Merge tag 'wq-for-7.1' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq

Pull workqueue updates from Tejun Heo:

 - New default WQ_AFFN_CACHE_SHARD affinity scope subdivides LLCs into
   smaller shards to improve scalability on machines with many CPUs per
   LLC

 - Misc:
    - system_dfl_long_wq for long unbound works
    - devm_alloc_workqueue() for device-managed allocation
    - sysfs exposure for ordered workqueues and the EFI workqueue
    - removal of HK_TYPE_WQ from wq_unbound_cpumask
    - various small fixes

* tag 'wq-for-7.1' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq: (21 commits)
  workqueue: validate cpumask_first() result in llc_populate_cpu_shard_id()
  workqueue: use NR_STD_WORKER_POOLS instead of hardcoded value
  workqueue: avoid unguarded 64-bit division
  docs: workqueue: document WQ_AFFN_CACHE_SHARD affinity scope
  workqueue: add test_workqueue benchmark module
  tools/workqueue: add CACHE_SHARD support to wq_dump.py
  workqueue: set WQ_AFFN_CACHE_SHARD as the default affinity scope
  workqueue: add WQ_AFFN_CACHE_SHARD affinity scope
  workqueue: fix typo in WQ_AFFN_SMT comment
  workqueue: Remove HK_TYPE_WQ from affecting wq_unbound_cpumask
  workqueue: unlink pwqs from wq->pwqs list in alloc_and_link_pwqs() error path
  workqueue: Remove NULL wq WARN in __queue_delayed_work()
  workqueue: fix parse_affn_scope() prefix matching bug
  workqueue: devres: Add device-managed allocate workqueue
  workqueue: Add system_dfl_long_wq for long unbound works
  tools/workqueue/wq_dump.py: add NODE prefix to all node columns
  tools/workqueue/wq_dump.py: fix column alignment in node_nr/max_active section
  tools/workqueue/wq_dump.py: remove backslash separator from node_nr/max_active header
  efi: Allow to expose the workqueue via sysfs
  workqueue: Allow to expose ordered workqueues via sysfs
  ...

10 files changed, 629 insertions, 51 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f2ce1f4975c1..b9a2c649e411 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -8543,7 +8543,8 @@ Kernel parameters
         workqueue.default_affinity_scope=
 			Select the default affinity scope to use for unbound
 			workqueues. Can be one of "cpu", "smt", "cache",
-			"numa" and "system". Default is "cache". For more
+			"cache_shard", "numa" and "system". Default is
+			"cache_shard". For more
 			information, see the Affinity Scopes section in
 			Documentation/core-api/workqueue.rst.
 
diff --git a/Documentation/core-api/workqueue.rst b/Documentation/core-api/workqueue.rst
index 165ca73e8351..411e1b28b8de 100644
--- a/Documentation/core-api/workqueue.rst
+++ b/Documentation/core-api/workqueue.rst
@@ -378,9 +378,9 @@ Affinity Scopes
 
 An unbound workqueue groups CPUs according to its affinity scope to improve
 cache locality. For example, if a workqueue is using the default affinity
-scope of "cache", it will group CPUs according to last level cache
-boundaries. A work item queued on the workqueue will be assigned to a worker
-on one of the CPUs which share the last level cache with the issuing CPU.
+scope of "cache_shard", it will group CPUs into sub-LLC shards. A work item
+queued on the workqueue will be assigned to a worker on one of the CPUs
+within the same shard as the issuing CPU.
 Once started, the worker may or may not be allowed to move outside the scope
 depending on the ``affinity_strict`` setting of the scope.
 
@@ -402,7 +402,13 @@ Workqueue currently supports the following affinity scopes.
 ``cache``
   CPUs are grouped according to cache boundaries. Which specific cache
   boundary is used is determined by the arch code. L3 is used in a lot of
-  cases. This is the default affinity scope.
+  cases.
+
+``cache_shard``
+  CPUs are grouped into sub-LLC shards of at most ``wq_cache_shard_size``
+  cores (default 8, tunable via the ``workqueue.cache_shard_size`` boot
+  parameter). Shards are always split on core (SMT group) boundaries.
+  This is the default affinity scope.
 
