On Tue, Jun 08, 2021 at 05:12:37PM -0700, Andrew Morton wrote: > On Tue, 8 Jun 2021 16:02:25 -0700 Roman Gushchin <guro@xxxxxx> 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. > > > > ... > > > > +/* > > + * 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 > > Can't we do 1024/sizeof(struct inode_switch_wbs_context)? It must be something like DIV_ROUND_DOWN_ULL(1024 - sizeof(struct inode_switch_wbs_context), sizeof(struct inode *)) + 1 But honestly 1024 came out of a thin air too, so I'm not sure it worth it. I liked the number 128 but then made it fit into the closest kmalloc cache. Btw, thank you for picking these patches up!
