workqueue: Implement non-strict affinity scope for unbound workqueues

An unbound workqueue can be served by multiple worker_pools to improve
locality. The segmentation is achieved by grouping CPUs into pods. By
default, the cache boundaries according to cpus_share_cache() define the
CPUs are grouped. Let's a workqueue is allowed to run on all CPUs and the
system has two L3 caches. The workqueue would be mapped to two worker_pools
each serving one L3 cache domains.

While this improves locality, because the pod boundaries are strict, it
limits the total bandwidth a given issuer can consume. For example, let's
say there is a thread pinned to a CPU issuing enough work items to saturate
the whole machine. With the machine segmented into two pods, no matter how
many work items it issues, it can only use half of the CPUs on the system.

While this limitation has existed for a very long time, it wasn't very
pronounced because the affinity grouping used to be always by NUMA nodes.
With cache boundaries as the default and support for even finer grained
scopes (smt and cpu), it is now an a lot more pressing problem.

This patch implements non-strict affinity scope where the pod boundaries
aren't enforced strictly. Going back to the previous example, the workqueue
would still be mapped to two worker_pools; however, the affinity enforcement
would be soft. The workers in both pools would have their cpus_allowed set
to the whole machine thus allowing the scheduler to migrate them anywhere on
the machine. However, whenever an idle worker is woken up, the workqueue
code asks the scheduler to bring back the task within the pod if the worker
is outside. ie. work items start executing within its affinity scope but can
be migrated outside as the scheduler sees fit. This removes the hard cap on
utilization while maintaining the benefits of affinity scopes.

After the earlier ->__pod_cpumask changes, the implementation is pretty
simple. When non-strict which is the new default:

* pool_allowed_cpus() returns @pool->attrs->cpumask instead of
  ->__pod_cpumask so that the workers are allowed to run on any CPU that
  the associated workqueues allow.

* If the idle worker task's ->wake_cpu is outside the pod, kick_pool() sets
  the field to a CPU within the pod.

This would be the first use of task_struct->wake_cpu outside scheduler
proper, so it isn't clear whether this would be acceptable. However, other
methods of migrating tasks are significantly more expensive and are likely
prohibitively so if we want to do this on every work item. This needs
discussion with scheduler folks.

There is also a race window where setting ->wake_cpu wouldn't be effective
as the target task is still on CPU. However, the window is pretty small and
this being a best-effort optimization, it doesn't seem to warrant more
complexity at the moment.

While the non-strict cache affinity scopes seem to be the best option, the
performance picture interacts with the affinity scope and is a bit
complicated to fully discuss in this patch, so the behavior is made easily
selectable through wqattrs and sysfs and the next patch will add
documentation to discuss performance implications.

v2: pool->attrs->affn_strict is set to true for per-cpu worker_pools.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 72 insertions, 2 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index e61b4291bec8..6f6f4f37ceb3 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -211,6 +211,7 @@ enum pool_workqueue_stats {
 	PWQ_STAT_CPU_TIME,	/* total CPU time consumed */
 	PWQ_STAT_CPU_INTENSIVE,	/* wq_cpu_intensive_thresh_us violations */
 	PWQ_STAT_CM_WAKEUP,	/* concurrency-management worker wakeups */
+	PWQ_STAT_REPATRIATED,	/* unbound workers brought back into scope */
 	PWQ_STAT_MAYDAY,	/* maydays to rescuer */
 	PWQ_STAT_RESCUED,	/* linked work items executed by rescuer */
 
@@ -1103,13 +1104,41 @@ static bool assign_work(struct work_struct *work, struct worker *worker,
 static bool kick_pool(struct worker_pool *pool)
 {
 	struct worker *worker = first_idle_worker(pool);
+	struct task_struct *p;
 
 	lockdep_assert_held(&pool->lock);
 
 	if (!need_more_worker(pool) || !worker)
 		return false;
 
-	wake_up_process(worker->task);
+	p = worker->task;
+
+#ifdef CONFIG_SMP
+	/*
+	 * Idle @worker is about to execute @work and waking up provides an
+	 * opportunity to migrate @worker at a lower cost by setting the task's
+	 * wake_cpu field. Let's see if we want to move @worker to improve
+	 * execution locality.
+	 *
+	 * We're waking the worker that went idle the latest and there's some
+	 * chance that @worker is marked idle but hasn't gone off CPU yet. If
+	 * so, setting the wake_cpu won't do anything. As this is a best-effort
+	 * optimization and the race window is narrow, let's leave as-is for
+	 * now. If this becomes pronounced, we can skip over workers which are
+	 * still on cpu when picking an idle worker.
+	 *
+	 * If @pool has non-strict affinity, @worker might have ended up outside
+	 * its affinity scope. Repatriate.
+	 */
+	if (!pool->attrs->affn_strict &&
+	    !cpumask_test_cpu(p->wake_cpu, pool->attrs->__pod_cpumask)) {
+		struct work_struct *work = list_first_entry(&pool->worklist,
+						struct work_struct, entry);
+		p->wake_cpu = cpumask_any_distribute(pool->attrs->__pod_cpumask);
+		get_work_pwq(work)->stats[PWQ_STAT_REPATRIATED]++;
+	}
+#endif
+	wake_up_process(p);
 	return true;
 }
 