 ``numa``
   CPUs are grouped according to NUMA boundaries.
diff --git a/Documentation/driver-api/driver-model/devres.rst b/Documentation/driver-api/driver-model/devres.rst
index 7d2b897d66fa..017fb155a5bc 100644
--- a/Documentation/driver-api/driver-model/devres.rst
+++ b/Documentation/driver-api/driver-model/devres.rst
@@ -464,3 +464,7 @@ SPI
 
 WATCHDOG
   devm_watchdog_register_device()
+
+WORKQUEUE
+  devm_alloc_workqueue()
+  devm_alloc_ordered_workqueue()
diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index b2fb92a4bbd1..3dab284a7754 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -423,7 +423,7 @@ static int __init efisubsys_init(void)
 		 * ordered workqueue (which creates only one execution context)
 		 * should suffice for all our needs.
 		 */
-		efi_rts_wq = alloc_ordered_workqueue("efi_rts_wq", 0);
+		efi_rts_wq = alloc_ordered_workqueue("efi_runtime", WQ_SYSFS);
 		if (!efi_rts_wq) {
 			pr_err("Creating efi_rts_wq failed, EFI runtime services disabled.\n");
 			clear_bit(EFI_RUNTIME_SERVICES, &efi.flags);
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index a4749f56398f..ab6cb70ca1a5 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -131,8 +131,9 @@ struct rcu_work {
 enum wq_affn_scope {
 	WQ_AFFN_DFL,			/* use system default */
 	WQ_AFFN_CPU,			/* one pod per CPU */
-	WQ_AFFN_SMT,			/* one pod poer SMT */
+	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 */
 
@@ -440,6 +441,9 @@ enum wq_consts {
  * system_long_wq is similar to system_percpu_wq but may host long running
  * works.  Queue flushing might take relatively long.
  *
+ * system_dfl_long_wq is similar to system_dfl_wq but it may host long running
+ * works.
+ *
  * system_dfl_wq is unbound workqueue.  Workers are not bound to
  * any specific CPU, not concurrency managed, and all queued works are
  * executed immediately as long as max_active limit is not reached and
@@ -468,6 +472,7 @@ extern struct workqueue_struct *system_power_efficient_wq;
 extern struct workqueue_struct *system_freezable_power_efficient_wq;
 extern struct workqueue_struct *system_bh_wq;
 extern struct workqueue_struct *system_bh_highpri_wq;
+extern struct workqueue_struct *system_dfl_long_wq;
 
 void workqueue_softirq_action(bool highpri);
 void workqueue_softirq_dead(unsigned int cpu);
@@ -512,6 +517,26 @@ __printf(1, 4) struct workqueue_struct *
 alloc_workqueue_noprof(const char *fmt, unsigned int flags, int max_active, ...);
 #define alloc_workqueue(...)	alloc_hooks(alloc_workqueue_noprof(__VA_ARGS__))
 
+/**
+ * devm_alloc_workqueue - Resource-managed allocate a workqueue
+ * @dev: Device to allocate workqueue for
+ * @fmt: printf format for the name of the workqueue
+ * @flags: WQ_* flags
+ * @max_active: max in-flight work items, 0 for default
+ * @...: args for @fmt
+ *
+ * Resource managed workqueue, see alloc_workqueue() for details.
+ *
+ * The workqueue will be automatically destroyed on driver detach.  Typically
+ * this should be used in drivers already relying on devm interafaces.
+ *
+ * RETURNS:
+ * Pointer to the allocated workqueue on success, %NULL on failure.
+ */
+__printf(2, 5) struct workqueue_struct *
+devm_alloc_workqueue(struct device *dev, const char *fmt, unsigned int flags,
+		     int max_active, ...);
+
 #ifdef CONFIG_LOCKDEP
 /**
  * alloc_workqueue_lockdep_map - allocate a workqueue with user-defined lockdep_map
@@ -568,6 +593,8 @@ alloc_workqueue_lockdep_map(const char *fmt, unsigned int flags, int max_active,
  */
 #define alloc_ordered_workqueue(fmt, flags, args...)			\
 	alloc_workqueue(fmt, WQ_UNBOUND | __WQ_ORDERED | (flags), 1, ##args)
+#define devm_alloc_ordered_workqueue(dev, fmt, flags, args...)		\
+	devm_alloc_workqueue(dev, fmt, WQ_UNBOUND | __WQ_ORDERED | (flags), 1, ##args)
 
 #define create_workqueue(name)						\
 	alloc_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM | WQ_PERCPU, 1, (name))
@@ -712,14 +739,14 @@ static inline bool schedule_work_on(int cpu, struct work_struct *work)
 }
 
