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sched_ext: Use cgroup_lock/unlock() to synchronize against cgroup operations

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SCX hooks into CPU cgroup controller operations and read-locks
scx_cgroup_rwsem to exclude them while enabling and disable schedulers.
While this works, it's unnecessarily complicated given that
cgroup_[un]lock() are available and thus the cgroup operations can be locked
out that way.

Drop scx_cgroup_rwsem locking from the tg on/offline and cgroup [can_]attach
operations. Instead, grab cgroup_lock() from scx_cgroup_lock(). Drop
scx_cgroup_finish_attach() which is no longer necessary. Drop the now
unnecessary rcu locking and css ref bumping in scx_cgroup_init() and
scx_cgroup_exit().

As scx_cgroup_set_weight/bandwidth() paths aren't protected by
cgroup_lock(), rename scx_cgroup_rwsem to scx_cgroup_ops_rwsem and retain
the locking there.

This is overall simpler and will also allow enable/disable paths to
synchronize against cgroup changes independent of the CPU controller.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
Tejun Heo 2025-09-03 11:36:07 -10:00
parent bcb7c23056
commit a5bd6ba30b
3 changed files with 14 additions and 56 deletions
Download patch file Download diff file Expand all files Collapse all files

2
kernel/sched/core.c
View file

@ -9362,8 +9362,6 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset)
cgroup_taskset_for_each(task, css, tset)
sched_move_task(task, false);
scx_cgroup_finish_attach();
}
static void cpu_cgroup_cancel_attach(struct cgroup_taskset *tset)

