Re: [RFC PATCH] rcu: move SRCU grace period work to power efficient workqueue

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 02/13/2014 08:33 AM, Paul E. McKenney wrote:
> On Thu, Feb 13, 2014 at 12:04:57AM +0100, Frederic Weisbecker wrote:
>> On Wed, Feb 12, 2014 at 11:59:22AM -0800, Paul E. McKenney wrote:
>>> On Wed, Feb 12, 2014 at 02:23:54PM -0500, Tejun Heo wrote:
>>>> Hello,
>>>>
>>>> On Wed, Feb 12, 2014 at 11:02:41AM -0800, Paul E. McKenney wrote:
>>>>> +2.	Use the /sys/devices/virtual/workqueue/*/cpumask sysfs files
>>>>> +	to force the WQ_SYSFS workqueues to run on the specified set
>>>>> +	of CPUs.  The set of WQ_SYSFS workqueues can be displayed using
>>>>> +	"ls sys/devices/virtual/workqueue".
>>>>
>>>> One thing to be careful about is that once published, it becomes part
>>>> of userland visible interface.  Maybe adding some words warning
>>>> against sprinkling WQ_SYSFS willy-nilly is a good idea?
>>>
>>> Good point!  How about the following?
>>>
>>> 							Thanx, Paul
>>>
>>> ------------------------------------------------------------------------
>>>
>>> Documentation/kernel-per-CPU-kthreads.txt: Workqueue affinity
>>>
>>> This commit documents the ability to apply CPU affinity to WQ_SYSFS
>>> workqueues, thus offloading them from the desired worker CPUs.
>>>
>>> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>>> Cc: Tejun Heo <tj@kernel.org>
>>>
>>> diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
>>> index 827104fb9364..214da3a47a68 100644
>>> --- a/Documentation/kernel-per-CPU-kthreads.txt
>>> +++ b/Documentation/kernel-per-CPU-kthreads.txt
>>> @@ -162,7 +162,16 @@ Purpose: Execute workqueue requests
>>>  To reduce its OS jitter, do any of the following:
>>>  1.	Run your workload at a real-time priority, which will allow
>>>  	preempting the kworker daemons.
>>> -2.	Do any of the following needed to avoid jitter that your
>>> +2.	Use the /sys/devices/virtual/workqueue/*/cpumask sysfs files
>>> +	to force the WQ_SYSFS workqueues to run on the specified set
>>> +	of CPUs.  The set of WQ_SYSFS workqueues can be displayed using
>>> +	"ls sys/devices/virtual/workqueue".  That said, the workqueues
>>> +	maintainer would like to caution people against indiscriminately
>>> +	sprinkling WQ_SYSFS across all the workqueues.  The reason for
>>> +	caution is that it is easy to add WQ_SYSFS, but because sysfs
>>> +	is part of the formal user/kernel API, it can be nearly impossible
>>> +	to remove it, even if its addition was a mistake.
>>> +3.	Do any of the following needed to avoid jitter that your
>>
>> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
>>
>> I just suggest we append a small explanation about what WQ_SYSFS is about.
>> Like:
> 
> Fair point!  I wordsmithed it into the following.  Seem reasonable?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> Documentation/kernel-per-CPU-kthreads.txt: Workqueue affinity
> 
> This commit documents the ability to apply CPU affinity to WQ_SYSFS
> workqueues, thus offloading them from the desired worker CPUs.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Reviewed-by: Tejun Heo <tj@kernel.org>
> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> 
> diff --git a/Documentation/kernel-per-CPU-kthreads.txt b/Documentation/kernel-per-CPU-kthreads.txt
> index 827104fb9364..f3cd299fcc41 100644
> --- a/Documentation/kernel-per-CPU-kthreads.txt
> +++ b/Documentation/kernel-per-CPU-kthreads.txt
> @@ -162,7 +162,18 @@ Purpose: Execute workqueue requests
>  To reduce its OS jitter, do any of the following:
>  1.	Run your workload at a real-time priority, which will allow
>  	preempting the kworker daemons.
> -2.	Do any of the following needed to avoid jitter that your
> +2.	A given workqueue can be made visible in the sysfs filesystem
> +	by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
> +	Such a workqueue can be confined to a given subset of the
> +	CPUs using the /sys/devices/virtual/workqueue/*/cpumask sysfs
> +	files.	The set of WQ_SYSFS workqueues can be displayed using
> +	"ls sys/devices/virtual/workqueue".  That said, the workqueues
> +	maintainer would like to caution people against indiscriminately
> +	sprinkling WQ_SYSFS across all the workqueues.	The reason for
> +	caution is that it is easy to add WQ_SYSFS, but because sysfs is
> +	part of the formal user/kernel API, it can be nearly impossible
> +	to remove it, even if its addition was a mistake.
> +3.	Do any of the following needed to avoid jitter that your
>  	application cannot tolerate:
>  	a.	Build your kernel with CONFIG_SLUB=y rather than
>  		CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
> 
> 


Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>