 /**
- * schedule_work - put work task in global workqueue
+ * schedule_work - put work task in per-CPU workqueue
  * @work: job to be done
  *
- * Returns %false if @work was already on the kernel-global workqueue and
+ * Returns %false if @work was already on the system per-CPU workqueue and
  * %true otherwise.
  *
- * This puts a job in the kernel-global workqueue if it was not already
- * queued and leaves it in the same position on the kernel-global
+ * This puts a job in the system per-CPU workqueue if it was not already
+ * queued and leaves it in the same position on the system per-CPU
  * workqueue otherwise.
  *
  * Shares the same memory-ordering properties of queue_work(), cf. the
@@ -783,6 +810,8 @@ extern void __warn_flushing_systemwide_wq(void)
 	     _wq == system_highpri_wq) ||				\
 	    (__builtin_constant_p(_wq == system_long_wq) &&		\
 	     _wq == system_long_wq) ||					\
+	    (__builtin_constant_p(_wq == system_dfl_long_wq) &&		\
+	     _wq == system_dfl_long_wq) ||					\
 	    (__builtin_constant_p(_wq == system_dfl_wq) &&		\
 	     _wq == system_dfl_wq) ||				\
 	    (__builtin_constant_p(_wq == system_freezable_wq) &&	\
@@ -796,12 +825,12 @@ extern void __warn_flushing_systemwide_wq(void)
 })
 
 /**
- * schedule_delayed_work_on - queue work in global workqueue on CPU after delay
+ * schedule_delayed_work_on - queue work in per-CPU workqueue on CPU after delay
  * @cpu: cpu to use
  * @dwork: job to be done
  * @delay: number of jiffies to wait
  *
- * After waiting for a given time this puts a job in the kernel-global
+ * After waiting for a given time this puts a job in the system per-CPU
  * workqueue on the specified CPU.
  */
 static inline bool schedule_delayed_work_on(int cpu, struct delayed_work *dwork,
@@ -811,11 +840,11 @@ static inline bool schedule_delayed_work_on(int cpu, struct delayed_work *dwork,
 }
 
 /**
- * schedule_delayed_work - put work task in global workqueue after delay
+ * schedule_delayed_work - put work task in per-CPU workqueue after delay
  * @dwork: job to be done
  * @delay: number of jiffies to wait or 0 for immediate execution
  *
- * After waiting for a given time this puts a job in the kernel-global
+ * After waiting for a given time this puts a job in the system per-CPU
  * workqueue.
  */
 static inline bool schedule_delayed_work(struct delayed_work *dwork,
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c6ea96d5b716..5f747f241a5f 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -41,6 +41,7 @@
 #include <linux/mempolicy.h>
 #include <linux/freezer.h>
 #include <linux/debug_locks.h>
+#include <linux/device/devres.h>
 #include <linux/lockdep.h>
 #include <linux/idr.h>
 #include <linux/jhash.h>
@@ -130,6 +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
@@ -404,11 +413,12 @@ struct work_offq_data {
 	u32			flags;
 };
 
-static const char *wq_affn_names[WQ_AFFN_NR_TYPES] = {
+static const char * const wq_affn_names[WQ_AFFN_NR_TYPES] = {
 	[WQ_AFFN_DFL]		= "default",
 	[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",
 };
@@ -431,13 +441,16 @@ 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;
 
 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;
@@ -530,6 +543,8 @@ struct workqueue_struct *system_bh_wq;
 EXPORT_SYMBOL_GPL(system_bh_wq);
 struct workqueue_struct *system_bh_highpri_wq;
 EXPORT_SYMBOL_GPL(system_bh_highpri_wq);
+struct workqueue_struct *system_dfl_long_wq __ro_after_init;
+EXPORT_SYMBOL_GPL(system_dfl_long_wq);
 
 static int worker_thread(void *__worker);
 static void workqueue_sysfs_unregister(struct workqueue_struct *wq);
@@ -2519,7 +2534,6 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 	struct timer_list *timer = &dwork->timer;
 	struct work_struct *work = &dwork->work;
 
-	WARN_ON_ONCE(!wq);
 	WARN_ON_ONCE(timer->function != delayed_work_timer_fn);
 	WARN_ON_ONCE(timer_pending(timer));
 	WARN_ON_ONCE(!list_empty(&work->entry));
@@ -5635,8 +5649,16 @@ enomem:
 		for_each_possible_cpu(cpu) {
 			struct pool_workqueue *pwq = *per_cpu_ptr(wq->cpu_pwq, cpu);
 
