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Commit f303fccb authored by Tejun Heo's avatar Tejun Heo
Browse files

workqueue: implement "workqueue.debug_force_rr_cpu" debug feature 



Workqueue used to guarantee local execution for work items queued
without explicit target CPU.  The guarantee is gone now which can
break some usages in subtle ways.  To flush out those cases, this
patch implements a debug feature which forces round-robin CPU
selection for all such work items.

The debug feature defaults to off and can be enabled with a kernel
parameter.  The default can be flipped with a debug config option.

If you hit this commit during bisection, please refer to 041bd12e
("Revert "workqueue: make sure delayed work run in local cpu"") for
more information and ping me.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent ef557180

Documentation/kernel-parameters.txt

+11 −0
Original line number Diff line number Diff line
@@ -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.

kernel/workqueue.c

+21 −2
Original line number Diff line number Diff line
@@ -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 (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;

lib/Kconfig.debug

+15 −0
Original line number Diff line number Diff line
@@ -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