cgroup: remove per-cpu per-subsystem locks

The rstat update side used to insert the cgroup whose stats are updated
in the update tree and the read side flush the update tree to get the
latest uptodate stats. The per-cpu per-subsystem locks were used to
synchronize the update and flush side. However now the update side does
not access update tree but uses per-cpu lockless lists. So there is no
need for locks to synchronize update and flush side. Let's remove them.

Suggested-by: JP Kobryn <[e-mail]>
Signed-off-by: Shakeel Butt <[e-mail]>
Tested-by: JP Kobryn <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

include/linux/cgroup-defs.h

···
 	 * Child cgroups with stat updates on this cpu since the last read
 	 * are linked on the parent's ->updated_children through
 	 * ->updated_next. updated_children is terminated by its container css.
-	 *
-	 * In addition to being more compact, singly-linked list pointing to
-	 * the css makes it unnecessary for each per-cpu struct to point back
-	 * to the associated css.
-	 *
-	 * Protected by per-cpu css->ss->rstat_ss_cpu_lock.
 	 */
 	struct cgroup_subsys_state *updated_children;
 	struct cgroup_subsys_state *updated_next;	/* NULL if not on the list */
···
 	unsigned int depends_on;
 
 	spinlock_t rstat_ss_lock;
-	raw_spinlock_t __percpu *rstat_ss_cpu_lock;
 	struct llist_head __percpu *lhead; /* lockless update list head */
 };
 

include/trace/events/cgroup.h

···
 	TP_ARGS(cgrp, cpu, contended)
 );
 
-/*
- * Related to per CPU locks:
- * global rstat_base_cpu_lock for base stats
- * cgroup_subsys::rstat_ss_cpu_lock for subsystem stats
- */
-DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_lock_contended,
-
-	TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
-
-	TP_ARGS(cgrp, cpu, contended)
-);
-
-DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_lock_contended_fastpath,
-
-	TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
-
-	TP_ARGS(cgrp, cpu, contended)
-);
-
-DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_locked,
-
-	TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
-
-	TP_ARGS(cgrp, cpu, contended)
-);
-
-DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_locked_fastpath,
-
-	TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
-
-	TP_ARGS(cgrp, cpu, contended)
-);
-
-DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_unlock,
-
-	TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
-
-	TP_ARGS(cgrp, cpu, contended)
-);
-
-DEFINE_EVENT(cgroup_rstat, cgroup_rstat_cpu_unlock_fastpath,
-
-	TP_PROTO(struct cgroup *cgrp, int cpu, bool contended),
-
-	TP_ARGS(cgrp, cpu, contended)
-);
-
 #endif /* _TRACE_CGROUP_H */
 
 /* This part must be outside protection */

kernel/cgroup/rstat.c

···
 #include <trace/events/cgroup.h>
 
 static DEFINE_SPINLOCK(rstat_base_lock);
-static DEFINE_PER_CPU(raw_spinlock_t, rstat_base_cpu_lock);
 static DEFINE_PER_CPU(struct llist_head, rstat_backlog_list);
 
