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workqueue: add WQ_AFFN_CACHE_SHARD affinity scope

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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>
This commit is contained in:
Breno Leitao
2026-04-01 06:03:53 -07:00
committed by Tejun Heo
parent 9dc42c9070
commit 5920d046f7
2 changed files with 184 additions and 0 deletions
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include/linux/workqueue.h
+1
View File
@@ -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 */
kernel/workqueue.c
+183
View File
@@ -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;