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SubjectRe: [PATCHSET v2 cgroup/for-3.15] cgroup: update task migration path
On 2014/2/14 4:28, Tejun Heo wrote:
> Hello,
> This is v2 of update-task-migration-path patchset. Changes from v1[L]
> are
> * Rebased on top of "[PATCH cgroup/for-3.14-fixes] cgroup: update
> cgroup_enable_task_cg_lists() to grab siglock"
> * 0005-cgroup-update-how-a-newly-forked-task-gets-associate.patch and
> 0006-cgroup-drop-task_lock-protection-around-task-cgroups.patch
> added to address the race between migration and fork paths.
> Currently, when migrating a task or process from one cgroup to
> another, a flex_array is used to keep track of the target tasks and
> associated css_sets. The current implementation has several issues.
> * flex_array size is limited. Given the current data structure, the
> limit is ~87k on 64bit, which is pretty high but not impossible to
> hit.
> * If multiple targets are being migrated, as migrating each target
> involves memory allocation, it can fail at any point. cgroup core
> doesn't keep track of enough state to roll back partial migration
> either, so it ends up aborting with some targets migrated with no
> way of finding out which. While this isn't a big issue now, we're
> gonna be making more use of multi-target migration.
> * Fork path could race against migration path and it was impossible to
> implement a mechanism to migrate all tasks of a cgroup to another
> because migration path can't tell whether there are just forked
> tasks pointing to the css_set but not linked yet.
> This patchset updates task migration path such that
> * task->cg_list and css_sets are also used to keep track of targets
> during migration so that no extra memory allocation is necessary to
> keep track of migration targets.
> * Migration is split into several stages so that all preparations
> which may fail can be performed for all targets before actually
> starting migrating tasks. Ignoring ->can_attach() failure, this can
> guarantee all-or-nothing semantics of multi-target migration.
> * Newly forked tasks are now atomically associated with and linked to
> the parent's css_set in cgroup_post_fork(). This guarantees that
> the new task either is visible in the source cgroup once the
> parent's migration is complete or ends up in the target cgroup in
> the first place. This means that just keeping moving tasks out of a
> cgroup until it's empty is guaranteed to migrate all tasks.
> This patchset contains the following seven patches.
> 0001-cgroup-add-css_set-mg_tasks.patch
> 0002-cgroup-use-css_set-mg_tasks-to-track-target-tasks-du.patch
> 0003-cgroup-separate-out-cset_group_from_root-from-task_c.patch
> 0004-cgroup-split-process-task-migration-into-four-steps.patch
> 0005-cgroup-update-how-a-newly-forked-task-gets-associate.patch
> 0006-cgroup-drop-task_lock-protection-around-task-cgroups.patch
> 0007-cgroup-update-cgroup_transfer_tasks-to-either-succee.patch

Acked-by: Li Zefan <>

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