-			if (pwq)
+			if (pwq) {
+				/*
+				 * Unlink pwq from wq->pwqs since link_pwq()
+				 * may have already added it. wq->mutex is not
+				 * needed as the wq has not been published yet.
+				 */
+				if (!list_empty(&pwq->pwqs_node))
+					list_del_rcu(&pwq->pwqs_node);
 				kmem_cache_free(pwq_cache, pwq);
+			}
 		}
 		free_percpu(wq->cpu_pwq);
 		wq->cpu_pwq = NULL;
@@ -5904,6 +5926,33 @@ struct workqueue_struct *alloc_workqueue_noprof(const char *fmt,
 }
 EXPORT_SYMBOL_GPL(alloc_workqueue_noprof);
 
+static void devm_workqueue_release(void *res)
+{
+	destroy_workqueue(res);
+}
+
+__printf(2, 5) struct workqueue_struct *
+devm_alloc_workqueue(struct device *dev, const char *fmt, unsigned int flags,
+		     int max_active, ...)
+{
+	struct workqueue_struct *wq;
+	va_list args;
+	int ret;
+
+	va_start(args, max_active);
+	wq = alloc_workqueue(fmt, flags, max_active, args);
+	va_end(args);
+	if (!wq)
+		return NULL;
+
+	ret = devm_add_action_or_reset(dev, devm_workqueue_release, wq);
+	if (ret)
+		return NULL;
+
+	return wq;
+}
+EXPORT_SYMBOL_GPL(devm_alloc_workqueue);
+
 #ifdef CONFIG_LOCKDEP
 __printf(1, 5)
 struct workqueue_struct *
@@ -7059,7 +7108,7 @@ int workqueue_unbound_housekeeping_update(const struct cpumask *hk)
 	/*
 	 * If the operation fails, it will fall back to
 	 * wq_requested_unbound_cpumask which is initially set to
-	 * (HK_TYPE_WQ ∩ HK_TYPE_DOMAIN) house keeping mask and rewritten
+	 * HK_TYPE_DOMAIN house keeping mask and rewritten
 	 * by any subsequent write to workqueue/cpumask sysfs file.
 	 */
 	if (!cpumask_and(cpumask, wq_requested_unbound_cpumask, hk))
@@ -7078,13 +7127,7 @@ int workqueue_unbound_housekeeping_update(const struct cpumask *hk)
 
 static int parse_affn_scope(const char *val)
 {
-	int i;
-
-	for (i = 0; i < ARRAY_SIZE(wq_affn_names); i++) {
-		if (!strncasecmp(val, wq_affn_names[i], strlen(wq_affn_names[i])))
-			return i;
-	}
-	return -EINVAL;
+	return sysfs_match_string(wq_affn_names, val);
 }
 
 static int wq_affn_dfl_set(const char *val, const struct kernel_param *kp)
@@ -7191,7 +7234,26 @@ static struct attribute *wq_sysfs_attrs[] = {
 	&dev_attr_max_active.attr,
 	NULL,
 };
-ATTRIBUTE_GROUPS(wq_sysfs);
+
+static umode_t wq_sysfs_is_visible(struct kobject *kobj, struct attribute *a, int n)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct workqueue_struct *wq = dev_to_wq(dev);
+
+	/*
+	 * Adjusting max_active breaks ordering guarantee. Changing it has no
+	 * effect on BH worker. Limit max_active to RO in such case.
+	 */
+	if (wq->flags & (WQ_BH | __WQ_ORDERED))
+		return 0444;
+	return a->mode;
+}
+
+static const struct attribute_group wq_sysfs_group = {
+	.is_visible = wq_sysfs_is_visible,
+	.attrs = wq_sysfs_attrs,
+};
+__ATTRIBUTE_GROUPS(wq_sysfs);
 
 static ssize_t wq_nice_show(struct device *dev, struct device_attribute *attr,
 			    char *buf)
@@ -7494,13 +7556,6 @@ int workqueue_sysfs_register(struct workqueue_struct *wq)
 	struct wq_device *wq_dev;
 	int ret;
 
-	/*
-	 * Adjusting max_active breaks ordering guarantee.  Disallow exposing
-	 * ordered workqueues.
-	 */
-	if (WARN_ON(wq->flags & __WQ_ORDERED))
-		return -EINVAL;
-
 	wq->wq_dev = wq_dev = kzalloc_obj(*wq_dev);
 	if (!wq_dev)
 		return -ENOMEM;
@@ -7877,8 +7932,8 @@ void __init workqueue_init_early(void)
 {
 	struct wq_pod_type *pt = &wq_pod_types[WQ_AFFN_SYSTEM];
 	int std_nice[NR_STD_WORKER_POOLS] = { 0, HIGHPRI_NICE_LEVEL };
-	void (*irq_work_fns[2])(struct irq_work *) = { bh_pool_kick_normal,
-						       bh_pool_kick_highpri };
+	void (*irq_work_fns[NR_STD_WORKER_POOLS])(struct irq_work *) =
+		{ bh_pool_kick_normal, bh_pool_kick_highpri };
 	int i, cpu;
 
