lkml.org 
[lkml]   [2016]   [Feb]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 3/3] workqueue: implement "workqueue.debug_force_rr_cpu" debug feature
On Tue 09-02-16 18:14:50, Tejun Heo wrote:
> 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.

Makes sense to me

>
> If you hit this commit during bisection, please refer to 041bd12e272c
> ("Revert "workqueue: make sure delayed work run in local cpu"") for
> more information and ping me.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
> Documentation/kernel-parameters.txt | 11 +++++++++++
> kernel/workqueue.c | 23 +++++++++++++++++++++--
> lib/Kconfig.debug | 15 +++++++++++++++
> 3 files changed, 47 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 87d40a7..cda2ead 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 0547746..51d77e7 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 ecb9e75..8bfd1ac 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
> --
> 2.5.0

--
Michal Hocko
SUSE Labs

\
 
 \ /
  Last update: 2016-02-15 14:41    [W:0.073 / U:27.108 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site