@@ -2051,7 +2080,10 @@ static struct worker *alloc_worker(int node)
 
 static cpumask_t *pool_allowed_cpus(struct worker_pool *pool)
 {
-	return pool->attrs->__pod_cpumask;
+	if (pool->cpu < 0 && pool->attrs->affn_strict)
+		return pool->attrs->__pod_cpumask;
+	else
+		return pool->attrs->cpumask;
 }
 
 /**
@@ -3715,6 +3747,7 @@ static void copy_workqueue_attrs(struct workqueue_attrs *to,
 	to->nice = from->nice;
 	cpumask_copy(to->cpumask, from->cpumask);
 	cpumask_copy(to->__pod_cpumask, from->__pod_cpumask);
+	to->affn_strict = from->affn_strict;
 
 	/*
 	 * Unlike hash and equality test, copying shouldn't ignore wq-only
@@ -3745,6 +3778,7 @@ static u32 wqattrs_hash(const struct workqueue_attrs *attrs)
 		     BITS_TO_LONGS(nr_cpumask_bits) * sizeof(long), hash);
 	hash = jhash(cpumask_bits(attrs->__pod_cpumask),
 		     BITS_TO_LONGS(nr_cpumask_bits) * sizeof(long), hash);
+	hash = jhash_1word(attrs->affn_strict, hash);
 	return hash;
 }
 
@@ -3758,6 +3792,8 @@ static bool wqattrs_equal(const struct workqueue_attrs *a,
 		return false;
 	if (!cpumask_equal(a->__pod_cpumask, b->__pod_cpumask))
 		return false;
+	if (a->affn_strict != b->affn_strict)
+		return false;
 	return true;
 }
 
@@ -5847,6 +5883,7 @@ module_param_cb(default_affinity_scope, &wq_affn_dfl_ops, NULL, 0644);
  *  nice		RW int	: nice value of the workers
  *  cpumask		RW mask	: bitmask of allowed CPUs for the workers
  *  affinity_scope	RW str  : worker CPU affinity scope (cache, numa, none)
+ *  affinity_strict	RW bool : worker CPU affinity is strict
  */
 struct wq_device {
 	struct workqueue_struct		*wq;
@@ -6026,10 +6063,42 @@ static ssize_t wq_affn_scope_store(struct device *dev,
 	return ret ?: count;
 }
 
+static ssize_t wq_affinity_strict_show(struct device *dev,
+				       struct device_attribute *attr, char *buf)
+{
+	struct workqueue_struct *wq = dev_to_wq(dev);
+
+	return scnprintf(buf, PAGE_SIZE, "%d\n",
+			 wq->unbound_attrs->affn_strict);
+}
+
+static ssize_t wq_affinity_strict_store(struct device *dev,
+					struct device_attribute *attr,
+					const char *buf, size_t count)
+{
+	struct workqueue_struct *wq = dev_to_wq(dev);
+	struct workqueue_attrs *attrs;
+	int v, ret = -ENOMEM;
+
+	if (sscanf(buf, "%d", &v) != 1)
+		return -EINVAL;
+
+	apply_wqattrs_lock();
+	attrs = wq_sysfs_prep_attrs(wq);
+	if (attrs) {
+		attrs->affn_strict = (bool)v;
+		ret = apply_workqueue_attrs_locked(wq, attrs);
+	}
+	apply_wqattrs_unlock();
+	free_workqueue_attrs(attrs);
+	return ret ?: count;
+}
+
 static struct device_attribute wq_sysfs_unbound_attrs[] = {
 	__ATTR(nice, 0644, wq_nice_show, wq_nice_store),
 	__ATTR(cpumask, 0644, wq_cpumask_show, wq_cpumask_store),
 	__ATTR(affinity_scope, 0644, wq_affn_scope_show, wq_affn_scope_store),
+	__ATTR(affinity_strict, 0644, wq_affinity_strict_show, wq_affinity_strict_store),
 	__ATTR_NULL,
 };
 
@@ -6452,6 +6521,7 @@ void __init workqueue_init_early(void)
 			cpumask_copy(pool->attrs->cpumask, cpumask_of(cpu));
 			cpumask_copy(pool->attrs->__pod_cpumask, cpumask_of(cpu));
 			pool->attrs->nice = std_nice[i++];
+			pool->attrs->affn_strict = true;
 			pool->node = cpu_to_node(cpu);
 
 			/* alloc pool ID */