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cgroup: use css_set->mg_dst_cgrp for the migration target cgroup

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Migration can be multi-target on the default hierarchy when a
controller is enabled - processes belonging to each child cgroup have
to be moved to the child cgroup itself to refresh css association.

This isn't a problem for cgroup_migrate_add_src() as each source
css_set still maps to single source and target cgroups; however,
cgroup_migrate_prepare_dst() is called once after all source css_sets
are added and thus might not have a single destination cgroup.  This
is currently worked around by specifying NULL for @dst_cgrp and using
the source's default cgroup as destination as the only multi-target
migration in use is self-targetting.  While this works, it's subtle
and clunky.

As all taget cgroups are already specified while preparing the source
css_sets, this clunkiness can easily be removed by recording the
target cgroup in each source css_set.  This patch adds
css_set->mg_dst_cgrp which is recorded on cgroup_migrate_src() and
used by cgroup_migrate_prepare_dst().  This also makes migration code
ready for arbitrary multi-target migration.

Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo
2016-03-08 11:51:26 -05:00
parent 37ff9f8f47
commit e4857982f4
2 changed files with 18 additions and 17 deletions
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9
include/linux/cgroup-defs.h
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@@ -191,12 +191,13 @@ struct css_set {
/*
* If this cset is acting as the source of migration the following
* two fields are set. mg_src_cgrp is the source cgroup of the
* on-going migration and mg_dst_cset is the destination cset the
* target tasks on this cset should be migrated to. Protected by
* cgroup_mutex.
* two fields are set. mg_src_cgrp and mg_dst_cgrp are
* respectively the source and destination cgroups of the on-going
* migration. mg_dst_cset is the destination cset the target tasks
* on this cset should be migrated to. Protected by cgroup_mutex.
*/
struct cgroup *mg_src_cgrp;
struct cgroup *mg_dst_cgrp;
struct css_set *mg_dst_cset;
/*