66
kernel/sched/ext.c
View file

@ -3055,7 +3055,7 @@ bool scx_can_stop_tick(struct rq *rq)
#ifdef CONFIG_EXT_GROUP_SCHED
DEFINE_STATIC_PERCPU_RWSEM(scx_cgroup_rwsem);
DEFINE_STATIC_PERCPU_RWSEM(scx_cgroup_ops_rwsem);
static bool scx_cgroup_enabled;
void scx_tg_init(struct task_group *tg)
@ -3072,8 +3072,6 @@ int scx_tg_online(struct task_group *tg)
WARN_ON_ONCE(tg->scx.flags & (SCX_TG_ONLINE | SCX_TG_INITED));
percpu_down_read(&scx_cgroup_rwsem);
if (scx_cgroup_enabled) {
if (SCX_HAS_OP(sch, cgroup_init)) {
struct scx_cgroup_init_args args =
@ -3093,7 +3091,6 @@ int scx_tg_online(struct task_group *tg)
tg->scx.flags |= SCX_TG_ONLINE;
}
percpu_up_read(&scx_cgroup_rwsem);
return ret;
}
@ -3103,15 +3100,11 @@ void scx_tg_offline(struct task_group *tg)
WARN_ON_ONCE(!(tg->scx.flags & SCX_TG_ONLINE));
percpu_down_read(&scx_cgroup_rwsem);
if (scx_cgroup_enabled && SCX_HAS_OP(sch, cgroup_exit) &&
(tg->scx.flags & SCX_TG_INITED))
SCX_CALL_OP(sch, SCX_KF_UNLOCKED, cgroup_exit, NULL,
tg->css.cgroup);
tg->scx.flags &= ~(SCX_TG_ONLINE | SCX_TG_INITED);
percpu_up_read(&scx_cgroup_rwsem);
}
int scx_cgroup_can_attach(struct cgroup_taskset *tset)
@ -3121,9 +3114,6 @@ int scx_cgroup_can_attach(struct cgroup_taskset *tset)
struct task_struct *p;
int ret;
/* released in scx_finish/cancel_attach() */
percpu_down_read(&scx_cgroup_rwsem);
if (!scx_cgroup_enabled)
return 0;
@ -3163,7 +3153,6 @@ err:
p->scx.cgrp_moving_from = NULL;
}
percpu_up_read(&scx_cgroup_rwsem);
return ops_sanitize_err(sch, "cgroup_prep_move", ret);
}
@ -3186,11 +3175,6 @@ void scx_cgroup_move_task(struct task_struct *p)
p->scx.cgrp_moving_from = NULL;
}
void scx_cgroup_finish_attach(void)
{
percpu_up_read(&scx_cgroup_rwsem);
}
void scx_cgroup_cancel_attach(struct cgroup_taskset *tset)
{
struct scx_sched *sch = scx_root;
@ -3198,7 +3182,7 @@ void scx_cgroup_cancel_attach(struct cgroup_taskset *tset)
struct task_struct *p;
if (!scx_cgroup_enabled)
goto out_unlock;
return;
cgroup_taskset_for_each(p, css, tset) {
if (SCX_HAS_OP(sch, cgroup_cancel_move) &&
@ -3207,15 +3191,13 @@ void scx_cgroup_cancel_attach(struct cgroup_taskset *tset)
p, p->scx.cgrp_moving_from, css->cgroup);
p->scx.cgrp_moving_from = NULL;
}
out_unlock:
percpu_up_read(&scx_cgroup_rwsem);
}
void scx_group_set_weight(struct task_group *tg, unsigned long weight)
{
struct scx_sched *sch = scx_root;
percpu_down_read(&scx_cgroup_rwsem);
percpu_down_read(&scx_cgroup_ops_rwsem);
if (scx_cgroup_enabled && SCX_HAS_OP(sch, cgroup_set_weight) &&
tg->scx.weight != weight)
@ -3224,7 +3206,7 @@ void scx_group_set_weight(struct task_group *tg, unsigned long weight)
tg->scx.weight = weight;
percpu_up_read(&scx_cgroup_rwsem);
percpu_up_read(&scx_cgroup_ops_rwsem);
}
void scx_group_set_idle(struct task_group *tg, bool idle)
@ -3237,7 +3219,7 @@ void scx_group_set_bandwidth(struct task_group *tg,
{
struct scx_sched *sch = scx_root;
percpu_down_read(&scx_cgroup_rwsem);
percpu_down_read(&scx_cgroup_ops_rwsem);
if (scx_cgroup_enabled && SCX_HAS_OP(sch, cgroup_set_bandwidth) &&
(tg->scx.bw_period_us != period_us ||
@ -3250,23 +3232,25 @@ void scx_group_set_bandwidth(struct task_group *tg,
tg->scx.bw_quota_us = quota_us;
tg->scx.bw_burst_us = burst_us;
percpu_up_read(&scx_cgroup_rwsem);
percpu_up_read(&scx_cgroup_ops_rwsem);
}
static void scx_cgroup_lock(void)
{
percpu_down_write(&scx_cgroup_rwsem);
percpu_down_write(&scx_cgroup_ops_rwsem);
cgroup_lock();
}
static void scx_cgroup_unlock(void)
{
percpu_up_write(&scx_cgroup_rwsem);
cgroup_unlock();
percpu_up_write(&scx_cgroup_ops_rwsem);
}
#else /* CONFIG_EXT_GROUP_SCHED */
static inline void scx_cgroup_lock(void) {}
static inline void scx_cgroup_unlock(void) {}
static void scx_cgroup_lock(void) {}
static void scx_cgroup_unlock(void) {}
#endif /* CONFIG_EXT_GROUP_SCHED */
@ -3382,15 +3366,12 @@ static void scx_cgroup_exit(struct scx_sched *sch)
{
struct cgroup_subsys_state *css;
percpu_rwsem_assert_held(&scx_cgroup_rwsem);
scx_cgroup_enabled = false;
/*
* scx_tg_on/offline() are excluded through scx_cgroup_rwsem. If we walk
* scx_tg_on/offline() are excluded through cgroup_lock(). If we walk
* cgroups and exit all the inited ones, all online cgroups are exited.
*/
rcu_read_lock();
css_for_each_descendant_post(css, &root_task_group.css) {
struct task_group *tg = css_tg(css);
@ -3401,17 +3382,9 @@ static void scx_cgroup_exit(struct scx_sched *sch)
if (!sch->ops.cgroup_exit)
continue;
if (WARN_ON_ONCE(!css_tryget(css)))
continue;
rcu_read_unlock();
SCX_CALL_OP(sch, SCX_KF_UNLOCKED, cgroup_exit, NULL,
css->cgroup);
rcu_read_lock();
css_put(css);
}
rcu_read_unlock();
}
static int scx_cgroup_init(struct scx_sched *sch)
@ -3419,13 +3392,10 @@ static int scx_cgroup_init(struct scx_sched *sch)
struct cgroup_subsys_state *css;
int ret;
percpu_rwsem_assert_held(&scx_cgroup_rwsem);
/*
* scx_tg_on/offline() are excluded through scx_cgroup_rwsem. If we walk
* scx_tg_on/offline() are excluded through cgroup_lock(). If we walk
* cgroups and init, all online cgroups are initialized.
*/
rcu_read_lock();
css_for_each_descendant_pre(css, &root_task_group.css) {
struct task_group *tg = css_tg(css);
struct scx_cgroup_init_args args = {
@ -3444,10 +3414,6 @@ static int scx_cgroup_init(struct scx_sched *sch)
continue;
}
if (WARN_ON_ONCE(!css_tryget(css)))
continue;
rcu_read_unlock();
ret = SCX_CALL_OP_RET(sch, SCX_KF_UNLOCKED, cgroup_init, NULL,
css->cgroup, &args);
if (ret) {
@ -3456,11 +3422,7 @@ static int scx_cgroup_init(struct scx_sched *sch)
return ret;
}
tg->scx.flags |= SCX_TG_INITED;
rcu_read_lock();
css_put(css);
}
rcu_read_unlock();
WARN_ON_ONCE(scx_cgroup_enabled);
scx_cgroup_enabled = true;

2
kernel/sched/ext.h
View file

@ -77,7 +77,6 @@ int scx_tg_online(struct task_group *tg);
void scx_tg_offline(struct task_group *tg);
int scx_cgroup_can_attach(struct cgroup_taskset *tset);
void scx_cgroup_move_task(struct task_struct *p);
void scx_cgroup_finish_attach(void);
void scx_cgroup_cancel_attach(struct cgroup_taskset *tset);
void scx_group_set_weight(struct task_group *tg, unsigned long cgrp_weight);
void scx_group_set_idle(struct task_group *tg, bool idle);
@ -88,7 +87,6 @@ static inline int scx_tg_online(struct task_group *tg) { return 0; }
static inline void scx_tg_offline(struct task_group *tg) {}
static inline int scx_cgroup_can_attach(struct cgroup_taskset *tset) { return 0; }
static inline void scx_cgroup_move_task(struct task_struct *p) {}
static inline void scx_cgroup_finish_attach(void) {}
static inline void scx_cgroup_cancel_attach(struct cgroup_taskset *tset) {}
static inline void scx_group_set_weight(struct task_group *tg, unsigned long cgrp_weight) {}
static inline void scx_group_set_idle(struct task_group *tg, bool idle) {}