From 5920d046f7ae3bf9cf51b9d915c1fff13d299d84 Mon Sep 17 00:00:00 2001 From: Breno Leitao Date: Wed, 1 Apr 2026 06:03:53 -0700 Subject: workqueue: add WQ_AFFN_CACHE_SHARD affinity scope On systems where many CPUs share one LLC, unbound workqueues using WQ_AFFN_CACHE collapse to a single worker pool, causing heavy spinlock contention on pool->lock. For example, Chuck Lever measured 39% of cycles lost to native_queued_spin_lock_slowpath on a 12-core shared-L3 NFS-over-RDMA system. The existing affinity hierarchy (cpu, smt, cache, numa, system) offers no intermediate option between per-LLC and per-SMT-core granularity. Add WQ_AFFN_CACHE_SHARD, which subdivides each LLC into groups of at most wq_cache_shard_size cores (default 8, tunable via boot parameter). Shards are always split on core (SMT group) boundaries so that Hyper-Threading siblings are never placed in different pods. Cores are distributed across shards as evenly as possible -- for example, 36 cores in a single LLC with max shard size 8 produces 5 shards of 8+7+7+7+7 cores. The implementation follows the same comparator pattern as other affinity scopes: precompute_cache_shard_ids() pre-fills the cpu_shard_id[] array from the already-initialized WQ_AFFN_CACHE and WQ_AFFN_SMT topology, and cpus_share_cache_shard() is passed to init_pod_type(). Benchmark on NVIDIA Grace (72 CPUs, single LLC, 50k items/thread), show cache_shard delivers ~5x the throughput and ~6.5x lower p50 latency compared to cache scope on this 72-core single-LLC system. Suggested-by: Tejun Heo Signed-off-by: Breno Leitao Signed-off-by: Tejun Heo --- include/linux/workqueue.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux/workqueue.h') diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h index 75634a09576a..ab6cb70ca1a5 100644 --- a/include/linux/workqueue.h +++ b/include/linux/workqueue.h @@ -133,6 +133,7 @@ enum wq_affn_scope { WQ_AFFN_CPU, /* one pod per CPU */ WQ_AFFN_SMT, /* one pod per SMT */ WQ_AFFN_CACHE, /* one pod per LLC */ + WQ_AFFN_CACHE_SHARD, /* synthetic sub-LLC shards */ WQ_AFFN_NUMA, /* one pod per NUMA node */ WQ_AFFN_SYSTEM, /* one pod across the whole system */ -- cgit v1.2.3