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workqueue: set WQ_AFFN_CACHE_SHARD as the default affinity scope

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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 <leitao@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Breno Leitao 2026-04-01 06:03:54 -07:00 committed by Tejun Heo
parent 5920d046f7
commit 4cdc8a7389
1 changed files with 1 additions and 1 deletions
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kernel/workqueue.c
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@ -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;