Re: [PATCH v8 8/8] writeback, cgroup: release dying cgwbs by switching attached inodes

From: Jan Kara
Date: Tue Jun 08 2021 - 04:55:01 EST

On Mon 07-06-21 18:31:23, Roman Gushchin wrote:
> Asynchronously try to release dying cgwbs by switching attached inodes
> to the nearest living ancestor 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.
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> Acked-by: Dennis Zhou <dennis@xxxxxxxxxx>

The patch looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

Just one codingstyle nit below.

> + if (!wb_tryget(wb))
> + continue;
> +
> + spin_unlock_irq(&cgwb_lock);
> + while ((cleanup_offline_cgwb(wb)))
^^ too many parentheses here...

> + cond_resched();
> + spin_lock_irq(&cgwb_lock);

Jan Kara <jack@xxxxxxxx>