 	BUILD_BUG_ON(__alignof__(struct pool_workqueue) < __alignof__(long long));
@@ -7890,7 +7945,6 @@ void __init workqueue_init_early(void)
 
 	cpumask_copy(wq_online_cpumask, cpu_online_mask);
 	cpumask_copy(wq_unbound_cpumask, cpu_possible_mask);
-	restrict_unbound_cpumask("HK_TYPE_WQ", housekeeping_cpumask(HK_TYPE_WQ));
 	restrict_unbound_cpumask("HK_TYPE_DOMAIN", housekeeping_cpumask(HK_TYPE_DOMAIN));
 	if (!cpumask_empty(&wq_cmdline_cpumask))
 		restrict_unbound_cpumask("workqueue.unbound_cpus", &wq_cmdline_cpumask);
@@ -7974,11 +8028,12 @@ void __init workqueue_init_early(void)
 	system_bh_wq = alloc_workqueue("events_bh", WQ_BH | WQ_PERCPU, 0);
 	system_bh_highpri_wq = alloc_workqueue("events_bh_highpri",
 					       WQ_BH | WQ_HIGHPRI | WQ_PERCPU, 0);
+	system_dfl_long_wq = alloc_workqueue("events_dfl_long", WQ_UNBOUND, WQ_MAX_ACTIVE);
 	BUG_ON(!system_wq || !system_percpu_wq|| !system_highpri_wq || !system_long_wq ||
 	       !system_unbound_wq || !system_freezable_wq || !system_dfl_wq ||
 	       !system_power_efficient_wq ||
 	       !system_freezable_power_efficient_wq ||
-	       !system_bh_wq || !system_bh_highpri_wq);
+	       !system_bh_wq || !system_bh_highpri_wq || !system_dfl_long_wq);
 }
 
 static void __init wq_cpu_intensive_thresh_init(void)
@@ -8144,6 +8199,186 @@ 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;
+	unsigned int leader;
+	/* 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.
+			 */
+			leader = cpumask_first(sibling_cpus);
+
+			/*
+			 * This check silences a Warray-bounds warning on UP
+			 * configs where NR_CPUS=1 makes cpu_shard_id[]
+			 * a single-element array, and the compiler can't
+			 * prove the index is always 0.
+			 */
+			if (WARN_ON_ONCE(leader >= nr_cpu_ids))
+				continue;
+			cpu_shard_id[c] = cpu_shard_id[leader];
+		}
+	}
+
+	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
  *
@@ -8159,6 +8394,8 @@ 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;
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index aac60b6cfa4b..77c3774c1c49 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2636,6 +2636,16 @@ config TEST_VMALLOC
 
 	  If unsure, say N.
 
