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sched_ext: Add scx_cgroup_sched() for cgrp->scx_sched reads

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cgrp->scx_sched is __rcu and published with rcu_assign_pointer() but every
reader loads it with a plain access, so sparse flags all of them. The reads
are lock-protected: enable/disable paths rewrite the field under all of
scx_enable_mutex, scx_fork_rwsem and cgroup_mutex, and cgroup creation
inherits the parent's sched under cgroup_mutex before the new cgroup is
reachable, so holding any one of the three locks makes the read stable.

Add scx_cgroup_sched() which states the protection with
rcu_dereference_check() and convert the readers. No functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo 2026-07-23 15:07:56 -10:00
parent 3a21e34eb2
commit 7947442047
3 changed files with 26 additions and 10 deletions
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6
kernel/sched/ext/ext.c
View File

@ -3807,7 +3807,7 @@ int scx_fork(struct task_struct *p, struct kernel_clone_args *kargs)
if (scx_init_task_enabled) {
#ifdef CONFIG_EXT_SUB_SCHED
struct scx_sched *sch = kargs->cset->dfl_cgrp->scx_sched;
struct scx_sched *sch = scx_cgroup_sched(kargs->cset->dfl_cgrp);
#else
struct scx_sched *sch = scx_root;
#endif
@ -4461,7 +4461,7 @@ int scx_tg_online(struct task_group *tg)
* always belongs to the root sched.
*/
if (cgroup_on_dfl(tg->css.cgroup))
sch = tg->css.cgroup->scx_sched;
sch = scx_cgroup_sched(tg->css.cgroup);
else
sch = scx_tg_sched(&root_task_group);
@ -4542,7 +4542,7 @@ int scx_cgroup_can_attach(struct cgroup_taskset *tset)
* cgroup's sched and is reported through the
* exit_task/init_task pair that the re-homing generates.
*/
if (!sch || sch != task_css_set(p)->mg_dst_cset->dfl_cgrp->scx_sched)
if (!sch || sch != scx_cgroup_sched(task_css_set(p)->mg_dst_cset->dfl_cgrp))
continue;
if (SCX_HAS_OP(sch, cgroup_prep_move)) {

16
kernel/sched/ext/sub.c
View File

@ -1204,7 +1204,7 @@ void scx_sub_disable(struct scx_sched *sch)
/* verify that a scheduler can be attached to @cgrp and return the parent */
static struct scx_sched *find_parent_sched(struct cgroup *cgrp)
{
struct scx_sched *parent = cgrp->scx_sched;
struct scx_sched *parent = scx_cgroup_sched(cgrp);
struct scx_sched *pos;
lockdep_assert_held(&scx_sched_lock);
@ -1578,7 +1578,7 @@ static s32 scx_cgroup_task_migrating(struct cgroup_task_migrate_ctx *ctx)
if (!scx_cgroup_enabled)
return NOTIFY_OK;
to = ctx->dst_dcgrp->scx_sched;
to = scx_cgroup_sched(ctx->dst_dcgrp);
if (scx_task_on_sched(to, p))
return NOTIFY_OK;
@ -1612,7 +1612,7 @@ static void scx_cgroup_task_migrated(struct cgroup_task_migrate_ctx *ctx)
if (!scx_cgroup_enabled)
return;
to = ctx->dst_dcgrp->scx_sched;
to = scx_cgroup_sched(ctx->dst_dcgrp);
if (scx_task_on_sched(to, p))
return;
@ -1639,7 +1639,7 @@ static void scx_cgroup_task_migrate_canceled(struct cgroup_task_migrate_ctx *ctx
if (!scx_cgroup_enabled)
return;
to = ctx->dst_dcgrp->scx_sched;
to = scx_cgroup_sched(ctx->dst_dcgrp);
if (scx_task_on_sched(to, p))
return;
@ -1653,6 +1653,7 @@ static s32 scx_cgroup_lifetime_notify(struct notifier_block *nb,
{
struct cgroup *cgrp = data;
struct cgroup *parent = cgroup_parent(cgrp);
struct scx_sched *sch;
if (!cgroup_on_dfl(cgrp))
return NOTIFY_OK;
@ -1661,12 +1662,13 @@ static s32 scx_cgroup_lifetime_notify(struct notifier_block *nb,
case CGROUP_LIFETIME_ONLINE:
/* inherit ->scx_sched from $parent */
if (parent)
rcu_assign_pointer(cgrp->scx_sched, parent->scx_sched);
rcu_assign_pointer(cgrp->scx_sched, scx_cgroup_sched(parent));
break;
case CGROUP_LIFETIME_OFFLINE:
/* if there is a sched attached, shoot it down */
if (cgrp->scx_sched && cgrp->scx_sched->cgrp == cgrp)
scx_exit(cgrp->scx_sched, SCX_EXIT_UNREG_KERN,
sch = scx_cgroup_sched(cgrp);
if (sch && sch->cgrp == cgrp)
scx_exit(sch, SCX_EXIT_UNREG_KERN,
SCX_ECODE_RSN_CGROUP_OFFLINE,
"cgroup %llu going offline", cgroup_id(cgrp));
break;

14
kernel/sched/ext/sub.h
View File

@ -39,6 +39,20 @@ struct scx_dispatch_q *scx_local_or_reject_dsq(struct scx_sched *sch, struct rq
bool scx_task_reenq_on_cap_revoke(struct rq *rq, struct task_struct *p);
void scx_reenq_reject(struct rq *rq);
/*
* cgrp->scx_sched is written by root/sub enable/disable under all of
* scx_enable_mutex, scx_fork_rwsem and cgroup_mutex. A new cgroup inherits the
* parent's sched under just cgroup_mutex but is not yet reachable by the other
* two lock holders. Any one of the three locks stabilizes the association.
*/
static inline struct scx_sched *scx_cgroup_sched(struct cgroup *cgrp)
{
return rcu_dereference_check(cgrp->scx_sched,
lockdep_is_held(&cgroup_mutex) ||
percpu_rwsem_is_held(&scx_fork_rwsem) ||
lockdep_is_held(&scx_enable_mutex));
}
static inline const char *sch_cgrp_path(struct scx_sched *sch)
{
return sch->cgrp_path;