 static void cgroup_base_stat_flush(struct cgroup *cgrp, int cpu);
···
 	if (ss)
 		return per_cpu_ptr(ss->lhead, cpu);
 	return per_cpu_ptr(&rstat_backlog_list, cpu);
-}
-
-static raw_spinlock_t *ss_rstat_cpu_lock(struct cgroup_subsys *ss, int cpu)
-{
-	if (ss)
-		return per_cpu_ptr(ss->rstat_ss_cpu_lock, cpu);
-
-	return per_cpu_ptr(&rstat_base_cpu_lock, cpu);
-}
-
-/*
- * Helper functions for rstat per CPU locks.
- *
- * This makes it easier to diagnose locking issues and contention in
- * production environments. The parameter @fast_path determine the
- * tracepoints being added, allowing us to diagnose "flush" related
- * operations without handling high-frequency fast-path "update" events.
- */
-static __always_inline
-unsigned long _css_rstat_cpu_lock(struct cgroup_subsys_state *css, int cpu,
-		const bool fast_path)
-{
-	struct cgroup *cgrp = css->cgroup;
-	raw_spinlock_t *cpu_lock;
-	unsigned long flags;
-	bool contended;
-
-	/*
-	 * The _irqsave() is needed because the locks used for flushing are
-	 * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring this lock
-	 * with the _irq() suffix only disables interrupts on a non-PREEMPT_RT
-	 * kernel. The raw_spinlock_t below disables interrupts on both
-	 * configurations. The _irqsave() ensures that interrupts are always
-	 * disabled and later restored.
-	 */
-	cpu_lock = ss_rstat_cpu_lock(css->ss, cpu);
-	contended = !raw_spin_trylock_irqsave(cpu_lock, flags);
-	if (contended) {
-		if (fast_path)
-			trace_cgroup_rstat_cpu_lock_contended_fastpath(cgrp, cpu, contended);
-		else
-			trace_cgroup_rstat_cpu_lock_contended(cgrp, cpu, contended);
-
-		raw_spin_lock_irqsave(cpu_lock, flags);
-	}
-
-	if (fast_path)
-		trace_cgroup_rstat_cpu_locked_fastpath(cgrp, cpu, contended);
-	else
-		trace_cgroup_rstat_cpu_locked(cgrp, cpu, contended);
-
-	return flags;
-}
-
-static __always_inline
-void _css_rstat_cpu_unlock(struct cgroup_subsys_state *css, int cpu,
-		unsigned long flags, const bool fast_path)
-{
-	struct cgroup *cgrp = css->cgroup;
-	raw_spinlock_t *cpu_lock;
-
-	if (fast_path)
-		trace_cgroup_rstat_cpu_unlock_fastpath(cgrp, cpu, false);
-	else
-		trace_cgroup_rstat_cpu_unlock(cgrp, cpu, false);
-
-	cpu_lock = ss_rstat_cpu_lock(css->ss, cpu);
-	raw_spin_unlock_irqrestore(cpu_lock, flags);
 }
 
 /**
···
 {
 	struct css_rstat_cpu *rstatc = css_rstat_cpu(root, cpu);
 	struct cgroup_subsys_state *head = NULL, *parent, *child;
-	unsigned long flags;
-
-	flags = _css_rstat_cpu_lock(root, cpu, false);
 
 	css_process_update_tree(root->ss, cpu);
 
 	/* Return NULL if this subtree is not on-list */
 	if (!rstatc->updated_next)
-		goto unlock_ret;
+		return NULL;
 
 	/*
 	 * Unlink @root from its parent. As the updated_children list is
···
 	rstatc->updated_children = root;
 	if (child != root)
 		head = css_rstat_push_children(head, child, cpu);
-unlock_ret:
-	_css_rstat_cpu_unlock(root, cpu, flags, false);
+
 	return head;
 }
 
···
 {
 	int cpu;
 
-#ifdef CONFIG_SMP
-	/*
-	 * On uniprocessor machines, arch_spinlock_t is defined as an empty
-	 * struct. Avoid allocating a size of zero by having this block
-	 * excluded in this case. It's acceptable to leave the subsystem locks
-	 * unitialized since the associated lock functions are no-ops in the
-	 * non-smp case.
-	 */
-	if (ss) {
-		ss->rstat_ss_cpu_lock = alloc_percpu(raw_spinlock_t);
-		if (!ss->rstat_ss_cpu_lock)
-			return -ENOMEM;
-	}
-#endif
-
 	if (ss) {
 		ss->lhead = alloc_percpu(struct llist_head);
-		if (!ss->lhead) {
-			free_percpu(ss->rstat_ss_cpu_lock);
+		if (!ss->lhead)
 			return -ENOMEM;
-		}
 	}
 
 	spin_lock_init(ss_rstat_lock(ss));
-	for_each_possible_cpu(cpu) {
-		raw_spin_lock_init(ss_rstat_cpu_lock(ss, cpu));
+	for_each_possible_cpu(cpu)
 		init_llist_head(ss_lhead_cpu(ss, cpu));
-	}
 
 	return 0;
 }