workqueue: set WQ_AFFN_CACHE_SHARD as the default affinity scope

Set WQ_AFFN_CACHE_SHARD as the default affinity scope for unbound
workqueues. On systems where many CPUs share one LLC, the previous
default (WQ_AFFN_CACHE) collapses all CPUs to a single worker pool,
causing heavy spinlock contention on pool->lock.

WQ_AFFN_CACHE_SHARD subdivides each LLC into smaller groups, providing
a better balance between locality and contention. Users can revert to
the previous behavior with workqueue.default_affinity_scope=cache.

On systems with 8 or fewer cores per LLC, CACHE_SHARD produces a single
shard covering the entire LLC, making it functionally identical to the
previous CACHE default. The sharding only activates when an LLC has more
than 8 cores.

Signed-off-by: Breno Leitao <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: leitao

kernel/workqueue.c

@@ -450,7 +450,7 @@ static bool wq_topo_initialized __read_mostly = false;
 static struct kmem_cache *pwq_cache;
 
 static struct wq_pod_type wq_pod_types[WQ_AFFN_NR_TYPES];
-static enum wq_affn_scope wq_affn_dfl = WQ_AFFN_CACHE;
+static enum wq_affn_scope wq_affn_dfl = WQ_AFFN_CACHE_SHARD;
 
 /* buf for wq_update_unbound_pod_attrs(), protected by CPU hotplug exclusion */
 static struct workqueue_attrs *unbound_wq_update_pwq_attrs_buf;