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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 <tj@kernel.org> Signed-off-by: Breno Leitao <leitao@debian.org> Signed-off-by: Tejun Heo <tj@kernel.org> |
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