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Subject[PATCH -v3] sched, numa: Use {cpu, pid} to create task groups for shared faults

Here's the latest; it seems to not crash and appears to actually do as
advertised.

---
Subject: sched, numa: Use {cpu, pid} to create task groups for shared faults
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue Jul 30 10:40:20 CEST 2013

A very simple/straight forward shared fault task grouping
implementation.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
include/linux/sched.h | 3
kernel/sched/core.c | 3
kernel/sched/fair.c | 174 +++++++++++++++++++++++++++++++++++++++++++++++---
kernel/sched/sched.h | 5 -
4 files changed, 171 insertions(+), 14 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1341,6 +1341,9 @@ struct task_struct {
u64 node_stamp; /* migration stamp */
struct callback_head numa_work;

+ struct list_head numa_entry;
+ struct numa_group *numa_group;
+
/*
* Exponential decaying average of faults on a per-node basis.
* Scheduling placement decisions are made based on the these counts.
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1733,6 +1733,9 @@ static void __sched_fork(struct task_str
p->numa_work.next = &p->numa_work;
p->numa_faults = NULL;
p->numa_faults_buffer = NULL;
+
+ INIT_LIST_HEAD(&p->numa_entry);
+ p->numa_group = NULL;
#endif /* CONFIG_NUMA_BALANCING */
}

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1149,6 +1149,17 @@ static void numa_migrate_preferred(struc
p->numa_migrate_retry = jiffies + HZ/10;
}

+struct numa_group {
+ atomic_t refcount;
+
+ spinlock_t lock; /* nr_tasks, tasks */
+ int nr_tasks;
+ struct list_head task_list;
+
+ struct rcu_head rcu;
+ atomic_long_t faults[0];
+};
+
static void task_numa_placement(struct task_struct *p)
{
int seq, nid, max_nid = -1;
@@ -1157,6 +1168,7 @@ static void task_numa_placement(struct t
seq = ACCESS_ONCE(p->mm->numa_scan_seq);
if (p->numa_scan_seq == seq)
return;
+
p->numa_scan_seq = seq;
p->numa_migrate_seq++;
p->numa_scan_period_max = task_scan_max(p);
@@ -1167,14 +1179,24 @@ static void task_numa_placement(struct t
int priv, i;

for (priv = 0; priv < 2; priv++) {
+ long diff;
+
i = task_faults_idx(nid, priv);

+ diff = -p->numa_faults[i];
+
/* Decay existing window, copy faults since last scan */
p->numa_faults[i] >>= 1;
p->numa_faults[i] += p->numa_faults_buffer[i];
p->numa_faults_buffer[i] = 0;

+ diff += p->numa_faults[i];
faults += p->numa_faults[i];
+
+ if (p->numa_group) {
+ /* safe because we can only change our own group */
+ atomic_long_add(diff, &p->numa_group->faults[i]);
+ }
}

if (faults > max_faults) {
@@ -1211,6 +1233,130 @@ static void task_numa_placement(struct t
}
}

