sched/mmcid: Avoid full tasklist walks

Chasing vfork()'ed tasks on a CID ownership mode switch requires a full
task list walk, which is obviously expensive on large systems.

Avoid that by keeping a list of tasks using a mm MMCID entity in mm::mm_cid
and walk this list instead. This removes the proven to be flaky counting
logic and avoids a full task list walk in the case of vfork()'ed tasks.

Fixes: fbd0e71 ("sched/mmcid: Provide CID ownership mode fixup functions")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260310202526.183824481@kernel.org

Author: Thomas Gleixner

include/linux/rseq_types.h

@@ -133,10 +133,12 @@ struct rseq_data { };
  * @active:	MM CID is active for the task
  * @cid:	The CID associated to the task either permanently or
  *		borrowed from the CPU
+ * @node:	Queued in the per MM MMCID list
  */
 struct sched_mm_cid {
 	unsigned int		active;
 	unsigned int		cid;
+	struct hlist_node	node;
 };
 
 /**
@@ -157,6 +159,7 @@ struct mm_cid_pcpu {
  * @work:		Regular work to handle the affinity mode change case
  * @lock:		Spinlock to protect against affinity setting which can't take @mutex
  * @mutex:		Mutex to serialize forks and exits related to this mm
+ * @user_list:		List of the MM CID users of a MM
  * @nr_cpus_allowed:	The number of CPUs in the per MM allowed CPUs map. The map
  *			is growth only.
  * @users:		The number of tasks sharing this MM. Separate from mm::mm_users
@@ -177,13 +180,14 @@ struct mm_mm_cid {
 
 	raw_spinlock_t		lock;
 	struct mutex		mutex;
+	struct hlist_head	user_list;
 
 	/* Low frequency modified */
 	unsigned int		nr_cpus_allowed;
 	unsigned int		users;
 	unsigned int		pcpu_thrs;
 	unsigned int		update_deferred;
-}____cacheline_aligned_in_smp;
+} ____cacheline_aligned;
 #else /* CONFIG_SCHED_MM_CID */
 struct mm_mm_cid { };
 struct sched_mm_cid { };

kernel/fork.c

@@ -1000,6 +1000,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 #ifdef CONFIG_SCHED_MM_CID
 	tsk->mm_cid.cid = MM_CID_UNSET;
 	tsk->mm_cid.active = 0;
+	INIT_HLIST_NODE(&tsk->mm_cid.node);
 #endif
 	return tsk;
 

kernel/sched/core.c

@@ -10620,65 +10620,32 @@ static inline void mm_cid_transit_to_cpu(struct task_struct *t, struct mm_cid_pc
 	}
 }
 
-static bool mm_cid_fixup_task_to_cpu(struct task_struct *t, struct mm_struct *mm)
+static void mm_cid_fixup_task_to_cpu(struct task_struct *t, struct mm_struct *mm)
 {
 	/* Remote access to mm::mm_cid::pcpu requires rq_lock */
 	guard(task_rq_lock)(t);
-	/* If the task is not active it is not in the users count */
-	if (!t->mm_cid.active)
-		return false;
 	if (cid_on_task(t->mm_cid.cid)) {
 		/* If running on the CPU, put the CID in transit mode, otherwise drop it */
 		if (task_rq(t)->curr == t)
 			mm_cid_transit_to_cpu(t, per_cpu_ptr(mm->mm_cid.pcpu, task_cpu(t)));
 		else
 			mm_unset_cid_on_task(t);
 	}
-	return true;
 }
 
-static void mm_cid_do_fixup_tasks_to_cpus(struct mm_struct *mm)
+static void mm_cid_fixup_tasks_to_cpus(void)
 {
-	struct task_struct *p, *t;
-	unsigned int users;
-
-	/*
-	 * This can obviously race with a concurrent affinity change, which
-	 * increases the number of allowed CPUs for this mm, but that does
-	 * not affect the mode and only changes the CID constraints. A
-	 * possible switch back to per task mode happens either in the
-	 * deferred handler function or in the next fork()/exit().
-	 *
-	 * The caller has already transferred so remove it from the users
-	 * count. The incoming task is already visible and has mm_cid.active,
-	 * but has task::mm_cid::cid == UNSET. Still it needs to be accounted
-	 * for. Concurrent fork()s might add more threads, but all of them have
-	 * task::mm_cid::active = 0, so they don't affect the accounting here.
-	 */
-	users = mm->mm_cid.users - 1;
-
-	guard(rcu)();
-	for_other_threads(current, t) {
-		if (mm_cid_fixup_task_to_cpu(t, mm))
-			users--;
-	}
+	struct mm_struct *mm = current->mm;
+	struct task_struct *t;
 
-	if (!users)
-		return;
+	lockdep_assert_held(&mm->mm_cid.mutex);
 
-	/* Happens only for VM_CLONE processes. */
-	for_each_process_thread(p, t) {
-		if (t == current || t->mm != mm)
-			continue;
-		mm_cid_fixup_task_to_cpu(t, mm);
+	hlist_for_each_entry(t, &mm->mm_cid.user_list, mm_cid.node) {
+		/* Current has already transferred before invoking the fixup. */
+		if (t != current)
+			mm_cid_fixup_task_to_cpu(t, mm);
 	}
-}
-
-static void mm_cid_fixup_tasks_to_cpus(void)
-{
-	struct mm_struct *mm = current->mm;
 
-	mm_cid_do_fixup_tasks_to_cpus(mm);
 	mm_cid_complete_transit(mm, MM_CID_ONCPU);
 }
 
@@ -10687,6 +10654,7 @@ static bool sched_mm_cid_add_user(struct task_struct *t, struct mm_struct *mm)
 	lockdep_assert_held(&mm->mm_cid.lock);
 
 	t->mm_cid.active = 1;
+	hlist_add_head(&t->mm_cid.node, &mm->mm_cid.user_list);
 	mm->mm_cid.users++;
 	return mm_update_max_cids(mm);
 }
@@ -10744,6 +10712,7 @@ static bool sched_mm_cid_remove_user(struct task_struct *t)
 	/* Clear the transition bit */
 	t->mm_cid.cid = cid_from_transit_cid(t->mm_cid.cid);
 	mm_unset_cid_on_task(t);
+	hlist_del_init(&t->mm_cid.node);
 	t->mm->mm_cid.users--;
 	return mm_update_max_cids(t->mm);
 }
@@ -10886,6 +10855,7 @@ void mm_init_cid(struct mm_struct *mm, struct task_struct *p)
 	mutex_init(&mm->mm_cid.mutex);
 	mm->mm_cid.irq_work = IRQ_WORK_INIT_HARD(mm_cid_irq_work);
 	INIT_WORK(&mm->mm_cid.work, mm_cid_work_fn);
+	INIT_HLIST_HEAD(&mm->mm_cid.user_list);
 	cpumask_copy(mm_cpus_allowed(mm), &p->cpus_mask);
 	bitmap_zero(mm_cidmask(mm), num_possible_cpus());
 }