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

[Roman Gushchin]

On Mon, Oct 07, 2019 at 02:01:44PM +0800, Hillf Danton wrote:
> 
> On Fri, 4 Oct 2019 15:11:04 -0700 Roman Gushchin wrote:
> > 
> > This is a RFC patch, which is not intended to be merged as is,
> > but hopefully will start a discussion which can result in a good
> > solution for the described problem.
> > --
> > 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 the inode was 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.
> 
> The diff below fixes e8a7abf5a5bd ("writeback: disassociate inodes
> from dying bdi_writebacks") by selecting new memcg_css id for dying
> bdi_writeback to switch to.
> Checking offline memcg is also added, which is perhaps needed in your
> case. Let us know if it makes sense in helping you cut dying cgroups
> down a bit.

Hello, Hillf!

Thank you for the patch! I'll be back with testing results in few days.
I doubt that it can completely solve the problem (if nobody is using
the inode for writing), but probably can make it less noticeable.

Thanks!

> 
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -552,6 +552,8 @@ out_free:
>  void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
>  				 struct inode *inode)
>  {
> +	int new_id = 0;
> +
>  	if (!inode_cgwb_enabled(inode)) {
>  		spin_unlock(&inode->i_lock);
>  		return;
> @@ -560,6 +562,22 @@ void wbc_attach_and_unlock_inode(struct
>  	wbc->wb = inode_to_wb(inode);
>  	wbc->inode = inode;
>  
> +	if (unlikely(wb_dying(wbc->wb)) ||
> +	    !mem_cgroup_from_css(wbc->wb->memcg_css)->cgwb_list.next) {
> +		int id = wbc->wb->memcg_css->id;
> +		/*
> +		 * any css id is fine in order to let dying/offline
> +		 * memcg reap
> +		 */
> +		if (id != wbc->wb_id && wbc->wb_id)
> +			new_id = wbc->wb_id;
> +		else if (id != wbc->wb_lcand_id && wbc->wb_lcand_id)
> +			new_id = wbc->wb_lcand_id;
> +		else if (id != wbc->wb_tcand_id && wbc->wb_tcand_id)
> +			new_id = wbc->wb_tcand_id;
> +		else
> +			new_id = inode_to_bdi(inode)->wb.memcg_css->id;
> +	}
>  	wbc->wb_id = wbc->wb->memcg_css->id;
>  	wbc->wb_lcand_id = inode->i_wb_frn_winner;
>  	wbc->wb_tcand_id = 0;
> @@ -574,8 +592,8 @@ void wbc_attach_and_unlock_inode(struct
>  	 * A dying wb indicates that the memcg-blkcg mapping has changed
>  	 * and a new wb is already serving the memcg.  Switch immediately.
>  	 */
> -	if (unlikely(wb_dying(wbc->wb)))
> -		inode_switch_wbs(inode, wbc->wb_id);
> +	if (new_id)
> +		inode_switch_wbs(inode, new_id);
>  }
>  EXPORT_SYMBOL_GPL(wbc_attach_and_unlock_inode);
>  
> --
> 
>