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writeback, cgroup: release dying cgwbs by switching attached inodes
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. [[email protected]: replace open-coded "115" with arithmetic] Link: https://lkml.kernel.org/r/YMEcSBcq/[email protected] [[email protected]: add smp_mb() to inode_prepare_wbs_switch()] Link: https://lkml.kernel.org/r/[email protected] [[email protected]: fix documentation] Link: https://lkml.kernel.org/r/[email protected] Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: Roman Gushchin <[email protected]> Signed-off-by: Matthew Wilcox (Oracle) <[email protected]> Acked-by: Tejun Heo <[email protected]> Acked-by: Dennis Zhou <[email protected]> Reviewed-by: Jan Kara <[email protected]> Cc: Alexander Viro <[email protected]> Cc: Dave Chinner <[email protected]> Cc: Jan Kara <[email protected]> Cc: Jens Axboe <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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