+static inline int get_numa_group(struct numa_group *grp)
+{
+ return atomic_inc_not_zero(&grp->refcount);
+}
+
+static inline void put_numa_group(struct numa_group *grp)
+{
+ if (atomic_dec_and_test(&grp->refcount))
+ kfree_rcu(grp, rcu);
+}
+
+static void double_lock(spinlock_t *l1, spinlock_t *l2)
+{
+ if (l1 > l2)
+ swap(l1, l2);
+
+ spin_lock(l1);
+ spin_lock_nested(l2, SINGLE_DEPTH_NESTING);
+}
+
+static void task_numa_group(struct task_struct *p, int cpu, int pid)
+{
+ struct numa_group *grp, *my_grp;
+ struct task_struct *tsk;
+ bool join = false;
+ int i;
+
+ if (unlikely(!p->numa_group)) {
+ unsigned int size = sizeof(struct numa_group) +
+ 2*nr_node_ids*sizeof(atomic_long_t);
+
+ grp = kzalloc(size, GFP_KERNEL | __GFP_NOWARN);
+ if (!grp)
+ return;
+
+ atomic_set(&grp->refcount, 1);
+ spin_lock_init(&grp->lock);
+ INIT_LIST_HEAD(&grp->task_list);
+
+ for (i = 0; i < 2*nr_node_ids; i++)
+ atomic_long_set(&grp->faults[i], p->numa_faults[i]);
+
+ list_add(&p->numa_entry, &grp->task_list);
+ grp->nr_tasks++;
+ rcu_assign_pointer(p->numa_group, grp);
+ }
+
+ rcu_read_lock();
+ tsk = ACCESS_ONCE(cpu_rq(cpu)->curr);
+
+ if ((tsk->pid & LAST__PID_MASK) != pid)
+ goto unlock;
+
+ grp = rcu_dereference(tsk->numa_group);
+ if (!grp)
+ goto unlock;
+
+ my_grp = p->numa_group;
+ if (grp == my_grp)
+ goto unlock;
+
+ /*
+ * Only join the other group if its bigger; if we're the bigger group,
+ * the other task will join us.
+ */
+ if (my_grp->nr_tasks > grp->nr_tasks)
+ goto unlock;
+
+ /*
+ * Tie-break on the grp address.
+ */
+ if (my_grp->nr_tasks == grp->nr_tasks && my_grp > grp)
+ goto unlock;
+
+ if (!get_numa_group(grp))
+ goto unlock;
+
+ join = true;
+
+unlock:
+ rcu_read_unlock();
+
+ if (!join)
+ return;
+
+ for (i = 0; i < 2*nr_node_ids; i++) {
+ atomic_long_sub(p->numa_faults[i], &my_grp->faults[i]);
+ atomic_long_add(p->numa_faults[i], &grp->faults[i]);
+ }
+
+ double_lock(&my_grp->lock, &grp->lock);
+
+ list_move(&p->numa_entry, &grp->task_list);
+ my_grp->nr_tasks--;
+ grp->nr_tasks++;
+
+ spin_unlock(&my_grp->lock);
+ spin_unlock(&grp->lock);
+
+ rcu_assign_pointer(p->numa_group, grp);
+
+ put_numa_group(my_grp);
+}
+
+void task_numa_free(struct task_struct *p)
+{
+ struct numa_group *grp = p->numa_group;
+ int i;
+
+ kfree(p->numa_faults);
+
+ if (grp) {
+ for (i = 0; i < 2*nr_node_ids; i++)
+ atomic_long_sub(p->numa_faults[i], &grp->faults[i]);
+
+ spin_lock(&grp->lock);
+ list_del(&p->numa_entry);
+ grp->nr_tasks--;
+ spin_unlock(&grp->lock);
+ rcu_assign_pointer(p->numa_group, NULL);
+ put_numa_group(grp);
+ }
+}
+
/*
* Got a PROT_NONE fault for a page on @node.
*/
@@ -1226,21 +1372,12 @@ void task_numa_fault(int last_cpupid, in
if (!p->mm)
return;

- /*
- * First accesses are treated as private, otherwise consider accesses
- * to be private if the accessing pid has not changed
- */
- if (!cpupid_pid_unset(last_cpupid))
- priv = ((p->pid & LAST__PID_MASK) == cpupid_to_pid(last_cpupid));
- else
- priv = 1;
-
/* Allocate buffer to track faults on a per-node basis */
if (unlikely(!p->numa_faults)) {
int size = sizeof(*p->numa_faults) * 2 * nr_node_ids;

/* numa_faults and numa_faults_buffer share the allocation */
- p->numa_faults = kzalloc(size * 2, GFP_KERNEL);
+ p->numa_faults = kzalloc(size * 2, GFP_KERNEL | __GFP_NOWARN);
if (!p->numa_faults)
return;

@@ -1249,6 +1386,23 @@ void task_numa_fault(int last_cpupid, in
}

/*
+ * First accesses are treated as private, otherwise consider accesses
+ * to be private if the accessing pid has not changed
+ */
+ if (unlikely(last_cpupid == (-1 & LAST_CPUPID_MASK))) {
+ priv = 1;
+ } else {
+ int cpu, pid;
+
+ cpu = cpupid_to_cpu(last_cpupid);
+ pid = cpupid_to_pid(last_cpupid);
+
+ priv = (pid == (p->pid & LAST__PID_MASK));
+ if (!priv)
+ task_numa_group(p, cpu, pid);
+ }
+
+ /*
* If pages are properly placed (did not migrate) then scan slower.
* This is reset periodically in case of phase changes
*
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -556,10 +556,7 @@ static inline u64 rq_clock_task(struct r
#ifdef CONFIG_NUMA_BALANCING
extern int migrate_task_to(struct task_struct *p, int cpu);
extern int migrate_swap(struct task_struct *, struct task_struct *);
-static inline void task_numa_free(struct task_struct *p)
-{
- kfree(p->numa_faults);
-}
+extern void task_numa_free(struct task_struct *p);
#else /* CONFIG_NUMA_BALANCING */
static inline void task_numa_free(struct task_struct *p)
{

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