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Subject[PATCH 35/52] sched: Use the ideal CPU to drive active balancing
Date
Use our shared/private distinction to allow the separate
handling of 'private' versus 'shared' workloads, which enables
the active-balancing of them:

- private tasks, via the sched_update_ideal_cpu_private() function,
try to 'spread' the system as evenly as possible.

- shared-access tasks that also share their mm (threads), via the
sched_update_ideal_cpu_shared() function, try to 'compress'
with other shared tasks on as few nodes as possible.

[ We'll be able to extend this grouping beyond threads in the
future, by using the existing p->shared_buddy directed graph. ]

Introduce the sched_rebalance_to() primitive to trigger active rebalancing.

The result of this patch is 2-3 times faster convergence and
much more stable convergence points.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
include/linux/sched.h | 1 +
kernel/sched/core.c | 19 ++++
kernel/sched/fair.c | 244 +++++++++++++++++++++++++++++++++++++++++++++---
kernel/sched/features.h | 7 +-
kernel/sched/sched.h | 1 +
5 files changed, 257 insertions(+), 15 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index c020bc7..6e52e21 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2019,6 +2019,7 @@ task_sched_runtime(struct task_struct *task);
/* sched_exec is called by processes performing an exec */
#ifdef CONFIG_SMP
extern void sched_exec(void);
+extern void sched_rebalance_to(int dest_cpu);
#else
#define sched_exec() {}
#endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 39cf991..34ce37e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2589,6 +2589,22 @@ unlock:
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
}

+/*
+ * sched_rebalance_to()
+ *
+ * Active load-balance to a target CPU.
+ */
+void sched_rebalance_to(int dest_cpu)
+{
+ struct task_struct *p = current;
+ struct migration_arg arg = { p, dest_cpu };
+
+ if (!cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)))
+ return;
+
+ stop_one_cpu(raw_smp_processor_id(), migration_cpu_stop, &arg);
+}
+
#endif

DEFINE_PER_CPU(struct kernel_stat, kstat);
@@ -4746,6 +4762,9 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
done:
ret = 1;
fail:
+#ifdef CONFIG_NUMA_BALANCING
+ rq_dest->curr_buddy = NULL;
+#endif
double_rq_unlock(rq_src, rq_dest);
raw_spin_unlock(&p->pi_lock);
return ret;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a5f3ad7..46d23c7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -848,6 +848,181 @@ static int task_ideal_cpu(struct task_struct *p)
return p->ideal_cpu;
}

