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workqueue: implement "workqueue.debug_force_rr_cpu" debug feature

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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 041bd12e27
("Revert "workqueue: make sure delayed work run in local cpu"") for
more information and ping me.

Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tejun Heo 2016-02-09 17:59:38 -05:00
parent ef55718044
commit f303fccb82
3 changed files with 47 additions and 2 deletions
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11
Documentation/kernel-parameters.txt
View file

@ -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.

23
kernel/workqueue.c
View file

@ -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;

15
lib/Kconfig.debug
View file

@ -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