+config TEST_WORKQUEUE
+	tristate "Test module for stress/performance analysis of workqueue"
+	default n
+	help
+	  This builds the "test_workqueue" module for benchmarking
+	  workqueue throughput under contention. Useful for evaluating
+	  affinity scope changes (e.g., cache_shard vs cache).
+
+	  If unsure, say N.
+
 config TEST_BPF
 	tristate "Test BPF filter functionality"
 	depends on m && NET
diff --git a/lib/Makefile b/lib/Makefile
index 1b9ee167517f..ea660cca04f4 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -79,6 +79,7 @@ UBSAN_SANITIZE_test_ubsan.o := y
 obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
 obj-$(CONFIG_TEST_LKM) += test_module.o
 obj-$(CONFIG_TEST_VMALLOC) += test_vmalloc.o
+obj-$(CONFIG_TEST_WORKQUEUE) += test_workqueue.o
 obj-$(CONFIG_TEST_RHASHTABLE) += test_rhashtable.o
 obj-$(CONFIG_TEST_STATIC_KEYS) += test_static_keys.o
 obj-$(CONFIG_TEST_STATIC_KEYS) += test_static_key_base.o
diff --git a/lib/test_workqueue.c b/lib/test_workqueue.c
new file mode 100644
index 000000000000..99e160bd5ad1
--- /dev/null
+++ b/lib/test_workqueue.c
@@ -0,0 +1,294 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Test module for stress and performance analysis of workqueue.
+ *
+ * Benchmarks queue_work() throughput on an unbound workqueue to measure
+ * pool->lock contention under different affinity scope configurations
+ * (e.g., cache vs cache_shard).
+ *
+ * The affinity scope is changed between runs via the workqueue's sysfs
+ * affinity_scope attribute (WQ_SYSFS).
+ *
+ * Copyright (c) 2026 Meta Platforms, Inc. and affiliates
+ * Copyright (c) 2026 Breno Leitao <leitao@debian.org>
+ *
+ */
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/workqueue.h>
+#include <linux/kthread.h>
+#include <linux/moduleparam.h>
+#include <linux/completion.h>
+#include <linux/atomic.h>
+#include <linux/slab.h>
+#include <linux/ktime.h>
+#include <linux/cpumask.h>
+#include <linux/sched.h>
+#include <linux/sort.h>
+#include <linux/fs.h>
+
+#define WQ_NAME "bench_wq"
+#define SCOPE_PATH "/sys/bus/workqueue/devices/" WQ_NAME "/affinity_scope"
+
+static int nr_threads;
+module_param(nr_threads, int, 0444);
+MODULE_PARM_DESC(nr_threads,
+		 "Number of threads to spawn (default: 0 = num_online_cpus())");
+
+static int wq_items = 50000;
+module_param(wq_items, int, 0444);
+MODULE_PARM_DESC(wq_items,
+		 "Number of work items each thread queues (default: 50000)");
+
+static struct workqueue_struct *bench_wq;
+static atomic_t threads_done;
+static DECLARE_COMPLETION(start_comp);
+static DECLARE_COMPLETION(all_done_comp);
+
+struct thread_ctx {
+	struct completion work_done;
+	struct work_struct work;
+	u64 *latencies;
+	int cpu;
+	int items;
+};
+
+static void bench_work_fn(struct work_struct *work)
+{
+	struct thread_ctx *ctx = container_of(work, struct thread_ctx, work);
+
+	complete(&ctx->work_done);
+}
+
+static int bench_kthread_fn(void *data)
+{
+	struct thread_ctx *ctx = data;
+	ktime_t t_start, t_end;
+	int i;
+
+	/* Wait for all threads to be ready */
+	wait_for_completion(&start_comp);
+
+	if (kthread_should_stop())
+		return 0;
+
+	for (i = 0; i < ctx->items; i++) {
+		reinit_completion(&ctx->work_done);
+		INIT_WORK(&ctx->work, bench_work_fn);
+
+		t_start = ktime_get();
+		queue_work(bench_wq, &ctx->work);
+		t_end = ktime_get();
+
+		ctx->latencies[i] = ktime_to_ns(ktime_sub(t_end, t_start));
+		wait_for_completion(&ctx->work_done);
+	}
+
+	if (atomic_dec_and_test(&threads_done))
+		complete(&all_done_comp);
+
+	/*
+	 * Wait for kthread_stop() so the module text isn't freed
+	 * while we're still executing.
+	 */
+	while (!kthread_should_stop())
+		schedule();
+
+	return 0;
+}
+
+static int cmp_u64(const void *a, const void *b)
+{
+	u64 va = *(const u64 *)a;
+	u64 vb = *(const u64 *)b;
+
+	if (va < vb)
+		return -1;
+	if (va > vb)
+		return 1;
+	return 0;
+}
+
+static int __init set_affn_scope(const char *scope)
+{
+	struct file *f;
+	loff_t pos = 0;
+	ssize_t ret;
+
+	f = filp_open(SCOPE_PATH, O_WRONLY, 0);
+	if (IS_ERR(f)) {
+		pr_err("test_workqueue: open %s failed: %ld\n",
+		       SCOPE_PATH, PTR_ERR(f));
+		return PTR_ERR(f);
+	}
+
+	ret = kernel_write(f, scope, strlen(scope), &pos);
+	filp_close(f, NULL);
+
+	if (ret < 0) {
+		pr_err("test_workqueue: write '%s' failed: %zd\n", scope, ret);
+		return ret;
+	}
+
+	return 0;
+}
+
+static int __init run_bench(int n_threads, const char *scope, const char *label)
+{
+	struct task_struct **tasks;
+	unsigned long total_items;