+
+static int sched_update_ideal_cpu_shared(struct task_struct *p)
+{
+ int full_buddies;
+ int max_buddies;
+ int target_cpu;
+ int ideal_cpu;
+ int this_cpu;
+ int this_node;
+ int best_node;
+ int buddies;
+ int node;
+ int cpu;
+
+ if (!sched_feat(PUSH_SHARED_BUDDIES))
+ return -1;
+
+ ideal_cpu = -1;
+ best_node = -1;
+ max_buddies = 0;
+ this_cpu = task_cpu(p);
+ this_node = cpu_to_node(this_cpu);
+
+ for_each_online_node(node) {
+ full_buddies = cpumask_weight(cpumask_of_node(node));
+
+ buddies = 0;
+ target_cpu = -1;
+
+ for_each_cpu(cpu, cpumask_of_node(node)) {
+ struct task_struct *curr;
+ struct rq *rq;
+
+ WARN_ON_ONCE(cpu_to_node(cpu) != node);
+
+ rq = cpu_rq(cpu);
+
+ /*
+ * Don't take the rq lock for scalability,
+ * we only rely on rq->curr statistically:
+ */
+ curr = ACCESS_ONCE(rq->curr);
+
+ if (curr == p) {
+ buddies += 1;
+ continue;
+ }
+
+ /* Pick up idle tasks immediately: */
+ if (curr == rq->idle && !rq->curr_buddy)
+ target_cpu = cpu;
+
+ /* Leave alone non-NUMA tasks: */
+ if (task_numa_shared(curr) < 0)
+ continue;
+
+ /* Private task is an easy target: */
+ if (task_numa_shared(curr) == 0) {
+ if (!rq->curr_buddy)
+ target_cpu = cpu;
+ continue;
+ }
+ if (curr->mm != p->mm) {
+ /* Leave alone different groups on their ideal node: */
+ if (cpu_to_node(curr->ideal_cpu) == node)
+ continue;
+ if (!rq->curr_buddy)
+ target_cpu = cpu;
+ continue;
+ }
+
+ buddies++;
+ }
+ WARN_ON_ONCE(buddies > full_buddies);
+
+ /* Don't go to a node that is already at full capacity: */
+ if (buddies == full_buddies)
+ continue;
+
+ if (!buddies)
+ continue;
+
+ if (buddies > max_buddies && target_cpu != -1) {
+ max_buddies = buddies;
+ best_node = node;
+ WARN_ON_ONCE(target_cpu == -1);
+ ideal_cpu = target_cpu;
+ }
+ }
+
+ WARN_ON_ONCE(best_node == -1 && ideal_cpu != -1);
+ WARN_ON_ONCE(best_node != -1 && ideal_cpu == -1);
+
+ this_node = cpu_to_node(this_cpu);
+
+ /* If we'd stay within this node then stay put: */
+ if (ideal_cpu == -1 || cpu_to_node(ideal_cpu) == this_node)
+ ideal_cpu = this_cpu;
+
+ return ideal_cpu;
+}
+
+static int sched_update_ideal_cpu_private(struct task_struct *p)
+{
+ int full_idles;
+ int this_idles;
+ int max_idles;
+ int target_cpu;
+ int ideal_cpu;
+ int best_node;
+ int this_node;
+ int this_cpu;
+ int idles;
+ int node;
+ int cpu;
+
+ if (!sched_feat(PUSH_PRIVATE_BUDDIES))
+ return -1;
+
+ ideal_cpu = -1;
+ best_node = -1;
+ max_idles = 0;
+ this_idles = 0;
+ this_cpu = task_cpu(p);
+ this_node = cpu_to_node(this_cpu);
+
+ for_each_online_node(node) {
+ full_idles = cpumask_weight(cpumask_of_node(node));
+
+ idles = 0;
+ target_cpu = -1;
+
+ for_each_cpu(cpu, cpumask_of_node(node)) {
+ struct rq *rq;
+
+ WARN_ON_ONCE(cpu_to_node(cpu) != node);
+
+ rq = cpu_rq(cpu);
+ if (rq->curr == rq->idle) {
+ if (!rq->curr_buddy)
+ target_cpu = cpu;
+ idles++;
+ }
+ }
+ WARN_ON_ONCE(idles > full_idles);
+
+ if (node == this_node)
+ this_idles = idles;
+
+ if (!idles)
+ continue;
+
+ if (idles > max_idles && target_cpu != -1) {
+ max_idles = idles;
+ best_node = node;
+ WARN_ON_ONCE(target_cpu == -1);
+ ideal_cpu = target_cpu;
+ }
+ }
+
+ WARN_ON_ONCE(best_node == -1 && ideal_cpu != -1);
+ WARN_ON_ONCE(best_node != -1 && ideal_cpu == -1);
+
+ /* If the target is not idle enough, skip: */
+ if (max_idles <= this_idles+1)
+ ideal_cpu = this_cpu;
+
+ /* If we'd stay within this node then stay put: */
+ if (ideal_cpu == -1 || cpu_to_node(ideal_cpu) == this_node)
+ ideal_cpu = this_cpu;
+
+ return ideal_cpu;
+}
+
+
/*
* Called for every full scan - here we consider switching to a new
* shared buddy, if the one we found during this scan is good enough:
@@ -862,7 +1037,6 @@ static void shared_fault_full_scan_done(struct task_struct *p)
WARN_ON_ONCE(!p->shared_buddy_curr);
p->shared_buddy = p->shared_buddy_curr;
p->shared_buddy_faults = p->shared_buddy_faults_curr;
- p->ideal_cpu = p->ideal_cpu_curr;

goto clear_buddy;
}
@@ -891,14 +1065,13 @@ static void task_numa_placement(struct task_struct *p)
unsigned long total[2] = { 0, 0 };
unsigned long faults, max_faults = 0;
int node, priv, shared, max_node = -1;
+ int this_node;

if (p->numa_scan_seq == seq)
return;

p->numa_scan_seq = seq;

- shared_fault_full_scan_done(p);
-
/*
* Update the fault average with the result of the latest
* scan:
@@ -926,10 +1099,7 @@ static void task_numa_placement(struct task_struct *p)
}
}

