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Commit aec3dfcb authored by Tejun Heo's avatar Tejun Heo
Browse files

cgroup: introduce effective cgroup_subsys_state 



In the planned default unified hierarchy, controllers may get
dynamically attached to and detached from a cgroup and a cgroup may
not have csses for all the controllers associated with the hierarchy.

When a cgroup doesn't have its own css for a given controller, the css
of the nearest ancestor with the controller enabled will be used,
which is called the effective css.  This patch introduces
cgroup_e_css() and for_each_e_css() to access the effective csses and
convert compare_css_sets(), find_existing_css_set() and
cgroup_migrate() to use the effective csses so that they can handle
cgroups with partial csses correctly.

This means that for two css_sets to be considered identical, they
should have both matching csses and cgroups.  compare_css_sets()
already compares both, not for correctness but for optimization.  As
this now becomes a matter of correctness, update the comments
accordingly.

For all !default hierarchies, cgroup_e_css() always equals
cgroup_css(), so this patch doesn't change behavior.

While at it, fix incorrect locking comment for for_each_css().

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent f392e51c

kernel/cgroup.c

+64 −19
Original line number Diff line number Diff line
@@ -208,6 +208,34 @@ static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp, 
		return &cgrp->dummy_css;

}



/**

 * cgroup_e_css - obtain a cgroup's effective css for the specified subsystem

 * @cgrp: the cgroup of interest

 * @ss: the subsystem of interest (%NULL returns the dummy_css)

 *

 * Similar to cgroup_css() but returns the effctive css, which is defined

 * as the matching css of the nearest ancestor including self which has @ss

 * enabled.  If @ss is associated with the hierarchy @cgrp is on, this

 * function is guaranteed to return non-NULL css.

 */

static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,

						struct cgroup_subsys *ss)

{

	lockdep_assert_held(&cgroup_mutex);



	if (!ss)

		return &cgrp->dummy_css;



	if (!(cgrp->root->subsys_mask & (1 << ss->id)))

		return NULL;



	while (cgrp->parent &&

	       !(cgrp->parent->child_subsys_mask & (1 << ss->id)))

		cgrp = cgrp->parent;



	return cgroup_css(cgrp, ss);

}



/* convenient tests for these bits */

static inline bool cgroup_is_dead(const struct cgroup *cgrp)

{
@@ -273,7 +301,7 @@ static int notify_on_release(const struct cgroup *cgrp) 
 * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end

 * @cgrp: the target cgroup to iterate css's of

 *

 * Should be called under cgroup_mutex.

 * Should be called under cgroup_[tree_]mutex.

 */

#define for_each_css(css, ssid, cgrp)					\

	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)	\
@@ -283,6 +311,20 @@ static int notify_on_release(const struct cgroup *cgrp) 
				lockdep_is_held(&cgroup_mutex)))) { }	\

		else



/**

 * for_each_e_css - iterate all effective css's of a cgroup

 * @css: the iteration cursor

 * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end

 * @cgrp: the target cgroup to iterate css's of

 *

 * Should be called under cgroup_[tree_]mutex.

 */

#define for_each_e_css(css, ssid, cgrp)					\

	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)	\

		if (!((css) = cgroup_e_css(cgrp, cgroup_subsys[(ssid)]))) \

			;						\

		else



/**

 * for_each_subsys - iterate all enabled cgroup subsystems

 * @ss: the iteration cursor
@@ -452,20 +494,20 @@ static bool compare_css_sets(struct css_set *cset, 
{

	struct list_head *l1, *l2;



	if (memcmp(template, cset->subsys, sizeof(cset->subsys))) {

		/* Not all subsystems matched */

	/*

	 * On the default hierarchy, there can be csets which are

	 * associated with the same set of cgroups but different csses.

	 * Let's first ensure that csses match.

	 */

	if (memcmp(template, cset->subsys, sizeof(cset->subsys)))

		return false;

	}



	/*

	 * Compare cgroup pointers in order to distinguish between

	 * different cgroups in heirarchies with no subsystems. We

	 * could get by with just this check alone (and skip the

	 * memcmp above) but on most setups the memcmp check will

	 * avoid the need for this more expensive check on almost all

	 * candidates.

	 * different cgroups in hierarchies.  As different cgroups may

	 * share the same effective css, this comparison is always

	 * necessary.

	 */



	l1 = &cset->cgrp_links;

	l2 = &old_cset->cgrp_links;

	while (1) {
@@ -530,13 +572,16 @@ static struct css_set *find_existing_css_set(struct css_set *old_cset, 
	 */

	for_each_subsys(ss, i) {

		if (root->subsys_mask & (1UL << i)) {

			/* Subsystem is in this hierarchy. So we want

			 * the subsystem state from the new

			 * cgroup */

			template[i] = cgroup_css(cgrp, ss);

			/*

			 * @ss is in this hierarchy, so we want the

			 * effective css from @cgrp.

			 */

			template[i] = cgroup_e_css(cgrp, ss);

		} else {

			/* Subsystem is not in this hierarchy, so we

			 * don't want to change the subsystem state */

			/*

			 * @ss is not in this hierarchy, so we don't want

			 * to change the css.

			 */

			template[i] = old_cset->subsys[i];

		}

	}
@@ -1969,7 +2014,7 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader, 
		return 0;



	/* check that we can legitimately attach to the cgroup */

	for_each_css(css, i, cgrp) {

	for_each_e_css(css, i, cgrp) {

		if (css->ss->can_attach) {

			ret = css->ss->can_attach(css, &tset);

			if (ret) {
@@ -1999,7 +2044,7 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader, 
	 */

	tset.csets = &tset.dst_csets;



	for_each_css(css, i, cgrp)

	for_each_e_css(css, i, cgrp)

		if (css->ss->attach)

			css->ss->attach(css, &tset);
@@ -2007,7 +2052,7 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader, 
	goto out_release_tset;



out_cancel_attach:

	for_each_css(css, i, cgrp) {

	for_each_e_css(css, i, cgrp) {

		if (css == failed_css)

			break;

		if (css->ss->cancel_attach)