Message-ID: <CAKohpo=5eTz2Kfded_D988wTeiQbKv35C36cPC9N1NXwnW4mzQ@mail.gmail.com>
Date:	Mon, 13 May 2013 13:59:33 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Amit Kucheria <amit.kucheria@...aro.org>, davem@...emloft.net,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Robin Randhawa <robin.randhawa@....com>,
	Charles Garcia-Tobin <charles.garcia-tobin@....com>,
	Steve Bannister <Steve.Bannister@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arvind Chauhan <arvind.chauhan@....com>,
	Patch Tracking <patches@...aro.org>, airlied@...hat.com,
	mingo@...hat.com, Jens Axboe <axboe@...nel.dk>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH V5 1/5] workqueues: Introduce new flag WQ_POWER_EFFICIENT
 for power oriented workqueues

On 27 April 2013 00:41, Tejun Heo <tj@...nel.org> wrote:
>
> It's already too late for the upcoming merge window, but things
> generally look good to me and I'll apply the patchset once wq/for-3.11
> opens.  One nitpick tho.
>
> On Thu, Apr 25, 2013 at 09:13:44AM +0530, Viresh Kumar wrote:
>> +     workqueue.power_efficient
>> +                     Workqueues can be performance or power-oriented.
>> +                     Currently, most workqueues are bound to the CPU they
>                                    ^^^^
>                                    per-cpu would be better
>
>> +                     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.
>
> The above description is confusing to me.  As have been discussed
> multiple times before, per-cpu workqueue in itself doesn't wake up the
> CPU physically.  The timer may but per-cpu workqueue doesn't.  It was
> confusing when this patchset was first posted and the above phrasing
> is still confusing.  What the patchset tries to do is preventing the
> scheduler from perceiving the CPU as active due to the activated
> worker thread pinned to that CPU, right?  The knob doesn't really do
> anything about waking up the processor in itself.  It just avoids
> feeding the scheduler with noisy activation events and allows it to
> allocate work item execution according to the scheduler's view of CPU
> active/idleness.  As the scheduler has longer / larger scope of
> overall CPU activities and means to regulate them, this leads to more
> power-efficient allocation of work item executions, right?  It'd be
> really great if the descriptions and the comment above the flag makes
> this abundantly clear because it's not something too apparent.

Sorry for the long delay for such a small change. I went on long leaves..

I have added following to make things more clear at places:
(Idle from scheduler's perspective. Which may or may not be physically idle)..

Let me know if it is still unclear..

And this is the new patch: (Attached it too for applying cleanly)

---------x---------------x-------------------

From: Viresh Kumar <viresh.kumar@...aro.org>
Date: Mon, 8 Apr 2013 16:45:40 +0530
Subject: [PATCH V5 resent 1/5] workqueues: Introduce new flag
WQ_POWER_EFFICIENT for power oriented workqueues

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 (Idle from
scheduler's perspective. Which may or may not be physically idle) just to
process some work. To save power, we can allow the work to be rescheduled on a
core that is already awake.

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@...aro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@...aro.org>
---
 Documentation/kernel-parameters.txt | 18 ++++++++++++++++++
 include/linux/workqueue.h           |  3 +++
 kernel/power/Kconfig                | 21 +++++++++++++++++++++
 kernel/workqueue.c                  | 11 +++++++++++
 4 files changed, 53 insertions(+)

diff --git a/Documentation/kernel-parameters.txt
b/Documentation/kernel-parameters.txt
index c3bfacb..9a991b6 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3320,6 +3320,24 @@ 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.
+			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 (Idle from scheduler's perspective.
+			Which may or may not be physically idle) just to process
+			some work. To save power, we can allow the work to be
+			rescheduled on a core that is already awake.
+
+			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.
+
 	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..e8c2b77 100644
--- a/kernel/power/Kconfig
+++ b/kernel/power/Kconfig
@@ -263,6 +263,27 @@ 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. 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 (Idle from scheduler's perspective. Which may
+	  or may not be physically idle) just to process some work. To save
+	  power, we can allow the work to be rescheduled on a core that is
+	  already awake.
+
+	  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.
+
+	  If in doubt, say N.
+
 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 1ae6028..9d8753e 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]);

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