Re: [PATCH V5 1/5] workqueues: Introduce new flag WQ_POWER_EFFICIENTfor power oriented workqueues

[Amit Kucheria]

On Wed, Apr 24, 2013 at 5:12 PM, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> Workqueues can be performance or power oriented. For performance we may want to
> keep them running on a single cpu, so that it remains cache hot. For power we
> can give scheduler the liberty to choose target cpu for running work handler.

Consider the following rewording:

Workqueues can be performance or power-oriented. Currently, most
workqueues are bound to the CPU they were created on. This gives good
performance (due to cache effects) at the cost of potentially waking
up otherwise idle cores just to process some work. To save power, we
can allow the work to be rescheduled on a core that is already awake.

> Later one (Power oriented WQ) can be achieved if the workqueue is allocated with
> WQ_UNBOUND flag. Enabling CONFIG_WQ_POWER_EFFICIENT will set
> 'wq_power_efficient' to 'true'. Setting 'power_efficient' boot param will
> override value of 'wq_power_efficient' variable. When 'wq_power_efficient' is
> set to 'true', we will convert WQ_POWER_EFFICIENT flag to WQ_UNBOUND on wq
> allocation. And so scheduler will have the liberty to choose where to run this
> work.

Consider the following rewording which is easier to understand IMO:

Workqueues created with the WQ_UNBOUND flag will allow some power
savings. However, we don't change the default behaviour of the system.
To enable power-saving behaviour, a new config option
CONFIG_WQ_POWER_EFFICIENT needs to be turned on. This option can also
be overridden by the workqueue.power_efficient boot parameter.



> Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> ---
>  Documentation/kernel-parameters.txt | 17 +++++++++++++++++
>  include/linux/workqueue.h           |  3 +++
>  kernel/power/Kconfig                | 19 +++++++++++++++++++
>  kernel/workqueue.c                  | 11 +++++++++++
>  4 files changed, 50 insertions(+)
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index c4fa000..a9040fa 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -3312,6 +3312,23 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>                         that this also can be controlled per-workqueue for
>                         workqueues visible under /sys/bus/workqueue/.
>
> +       workqueue.power_efficient
> +                       Workqueues can be performance or power oriented. For
> +                       performance we may want to keep them running on a single
> +                       cpu, so that it remains cache hot. For power we can give
> +                       scheduler the liberty to choose target cpu for running
> +                       work handler.
> +
> +                       Later one (Power oriented WQ) can be achieved if the
> +                       workqueue is allocated with WQ_UNBOUND flag. Enabling
> +                       CONFIG_WQ_POWER_EFFICIENT will set 'wq_power_efficient'
> +                       to 'true'. Setting 'power_efficient' boot param will
> +                       override value of 'wq_power_efficient' variable. When
> +                       'wq_power_efficient' is set to 'true', we will convert
> +                       WQ_POWER_EFFICIENT flag to WQ_UNBOUND on wq allocation.
> +                       And so scheduler will have the liberty to choose where
> +                       to run this work.
> +

Rewrite as above.

>         x2apic_phys     [X86-64,APIC] Use x2apic physical mode instead of
>                         default x2apic cluster mode on platforms
>                         supporting x2apic.
> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> index 623488f..83fa570 100644
> --- a/include/linux/workqueue.h
> +++ b/include/linux/workqueue.h
> @@ -302,6 +302,9 @@ enum {
>         WQ_HIGHPRI              = 1 << 4, /* high priority */
>         WQ_CPU_INTENSIVE        = 1 << 5, /* cpu instensive workqueue */
>         WQ_SYSFS                = 1 << 6, /* visible in sysfs, see wq_sysfs_register() */
> +       WQ_POWER_EFFICIENT      = 1 << 7, /* WQ_UNBOUND, for power
> +                                          * saving, if wq_power_efficient is
> +                                          * enabled. Unused otherwise. */
>
>         __WQ_DRAINING           = 1 << 16, /* internal: workqueue is draining */
>         __WQ_ORDERED            = 1 << 17, /* internal: workqueue is ordered */
> diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
> index 5dfdc9e..e1e9c8b 100644
> --- a/kernel/power/Kconfig
> +++ b/kernel/power/Kconfig
> @@ -263,6 +263,25 @@ config PM_GENERIC_DOMAINS
>         bool
>         depends on PM
>
> +config WQ_POWER_EFFICIENT
> +       bool "Workqueue allocated as UNBOUND (by default) for power efficiency"
> +       depends on PM

default n

> +       help
> +         Workqueues can be performance or power oriented. For performance we
> +         may want to keep them running on a single cpu, so that it remains
> +         cache hot. For power we can give scheduler the liberty to choose
> +         target cpu for running work handler.
> +
> +         Later one (Power oriented WQ) can be achieved if the workqueue is
> +         allocated with WQ_UNBOUND flag. Enabling CONFIG_WQ_POWER_EFFICIENT
> +         will set 'wq_power_efficient' to 'true'. Setting 'power_efficient'
> +         boot param will override value of 'wq_power_efficient' variable. When
> +         'wq_power_efficient' is set to 'true', we will convert
> +         WQ_POWER_EFFICIENT flag to WQ_UNBOUND on wq allocation. And so
> +         scheduler will have the liberty to choose where to run this work.
> +
> +         If in doubt, say N.
> +

Rewrite as above

>  config PM_GENERIC_DOMAINS_SLEEP
>         def_bool y
>         depends on PM_SLEEP && PM_GENERIC_DOMAINS
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 4aa9f5b..a327027 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -272,6 +272,14 @@ static cpumask_var_t *wq_numa_possible_cpumask;
>  static bool wq_disable_numa;
>  module_param_named(disable_numa, wq_disable_numa, bool, 0444);
>
> +#ifdef CONFIG_WQ_POWER_EFFICIENT
> +static bool wq_power_efficient = true;
> +#else
> +static bool wq_power_efficient;
> +#endif
> +
> +module_param_named(power_efficient, wq_power_efficient, bool, 0444);
> +
>  static bool wq_numa_enabled;           /* unbound NUMA affinity enabled */
>
>  /* buf for wq_update_unbound_numa_attrs(), protected by CPU hotplug exclusion */
> @@ -4085,6 +4093,9 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
>         struct workqueue_struct *wq;
>         struct pool_workqueue *pwq;
>
> +       if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient)
> +               flags |= WQ_UNBOUND;
> +
>         /* allocate wq and format name */
>         if (flags & WQ_UNBOUND)
>                 tbl_size = wq_numa_tbl_len * sizeof(wq->numa_pwq_tbl[0]);
> --
> 1.7.12.rc2.18.g61b472e
>
>
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