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    SubjectRe: [PATCH v9 8/8] writeback, cgroup: release dying cgwbs by switching attached inodes
    On Tue, 8 Jun 2021 16:02:25 -0700 Roman Gushchin <guro@fb.com> 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)?


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