context fit into 1024 bytes kmalloc.
> + */
> +#define WB_MAX_INODES_PER_ISW	115
> +
>  static atomic_t isw_nr_in_flight = ATOMIC_INIT(0);
>  static struct workqueue_struct *isw_wq;
>  
> @@ -502,6 +508,24 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
>  	atomic_dec(&isw_nr_in_flight);
>  }
>  
> +static bool inode_prepare_wbs_switch(struct inode *inode,
> +				     struct bdi_writeback *new_wb)
> +{
> +	/* while holding I_WB_SWITCH, no one else can update the association */
> +	spin_lock(&inode->i_lock);
> +	if (!(inode->i_sb->s_flags & SB_ACTIVE) ||
> +	    inode->i_state & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) ||
> +	    inode_to_wb(inode) == new_wb) {
> +		spin_unlock(&inode->i_lock);
> +		return false;
> +	}
> +	inode->i_state |= I_WB_SWITCH;
> +	__iget(inode);
> +	spin_unlock(&inode->i_lock);
> +
> +	return true;
> +}
> +
>  /**
>   * inode_switch_wbs - change the wb association of an inode
>   * @inode: target inode
> @@ -537,17 +561,8 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	if (!isw->new_wb)
>  		goto out_free;
>  
> -	/* while holding I_WB_SWITCH, no one else can update the association */
> -	spin_lock(&inode->i_lock);
> -	if (!(inode->i_sb->s_flags & SB_ACTIVE) ||
> -	    inode->i_state & (I_WB_SWITCH | I_FREEING | I_WILL_FREE) ||
> -	    inode_to_wb(inode) == isw->new_wb) {
> -		spin_unlock(&inode->i_lock);
> +	if (!inode_prepare_wbs_switch(inode, isw->new_wb))
>  		goto out_free;
> -	}
> -	inode->i_state |= I_WB_SWITCH;
> -	__iget(inode);
> -	spin_unlock(&inode->i_lock);
>  
>  	isw->inodes[0] = inode;
>  
> @@ -569,6 +584,64 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
>  	kfree(isw);
>  }
>  
> +/**
> + * cleanup_offline_cgwb - detach associated inodes
> + * @wb: target wb
> + *
> + * Switch all inodes attached to @wb to the bdi's root wb in order to eventually
> + * release the dying @wb.  Returns %true if not all inodes were switched and
> + * the function has to be restarted.
> + */
> +bool cleanup_offline_cgwb(struct bdi_writeback *wb)
> +{
> +	struct inode_switch_wbs_context *isw;
> +	struct inode *inode;
> +	int nr;
> +	bool restart = false;
> +
> +	isw = kzalloc(sizeof(*isw) + WB_MAX_INODES_PER_ISW *
> +		      sizeof(struct inode *), GFP_KERNEL);
> +	if (!isw)
> +		return restart;
> +
> +	/* no need to call wb_get() here: bdi's root wb is not refcounted */
> +	isw->new_wb = &wb->bdi->wb;
> +
> +	nr = 0;
> +	spin_lock(&wb->list_lock);
> +	list_for_each_entry(inode, &wb->b_attached, i_io_list) {
> +		if (!inode_prepare_wbs_switch(inode, isw->new_wb))
> +			continue;
> +
> +		isw->inodes[nr++] = inode;
> +
> +		if (nr >= WB_MAX_INODES_PER_ISW - 1) {
> +			restart = true;
> +			break;
> +		}
> +	}
> +	spin_unlock(&wb->list_lock);
> +
> +	/* no attached inodes? bail out */
> +	if (nr == 0) {
> +		kfree(isw);
> +		return restart;
> +	}
> +
> +	/*
> +	 * In addition to synchronizing among switchers, I_WB_SWITCH tells
> +	 * the RCU protected stat update paths to grab the i_page
> +	 * lock so that stat transfer can synchronize against them.
> +	 * Let's continue after I_WB_SWITCH is guaranteed to be visible.
> +	 */
> +	INIT_RCU_WORK(&isw->work, inode_switch_wbs_work_fn);
> +	queue_rcu_work(isw_wq, &isw->work);
> +
> +	atomic_inc(&isw_nr_in_flight);
> +
> +	return restart;
> +}
> +
>  /**
>   * wbc_attach_and_unlock_inode - associate wbc with target inode and unlock it
>   * @wbc: writeback_control of interest
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index 63f52ad2ce7a..1d7edad9914f 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -155,6 +155,7 @@ struct bdi_writeback {
>  	struct list_head memcg_node;	/* anchored at memcg->cgwb_list */
>  	struct list_head blkcg_node;	/* anchored at blkcg->cgwb_list */
>  	struct list_head b_attached;	/* attached inodes, protected by list_lock */
> +	struct list_head offline_node;	/* anchored at offline_cgwbs */
>  
>  	union {
>  		struct work_struct release_work;
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index 8e5c5bb16e2d..95de51c10248 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -221,6 +221,7 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
>  int cgroup_writeback_by_id(u64 bdi_id, int memcg_id, unsigned long nr_pages,
>  			   enum wb_reason reason, struct wb_completion *done);
>  void cgroup_writeback_umount(void);
> +bool cleanup_offline_cgwb(struct bdi_writeback *wb);
>  
>  /**
>   * inode_attach_wb - associate an inode with its wb
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 54c5dc4b8c24..53aee015dc49 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -371,12 +371,16 @@ static void wb_exit(struct bdi_writeback *wb)
>  #include <linux/memcontrol.h>
>  
>  /*
> - * cgwb_lock protects bdi->cgwb_tree, blkcg->cgwb_list, and memcg->cgwb_list.
