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


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