+	struct thread_ctx *ctxs;
+	u64 *all_latencies;
+	ktime_t start, end;
+	int cpu, i, j, ret;
+	s64 elapsed_us;
+
+	ret = set_affn_scope(scope);
+	if (ret)
+		return ret;
+
+	ctxs = kcalloc(n_threads, sizeof(*ctxs), GFP_KERNEL);
+	if (!ctxs)
+		return -ENOMEM;
+
+	tasks = kcalloc(n_threads, sizeof(*tasks), GFP_KERNEL);
+	if (!tasks) {
+		kfree(ctxs);
+		return -ENOMEM;
+	}
+
+	total_items = (unsigned long)n_threads * wq_items;
+	all_latencies = kvmalloc_array(total_items, sizeof(u64), GFP_KERNEL);
+	if (!all_latencies) {
+		kfree(tasks);
+		kfree(ctxs);
+		return -ENOMEM;
+	}
+
+	/* Allocate per-thread latency arrays */
+	for (i = 0; i < n_threads; i++) {
+		ctxs[i].latencies = kvmalloc_array(wq_items, sizeof(u64),
+						   GFP_KERNEL);
+		if (!ctxs[i].latencies) {
+			while (--i >= 0)
+				kvfree(ctxs[i].latencies);
+			kvfree(all_latencies);
+			kfree(tasks);
+			kfree(ctxs);
+			return -ENOMEM;
+		}
+	}
+
+	atomic_set(&threads_done, n_threads);
+	reinit_completion(&all_done_comp);
+	reinit_completion(&start_comp);
+
+	/* Create kthreads, each bound to a different online CPU */
+	i = 0;
+	for_each_online_cpu(cpu) {
+		if (i >= n_threads)
+			break;
+
+		ctxs[i].cpu = cpu;
+		ctxs[i].items = wq_items;
+		init_completion(&ctxs[i].work_done);
+
+		tasks[i] = kthread_create(bench_kthread_fn, &ctxs[i],
+					  "wq_bench/%d", cpu);
+		if (IS_ERR(tasks[i])) {
+			ret = PTR_ERR(tasks[i]);
+			pr_err("test_workqueue: failed to create kthread %d: %d\n",
+			       i, ret);
+			/* Unblock threads waiting on start_comp before stopping them */
+			complete_all(&start_comp);
+			while (--i >= 0)
+				kthread_stop(tasks[i]);
+			goto out_free;
+		}
+
+		kthread_bind(tasks[i], cpu);
+		wake_up_process(tasks[i]);
+		i++;
+	}
+
+	/* Start timing and release all threads */
+	start = ktime_get();
+	complete_all(&start_comp);
+
+	/* Wait for all threads to finish the benchmark */
+	wait_for_completion(&all_done_comp);
+
+	/* Drain any remaining work */
+	flush_workqueue(bench_wq);
+
+	/* Ensure all kthreads have fully exited before module memory is freed */
+	for (i = 0; i < n_threads; i++)
+		kthread_stop(tasks[i]);
+
+	end = ktime_get();
+	elapsed_us = ktime_us_delta(end, start);
+
+	/* Merge all per-thread latencies and sort for percentile calculation */
+	j = 0;
+	for (i = 0; i < n_threads; i++) {
+		memcpy(&all_latencies[j], ctxs[i].latencies,
+		       wq_items * sizeof(u64));
+		j += wq_items;
+	}
+
+	sort(all_latencies, total_items, sizeof(u64), cmp_u64, NULL);
+
+	pr_info("test_workqueue:   %-16s %llu items/sec\tp50=%llu\tp90=%llu\tp95=%llu ns\n",
+		label,
+		elapsed_us ? div_u64(total_items * 1000000ULL, elapsed_us) : 0,
+		all_latencies[total_items * 50 / 100],
+		all_latencies[total_items * 90 / 100],
+		all_latencies[total_items * 95 / 100]);
+
+	ret = 0;
+out_free:
+	for (i = 0; i < n_threads; i++)
+		kvfree(ctxs[i].latencies);
+	kvfree(all_latencies);
+	kfree(tasks);
+	kfree(ctxs);
+
+	return ret;
+}
+
+static const char * const bench_scopes[] = {
+	"cpu", "smt", "cache_shard", "cache", "numa", "system",
+};
+
+static int __init test_workqueue_init(void)
+{
+	int n_threads = min(nr_threads ?: num_online_cpus(), num_online_cpus());
+	int i;
+
+	if (wq_items <= 0) {
+		pr_err("test_workqueue: wq_items must be > 0\n");
+		return -EINVAL;
+	}
+
+	bench_wq = alloc_workqueue(WQ_NAME, WQ_UNBOUND | WQ_SYSFS, 0);
+	if (!bench_wq)
+		return -ENOMEM;
+
+	pr_info("test_workqueue: running %d threads, %d items/thread\n",
+		n_threads, wq_items);
+
+	for (i = 0; i < ARRAY_SIZE(bench_scopes); i++)
+		run_bench(n_threads, bench_scopes[i], bench_scopes[i]);
+
+	destroy_workqueue(bench_wq);
+
+	/* Return -EAGAIN so the module doesn't stay loaded after the benchmark */
+	return -EAGAIN;
+}
+
+module_init(test_workqueue_init);
+MODULE_AUTHOR("Breno Leitao <leitao@debian.org>");
+MODULE_DESCRIPTION("Stress/performance benchmark for workqueue subsystem");
+MODULE_LICENSE("GPL");
diff --git a/tools/workqueue/wq_dump.py b/tools/workqueue/wq_dump.py
index d29b918306b4..ce4161f52f2f 100644
--- a/tools/workqueue/wq_dump.py
+++ b/tools/workqueue/wq_dump.py
@@ -107,6 +107,7 @@ WQ_MEM_RECLAIM          = prog['WQ_MEM_RECLAIM']
 WQ_AFFN_CPU             = prog['WQ_AFFN_CPU']
 WQ_AFFN_SMT             = prog['WQ_AFFN_SMT']
 WQ_AFFN_CACHE           = prog['WQ_AFFN_CACHE']
+WQ_AFFN_CACHE_SHARD     = prog['WQ_AFFN_CACHE_SHARD']
 WQ_AFFN_NUMA            = prog['WQ_AFFN_NUMA']
 WQ_AFFN_SYSTEM          = prog['WQ_AFFN_SYSTEM']
 
