From: Breno Leitao Date: Wed, 1 Apr 2026 13:03:53 +0000 (-0700) Subject: workqueue: add WQ_AFFN_CACHE_SHARD affinity scope X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5920d046f7ae3bf9cf51b9d915c1fff13d299d84;p=thirdparty%2Fkernel%2Flinux.git 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 --- diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h index 75634a09576a8..ab6cb70ca1a52 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 */ diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 18c3fe90daca6..f4a23e1418a77 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -131,6 +131,14 @@ enum wq_internal_consts { WORKER_ID_LEN = 10 + WQ_NAME_LEN, /* "kworker/R-" + WQ_NAME_LEN */ }; +/* Layout of shards within one LLC pod */ +struct llc_shard_layout { + int nr_large_shards; /* number of large shards (cores_per_shard + 1) */ + int cores_per_shard; /* base number of cores per default shard */ + int nr_shards; /* total number of shards */ + /* nr_default shards = (nr_shards - nr_large_shards) */ +}; + /* * We don't want to trap softirq for too long. See MAX_SOFTIRQ_TIME and * MAX_SOFTIRQ_RESTART in kernel/softirq.c. These are macros because @@ -410,6 +418,7 @@ static const char * const wq_affn_names[WQ_AFFN_NR_TYPES] = { [WQ_AFFN_CPU] = "cpu", [WQ_AFFN_SMT] = "smt", [WQ_AFFN_CACHE] = "cache", + [WQ_AFFN_CACHE_SHARD] = "cache_shard", [WQ_AFFN_NUMA] = "numa", [WQ_AFFN_SYSTEM] = "system", }; @@ -432,6 +441,9 @@ module_param_named(cpu_intensive_warning_thresh, wq_cpu_intensive_warning_thresh static bool wq_power_efficient = IS_ENABLED(CONFIG_WQ_POWER_EFFICIENT_DEFAULT); module_param_named(power_efficient, wq_power_efficient, bool, 0444); +static unsigned int wq_cache_shard_size = 8; +module_param_named(cache_shard_size, wq_cache_shard_size, uint, 0444); + static bool wq_online; /* can kworkers be created yet? */ static bool wq_topo_initialized __read_mostly = false; @@ -8155,6 +8167,175 @@ static bool __init cpus_share_numa(int cpu0, int cpu1) return cpu_to_node(cpu0) == cpu_to_node(cpu1); } +/* Maps each CPU to its shard index within the LLC pod it belongs to */ +static int cpu_shard_id[NR_CPUS] __initdata; + +/** + * llc_count_cores - count distinct cores (SMT groups) within an LLC pod + * @pod_cpus: the cpumask of CPUs in the LLC pod + * @smt_pods: the SMT pod type, used to identify sibling groups + * + * A core is represented by the lowest-numbered CPU in its SMT group. Returns + * the number of distinct cores found in @pod_cpus. + */ +static int __init llc_count_cores(const struct cpumask *pod_cpus, + struct wq_pod_type *smt_pods) +{ + const struct cpumask *sibling_cpus; + int nr_cores = 0, c; + + /* + * Count distinct cores by only counting the first CPU in each + * SMT sibling group. + */ + for_each_cpu(c, pod_cpus) { + sibling_cpus = smt_pods->pod_cpus[smt_pods->cpu_pod[c]]; + if (cpumask_first(sibling_cpus) == c) + nr_cores++; + } + + return nr_cores; +} + +/* + * llc_shard_size - number of cores in a given shard + * + * Cores are spread as evenly as possible. The first @nr_large_shards shards are + * "large shards" with (cores_per_shard + 1) cores; the rest are "default + * shards" with cores_per_shard cores. + */ +static int __init llc_shard_size(int shard_id, int cores_per_shard, int nr_large_shards) +{ + /* The first @nr_large_shards shards are large shards */ + if (shard_id < nr_large_shards) + return cores_per_shard + 1; + + /* The remaining shards are default shards */ + return cores_per_shard; +} + +/* + * llc_calc_shard_layout - compute the shard layout for an LLC pod + * @nr_cores: number of distinct cores in the LLC pod + * + * Chooses the number of shards that keeps average shard size closest to + * wq_cache_shard_size. Returns a struct describing the total number of shards, + * the base size of each, and how many are large shards. + */ +static struct llc_shard_layout __init llc_calc_shard_layout(int nr_cores) +{ + struct llc_shard_layout layout; + + /* Ensure at least one shard; pick the count closest to the target size */ + layout.nr_shards = max(1, DIV_ROUND_CLOSEST(nr_cores, wq_cache_shard_size)); + layout.cores_per_shard = nr_cores / layout.nr_shards; + layout.nr_large_shards = nr_cores % layout.nr_shards; + + return layout; +} + +/* + * llc_shard_is_full - check whether a shard has reached its core capacity + * @cores_in_shard: number of cores already assigned to this shard + * @shard_id: index of the shard being checked + * @layout: the shard layout computed by llc_calc_shard_layout() + * + * Returns true if @cores_in_shard equals the expected size for @shard_id. + */ +static bool __init llc_shard_is_full(int cores_in_shard, int shard_id, + const struct llc_shard_layout *layout) +{ + return cores_in_shard == llc_shard_size(shard_id, layout->cores_per_shard, + layout->nr_large_shards); +} + +/** + * llc_populate_cpu_shard_id - populate cpu_shard_id[] for each CPU in an LLC pod + * @pod_cpus: the cpumask of CPUs in the LLC pod + * @smt_pods: the SMT pod type, used to identify sibling groups + * @nr_cores: number of distinct cores in @pod_cpus (from llc_count_cores()) + * + * Walks @pod_cpus in order. At each SMT group leader, advances to the next + * shard once the current shard is full. Results are written to cpu_shard_id[]. + */ +static void __init llc_populate_cpu_shard_id(const struct cpumask *pod_cpus, + struct wq_pod_type *smt_pods, + int nr_cores) +{ + struct llc_shard_layout layout = llc_calc_shard_layout(nr_cores); + const struct cpumask *sibling_cpus; + /* Count the number of cores in the current shard_id */ + int cores_in_shard = 0; + /* This is a cursor for the shards. Go from zero to nr_shards - 1*/ + int shard_id = 0; + int c; + + /* Iterate at every CPU for a given LLC pod, and assign it a shard */ + for_each_cpu(c, pod_cpus) { + sibling_cpus = smt_pods->pod_cpus[smt_pods->cpu_pod[c]]; + if (cpumask_first(sibling_cpus) == c) { + /* This is the CPU leader for the siblings */ + if (llc_shard_is_full(cores_in_shard, shard_id, &layout)) { + shard_id++; + cores_in_shard = 0; + } + cores_in_shard++; + cpu_shard_id[c] = shard_id; + } else { + /* + * The siblings' shard MUST be the same as the leader. + * never split threads in the same core. + */ + cpu_shard_id[c] = cpu_shard_id[cpumask_first(sibling_cpus)]; + } + } + + WARN_ON_ONCE(shard_id != (layout.nr_shards - 1)); +} + +/** + * precompute_cache_shard_ids - assign each CPU its shard index within its LLC + * + * Iterates over all LLC pods. For each pod, counts distinct cores then assigns + * shard indices to all CPUs in the pod. Must be called after WQ_AFFN_CACHE and + * WQ_AFFN_SMT have been initialized. + */ +static void __init precompute_cache_shard_ids(void) +{ + struct wq_pod_type *llc_pods = &wq_pod_types[WQ_AFFN_CACHE]; + struct wq_pod_type *smt_pods = &wq_pod_types[WQ_AFFN_SMT]; + const struct cpumask *cpus_sharing_llc; + int nr_cores; + int pod; + + if (!wq_cache_shard_size) { + pr_warn("workqueue: cache_shard_size must be > 0, setting to 1\n"); + wq_cache_shard_size = 1; + } + + for (pod = 0; pod < llc_pods->nr_pods; pod++) { + cpus_sharing_llc = llc_pods->pod_cpus[pod]; + + /* Number of cores in this given LLC */ + nr_cores = llc_count_cores(cpus_sharing_llc, smt_pods); + llc_populate_cpu_shard_id(cpus_sharing_llc, smt_pods, nr_cores); + } +} + +/* + * cpus_share_cache_shard - test whether two CPUs belong to the same cache shard + * + * Two CPUs share a cache shard if they are in the same LLC and have the same + * shard index. Used as the pod affinity callback for WQ_AFFN_CACHE_SHARD. + */ +static bool __init cpus_share_cache_shard(int cpu0, int cpu1) +{ + if (!cpus_share_cache(cpu0, cpu1)) + return false; + + return cpu_shard_id[cpu0] == cpu_shard_id[cpu1]; +} + /** * workqueue_init_topology - initialize CPU pods for unbound workqueues * @@ -8170,6 +8351,8 @@ void __init workqueue_init_topology(void) init_pod_type(&wq_pod_types[WQ_AFFN_CPU], cpus_dont_share); init_pod_type(&wq_pod_types[WQ_AFFN_SMT], cpus_share_smt); init_pod_type(&wq_pod_types[WQ_AFFN_CACHE], cpus_share_cache); + precompute_cache_shard_ids(); + init_pod_type(&wq_pod_types[WQ_AFFN_CACHE_SHARD], cpus_share_cache_shard); init_pod_type(&wq_pod_types[WQ_AFFN_NUMA], cpus_share_numa); wq_topo_initialized = true;