> - * bdi->cgwb_tree is also RCU protected.
> + * cgwb_lock protects bdi->cgwb_tree, blkcg->cgwb_list, offline_cgwbs and
> + * memcg->cgwb_list.  bdi->cgwb_tree is also RCU protected.
>   */
>  static DEFINE_SPINLOCK(cgwb_lock);
>  static struct workqueue_struct *cgwb_release_wq;
>  
> +static LIST_HEAD(offline_cgwbs);
> +static void cleanup_offline_cgwbs_workfn(struct work_struct *work);
> +static DECLARE_WORK(cleanup_offline_cgwbs_work, cleanup_offline_cgwbs_workfn);
> +
>  static void cgwb_release_workfn(struct work_struct *work)
>  {
>  	struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
> @@ -395,6 +399,11 @@ static void cgwb_release_workfn(struct work_struct *work)
>  
>  	fprop_local_destroy_percpu(&wb->memcg_completions);
>  	percpu_ref_exit(&wb->refcnt);
> +
> +	spin_lock_irq(&cgwb_lock);
> +	list_del(&wb->offline_node);
> +	spin_unlock_irq(&cgwb_lock);
> +
>  	wb_exit(wb);
>  	WARN_ON_ONCE(!list_empty(&wb->b_attached));
>  	kfree_rcu(wb, rcu);
> @@ -414,6 +423,7 @@ static void cgwb_kill(struct bdi_writeback *wb)
>  	WARN_ON(!radix_tree_delete(&wb->bdi->cgwb_tree, wb->memcg_css->id));
>  	list_del(&wb->memcg_node);
>  	list_del(&wb->blkcg_node);
> +	list_add(&wb->offline_node, &offline_cgwbs);
>  	percpu_ref_kill(&wb->refcnt);
>  }
>  
> @@ -635,6 +645,57 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
>  	mutex_unlock(&bdi->cgwb_release_mutex);
>  }
>  
> +/**
> + * cleanup_offline_cgwbs - try to release dying cgwbs
> + *
> + * Try to release dying cgwbs by switching attached inodes to the wb
> + * belonging to the root memory cgroup. Processed wbs are placed at the
> + * end of the list to guarantee the forward progress.
> + *
> + * Should be called with the acquired cgwb_lock lock, which might
> + * be released and re-acquired in the process.
> + */
> +static void cleanup_offline_cgwbs_workfn(struct work_struct *work)
> +{
> +	struct bdi_writeback *wb;
> +	LIST_HEAD(processed);
> +
> +	spin_lock_irq(&cgwb_lock);
> +
> +	while (!list_empty(&offline_cgwbs)) {
> +		wb = list_first_entry(&offline_cgwbs, struct bdi_writeback,
> +				      offline_node);
> +		list_move(&wb->offline_node, &processed);
> +
> +		/*
> +		 * If wb is dirty, cleaning up the writeback by switching
> +		 * attached inodes will result in an effective removal of any
> +		 * bandwidth restrictions, which isn't the goal.  Instead,
> +		 * it can be postponed until the next time, when all io
> +		 * will be likely completed.  If in the meantime some inodes
> +		 * will get re-dirtied, they should be eventually switched to
> +		 * a new cgwb.
> +		 */
> +		if (wb_has_dirty_io(wb))
> +			continue;
> +
> +		if (!wb_tryget(wb))
> +			continue;
> +
> +		spin_unlock_irq(&cgwb_lock);
> +		while ((cleanup_offline_cgwb(wb)))
> +			cond_resched();
> +		spin_lock_irq(&cgwb_lock);
> +
> +		wb_put(wb);
> +	}
> +
> +	if (!list_empty(&processed))
> +		list_splice_tail(&processed, &offline_cgwbs);
> +
> +	spin_unlock_irq(&cgwb_lock);
> +}
> +
>  /**
>   * wb_memcg_offline - kill all wb's associated with a memcg being offlined
>   * @memcg: memcg being offlined
> @@ -651,6 +712,8 @@ void wb_memcg_offline(struct mem_cgroup *memcg)
>  		cgwb_kill(wb);
>  	memcg_cgwb_list->next = NULL;	/* prevent new wb's */
>  	spin_unlock_irq(&cgwb_lock);
> +
> +	queue_work(system_unbound_wq, &cleanup_offline_cgwbs_work);
>  }
>  
>  /**
> -- 
> 2.31.1
>