3 files changed, 47 insertions, 2 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 87d40a72f6a1..cda2ead39093 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -4230,6 +4230,17 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         The default value of this parameter is determined by
                         the config option CONFIG_WQ_POWER_EFFICIENT_DEFAULT.
 
+        workqueue.debug_force_rr_cpu
+                        Workqueue used to implicitly guarantee that work
+                        items queued without explicit CPU specified are put
+                        on the local CPU.  This guarantee is no longer true
+                        and while local CPU is still preferred work items
+                        may be put on foreign CPUs.  This debug option
+                        forces round-robin CPU selection to flush out
+                        usages which depend on the now broken guarantee.
+                        When enabled, memory and cache locality will be
+                        impacted.
+
         x2apic_phys     [X86-64,APIC] Use x2apic physical mode instead of
                         default x2apic cluster mode on platforms
                         supporting x2apic.
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 054774605d2f..51d77e7c0989 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -307,6 +307,18 @@ static cpumask_var_t wq_unbound_cpumask;
 /* CPU where unbound work was last round robin scheduled from this CPU */
 static DEFINE_PER_CPU(int, wq_rr_cpu_last);
 
+/*
+ * Local execution of unbound work items is no longer guaranteed.  The
+ * following always forces round-robin CPU selection on unbound work items
+ * to uncover usages which depend on it.
+ */
+#ifdef CONFIG_DEBUG_WQ_FORCE_RR_CPU
+static bool wq_debug_force_rr_cpu = true;
+#else
+static bool wq_debug_force_rr_cpu = false;
+#endif
+module_param_named(debug_force_rr_cpu, wq_debug_force_rr_cpu, bool, 0644);
+
 /* the per-cpu worker pools */
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct worker_pool [NR_STD_WORKER_POOLS],
                                      cpu_worker_pools);
@@ -1309,10 +1321,17 @@ static bool is_chained_work(struct workqueue_struct *wq)
  */
 static int wq_select_unbound_cpu(int cpu)
 {
+        static bool printed_dbg_warning;
         int new_cpu;
 
-        if (cpumask_test_cpu(cpu, wq_unbound_cpumask))
-                return cpu;
+        if (likely(!wq_debug_force_rr_cpu)) {
+                if (cpumask_test_cpu(cpu, wq_unbound_cpumask))
+                        return cpu;
+        } else if (!printed_dbg_warning) {
+                pr_warn("workqueue: round-robin CPU selection forced, expect performance impact\n");
+                printed_dbg_warning = true;
+        }
+
         if (cpumask_empty(wq_unbound_cpumask))
                 return cpu;
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ecb9e75614bf..8bfd1aca7a3d 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1400,6 +1400,21 @@ config RCU_EQS_DEBUG
 
 endmenu # "RCU Debugging"
 
+config DEBUG_WQ_FORCE_RR_CPU
+        bool "Force round-robin CPU selection for unbound work items"
+        depends on DEBUG_KERNEL
+        default n
+        help
+          Workqueue used to implicitly guarantee that work items queued
+          without explicit CPU specified are put on the local CPU.  This
+          guarantee is no longer true and while local CPU is still
+          preferred work items may be put on foreign CPUs.  Kernel
+          parameter "workqueue.debug_force_rr_cpu" is added to force
+          round-robin CPU selection to flush out usages which depend on the
+          now broken guarantee.  This config option enables the debug
+          feature by default.  When enabled, memory and cache locality will
+          be impacted.
+
 config DEBUG_BLOCK_EXT_DEVT
         bool "Force extended block device numbers and spread them"
         depends on DEBUG_KERNEL