@@ -138,7 +139,7 @@ def print_pod_type(pt):
         print(f' [{cpu}]={pt.cpu_pod[cpu].value_()}', end='')
     print('')
 
-for affn in [WQ_AFFN_CPU, WQ_AFFN_SMT, WQ_AFFN_CACHE, WQ_AFFN_NUMA, WQ_AFFN_SYSTEM]:
+for affn in [WQ_AFFN_CPU, WQ_AFFN_SMT, WQ_AFFN_CACHE, WQ_AFFN_CACHE_SHARD, WQ_AFFN_NUMA, WQ_AFFN_SYSTEM]:
     print('')
     print(f'{wq_affn_names[affn].string_().decode().upper()}{" (default)" if affn == wq_affn_dfl else ""}')
     print_pod_type(wq_pod_types[affn])
@@ -227,15 +228,10 @@ if 'node_to_cpumask_map' in prog:
         print(f'NODE[{node:02}]={cpumask_str(node_to_cpumask_map[node])}')
     print('')
 
-    print(f'[{"workqueue":^{WQ_NAME_LEN-2}}\\ min max', end='')
-    first = True
+    print(f'[{"workqueue":^{WQ_NAME_LEN-1}} {"min":>4} {"max":>4}', end='')
     for node in for_each_node():
-        if first:
-            print(f'  NODE {node}', end='')
-            first = False
-        else:
-            print(f' {node:7}', end='')
-    print(f' {"dfl":>7} ]')
+        print(f' {"NODE " + str(node):>9}', end='')
+    print(f' {"dfl":>9} ]')
     print('')
 
     for wq in list_for_each_entry('struct workqueue_struct', workqueues.address_of_(), 'list'):
@@ -243,11 +239,11 @@ if 'node_to_cpumask_map' in prog:
             continue
 
         print(f'{wq.name.string_().decode():{WQ_NAME_LEN}} ', end='')
-        print(f'{wq.min_active.value_():3} {wq.max_active.value_():3}', end='')
+        print(f'{wq.min_active.value_():4} {wq.max_active.value_():4}', end='')
         for node in for_each_node():
             nna = wq.node_nr_active[node]
-            print(f' {nna.nr.counter.value_():3}/{nna.max.value_():3}', end='')
+            print(f' {f"{nna.nr.counter.value_()}/{nna.max.value_()}":>9}', end='')
         nna = wq.node_nr_active[nr_node_ids]
-        print(f' {nna.nr.counter.value_():3}/{nna.max.value_():3}')
+        print(f' {f"{nna.nr.counter.value_()}/{nna.max.value_()}":>9}')
 else:
     printf(f'node_to_cpumask_map not present, is NUMA enabled?')