- if (max_node != p->numa_max_node) {
- sched_setnuma(p, max_node, task_numa_shared(p));
- goto out_backoff;
- }
+ shared_fault_full_scan_done(p);

p->numa_migrate_seq++;
if (sched_feat(NUMA_SETTLE) &&
@@ -942,14 +1112,55 @@ static void task_numa_placement(struct task_struct *p)
* the impact of a little private memory accesses.
*/
shared = (total[0] >= total[1] / 2);
- if (shared != task_numa_shared(p)) {
- sched_setnuma(p, p->numa_max_node, shared);
+ if (shared)
+ p->ideal_cpu = sched_update_ideal_cpu_shared(p);
+ else
+ p->ideal_cpu = sched_update_ideal_cpu_private(p);
+
+ if (p->ideal_cpu >= 0) {
+ /* Filter migrations a bit - the same target twice in a row is picked: */
+ if (p->ideal_cpu == p->ideal_cpu_candidate) {
+ max_node = cpu_to_node(p->ideal_cpu);
+ } else {
+ p->ideal_cpu_candidate = p->ideal_cpu;
+ max_node = -1;
+ }
+ } else {
+ if (max_node < 0)
+ max_node = p->numa_max_node;
+ }
+
+ if (shared != task_numa_shared(p) || (max_node != -1 && max_node != p->numa_max_node)) {
+
p->numa_migrate_seq = 0;
- goto out_backoff;
+ /*
+ * Fix up node migration fault statistics artifact, as we
+ * migrate to another node we'll soon bring over our private
+ * working set - generating 'shared' faults as that happens.
+ * To counter-balance this effect, move this node's private
+ * stats to the new node.
+ */
+ if (max_node != -1 && p->numa_max_node != -1 && max_node != p->numa_max_node) {
+ int idx_oldnode = p->numa_max_node*2 + 1;
+ int idx_newnode = max_node*2 + 1;
+
+ p->numa_faults[idx_newnode] += p->numa_faults[idx_oldnode];
+ p->numa_faults[idx_oldnode] = 0;
+ }
+ sched_setnuma(p, max_node, shared);
+ } else {
+ /* node unchanged, back off: */
+ p->numa_scan_period = min(p->numa_scan_period * 2, sysctl_sched_numa_scan_period_max);
+ }
+
+ this_node = cpu_to_node(task_cpu(p));
+
+ if (max_node >= 0 && p->ideal_cpu >= 0 && max_node != this_node) {
+ struct rq *rq = cpu_rq(p->ideal_cpu);
+
+ rq->curr_buddy = p;
+ sched_rebalance_to(p->ideal_cpu);
}
- return;
-out_backoff:
- p->numa_scan_period = min(p->numa_scan_period * 2, sysctl_sched_numa_scan_period_max);
}

/*
@@ -1051,6 +1262,8 @@ void task_numa_fault(int node, int last_cpu, int pages)
int priv = (task_cpu(p) == last_cpu);
int idx = 2*node + priv;

+ WARN_ON_ONCE(last_cpu < 0 || node < 0);
+
if (unlikely(!p->numa_faults)) {
int entries = 2*nr_node_ids;
int size = sizeof(*p->numa_faults) * entries;
@@ -3545,6 +3758,11 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
if (p->nr_cpus_allowed == 1)
return prev_cpu;

+#ifdef CONFIG_NUMA_BALANCING
+ if (sched_feat(WAKE_ON_IDEAL_CPU) && p->ideal_cpu >= 0)
+ return p->ideal_cpu;
+#endif
+
if (sd_flag & SD_BALANCE_WAKE) {
if (cpumask_test_cpu(cpu, tsk_cpus_allowed(p)))
want_affine = 1;
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 737d2c8..c868a66 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -68,11 +68,14 @@ SCHED_FEAT(RT_RUNTIME_SHARE, true)
SCHED_FEAT(LB_MIN, false)
SCHED_FEAT(IDEAL_CPU, true)
SCHED_FEAT(IDEAL_CPU_THREAD_BIAS, false)
+SCHED_FEAT(PUSH_PRIVATE_BUDDIES, true)
+SCHED_FEAT(PUSH_SHARED_BUDDIES, true)
+SCHED_FEAT(WAKE_ON_IDEAL_CPU, false)

#ifdef CONFIG_NUMA_BALANCING
/* Do the working set probing faults: */
SCHED_FEAT(NUMA, true)
-SCHED_FEAT(NUMA_FAULTS_UP, true)
-SCHED_FEAT(NUMA_FAULTS_DOWN, true)
+SCHED_FEAT(NUMA_FAULTS_UP, false)
+SCHED_FEAT(NUMA_FAULTS_DOWN, false)
SCHED_FEAT(NUMA_SETTLE, true)
#endif
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index bb9475c..810a1a0 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -441,6 +441,7 @@ struct rq {
unsigned long numa_weight;
unsigned long nr_numa_running;
unsigned long nr_ideal_running;
+ struct task_struct *curr_buddy;
#endif
unsigned long nr_shared_running; /* 0 on non-NUMA */

--
1.7.11.7


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