sched/mmcid: Revert the complex CID management

The CID management is a complex beast, which affects both scheduling and
task migration. The compaction mechanism forces random tasks of a process
into task work on exit to user space causing latency spikes.

Revert back to the initial simple bitmap allocating mechanics, which are
known to have scalability issues as that allows to gradually build up a
replacement functionality in a reviewable way.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251119172549.068197830@linutronix.de

Author: KAGA-KOKO

include/linux/mm_types.h

@@ -922,13 +922,9 @@ struct vm_area_struct {
 #define vma_policy(vma) NULL
 #endif
 
-#ifdef CONFIG_SCHED_MM_CID
 struct mm_cid {
-	u64 time;
-	int cid;
-	int recent_cid;
+	unsigned int cid;
 };
-#endif
 
 /*
  * Opaque type representing current mm_struct flag state. Must be accessed via
@@ -1000,27 +996,13 @@ struct mm_struct {
 		 * runqueue locks.
 		 */
 		struct mm_cid __percpu *pcpu_cid;
-		/*
-		 * @mm_cid_next_scan: Next mm_cid scan (in jiffies).
-		 *
-		 * When the next mm_cid scan is due (in jiffies).
-		 */
-		unsigned long mm_cid_next_scan;
 		/**
 		 * @nr_cpus_allowed: Number of CPUs allowed for mm.
 		 *
 		 * Number of CPUs allowed in the union of all mm's
 		 * threads allowed CPUs.
 		 */
 		unsigned int nr_cpus_allowed;
-		/**
-		 * @max_nr_cid: Maximum number of allowed concurrency
-		 *              IDs allocated.
-		 *
-		 * Track the highest number of allowed concurrency IDs
-		 * allocated for the mm.
-		 */
-		atomic_t max_nr_cid;
 		/**
 		 * @cpus_allowed_lock: Lock protecting mm cpus_allowed.
 		 *
@@ -1371,35 +1353,7 @@ static inline void vma_iter_init(struct vma_iterator *vmi,
 
 #ifdef CONFIG_SCHED_MM_CID
 
-enum mm_cid_state {
-	MM_CID_UNSET = -1U,		/* Unset state has lazy_put flag set. */
-	MM_CID_LAZY_PUT = (1U << 31),
-};
-
-static inline bool mm_cid_is_unset(int cid)
-{
-	return cid == MM_CID_UNSET;
-}
-
-static inline bool mm_cid_is_lazy_put(int cid)
-{
-	return !mm_cid_is_unset(cid) && (cid & MM_CID_LAZY_PUT);
-}
-
-static inline bool mm_cid_is_valid(int cid)
-{
-	return !(cid & MM_CID_LAZY_PUT);
-}
-
-static inline int mm_cid_set_lazy_put(int cid)
-{
-	return cid | MM_CID_LAZY_PUT;
-}
-
-static inline int mm_cid_clear_lazy_put(int cid)
-{
-	return cid & ~MM_CID_LAZY_PUT;
-}
+#define	MM_CID_UNSET	(~0U)
 
 /*
  * mm_cpus_allowed: Union of all mm's threads allowed CPUs.
@@ -1432,11 +1386,8 @@ static inline void mm_init_cid(struct mm_struct *mm, struct task_struct *p)
 		struct mm_cid *pcpu_cid = per_cpu_ptr(mm->pcpu_cid, i);
 
 		pcpu_cid->cid = MM_CID_UNSET;
-		pcpu_cid->recent_cid = MM_CID_UNSET;
-		pcpu_cid->time = 0;
 	}
 	mm->nr_cpus_allowed = p->nr_cpus_allowed;
-	atomic_set(&mm->max_nr_cid, 0);
 	raw_spin_lock_init(&mm->cpus_allowed_lock);
 	cpumask_copy(mm_cpus_allowed(mm), &p->cpus_mask);
 	cpumask_clear(mm_cidmask(mm));

kernel/fork.c

@@ -955,10 +955,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 #endif
 
 #ifdef CONFIG_SCHED_MM_CID
-	tsk->mm_cid = -1;
-	tsk->last_mm_cid = -1;
+	tsk->mm_cid = MM_CID_UNSET;
+	tsk->last_mm_cid = MM_CID_UNSET;
 	tsk->mm_cid_active = 0;
-	tsk->migrate_from_cpu = -1;
 #endif
 	return tsk;