[PATCH 1/2] workqueue: don't always set __WQ_ORDERED implicitly

[Bob Liu]

Current code always set 'Unbound && max_active == 1' workqueues to ordered
implicitly, while this may be not an expected behaviour for some use cases.

E.g some scsi and iscsi workqueues(unbound && max_active = 1) want to be bind
to different cpu so as to get better isolation, but their cpumask can't be
changed because WQ_ORDERED is set implicitly.

This patch adds a flag __WQ_ORDERED_DISABLE and also
create_singlethread_workqueue_noorder() to offer an new option.

Signed-off-by: Bob Liu <bob...@oracle.com>
---
include/linux/workqueue.h | 4 ++++
kernel/workqueue.c | 4 +++-
2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index e48554e..4c86913 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -344,6 +344,7 @@ enum {
__WQ_ORDERED = 1 << 17, /* internal: workqueue is ordered */
__WQ_LEGACY = 1 << 18, /* internal: create*_workqueue() */
__WQ_ORDERED_EXPLICIT = 1 << 19, /* internal: alloc_ordered_workqueue() */
+ __WQ_ORDERED_DISABLE = 1 << 20, /* internal: don't set __WQ_ORDERED implicitly */

WQ_MAX_ACTIVE = 512, /* I like 512, better ideas? */
WQ_MAX_UNBOUND_PER_CPU = 4, /* 4 * #cpus for unbound wq */
@@ -433,6 +434,9 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
#define create_singlethread_workqueue(name) \
alloc_ordered_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, name)

+#define create_singlethread_workqueue_noorder(name) \
+ alloc_workqueue("%s", WQ_SYSFS | __WQ_LEGACY | WQ_MEM_RECLAIM | \
+ WQ_UNBOUND | __WQ_ORDERED_DISABLE, 1, (name))
extern void destroy_workqueue(struct workqueue_struct *wq);

struct workqueue_attrs *alloc_workqueue_attrs(void);
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 4e01c44..2167013 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4237,7 +4237,9 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
* on NUMA.
*/
if ((flags & WQ_UNBOUND) && max_active == 1)
- flags |= __WQ_ORDERED;
+ /* the caller may don't want __WQ_ORDERED to be set implicitly. */
+ if (!(flags & __WQ_ORDERED_DISABLE))
+ flags |= __WQ_ORDERED;

/* see the comment above the definition of WQ_POWER_EFFICIENT */
if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient)
--
2.9.5

[Bob Liu]

ping..

[Bob Liu]

On 6/28/20 11:54 PM, Lai Jiangshan wrote:

> On Thu, Jun 11, 2020 at 6:29 PM Bob Liu <bob...@oracle.com> wrote:
>>
>> Current code always set 'Unbound && max_active == 1' workqueues to ordered
>> implicitly, while this may be not an expected behaviour for some use cases.
>>
>> E.g some scsi and iscsi workqueues(unbound && max_active = 1) want to be bind
>> to different cpu so as to get better isolation, but their cpumask can't be
>> changed because WQ_ORDERED is set implicitly.
>

> Hello
>
> If I read the code correctly, the reason why their cpumask can't
> be changed is because __WQ_ORDERED_EXPLICIT, not __WQ_ORDERED.

>
>>
>> This patch adds a flag __WQ_ORDERED_DISABLE and also
>> create_singlethread_workqueue_noorder() to offer an new option.
>>
>> Signed-off-by: Bob Liu <bob...@oracle.com>
>> ---
>> include/linux/workqueue.h | 4 ++++
>> kernel/workqueue.c | 4 +++-
>> 2 files changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
>> index e48554e..4c86913 100644
>> --- a/include/linux/workqueue.h
>> +++ b/include/linux/workqueue.h
>> @@ -344,6 +344,7 @@ enum {
>> __WQ_ORDERED = 1 << 17, /* internal: workqueue is ordered */
>> __WQ_LEGACY = 1 << 18, /* internal: create*_workqueue() */
>> __WQ_ORDERED_EXPLICIT = 1 << 19, /* internal: alloc_ordered_workqueue() */
>> + __WQ_ORDERED_DISABLE = 1 << 20, /* internal: don't set __WQ_ORDERED implicitly */
>>
>> WQ_MAX_ACTIVE = 512, /* I like 512, better ideas? */
>> WQ_MAX_UNBOUND_PER_CPU = 4, /* 4 * #cpus for unbound wq */
>> @@ -433,6 +434,9 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
>> #define create_singlethread_workqueue(name) \
>> alloc_ordered_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, name)
>>
>> +#define create_singlethread_workqueue_noorder(name) \
>> + alloc_workqueue("%s", WQ_SYSFS | __WQ_LEGACY | WQ_MEM_RECLAIM | \
>> + WQ_UNBOUND | __WQ_ORDERED_DISABLE, 1, (name))
>

> I think using __WQ_ORDERED without __WQ_ORDERED_EXPLICIT is what you
> need, in which case cpumask is allowed to be changed.
>

I don't think so, see function workqueue_apply_unbound_cpumask():

wq_unbound_cpumask_store()
> workqueue_set_unbound_cpumask()
> workqueue_apply_unbound_cpumask() {
...
5276 /* creating multiple pwqs breaks ordering guarantee */
5277 if (wq->flags & __WQ_ORDERED)
5278 continue;
^^^^
Here will skip apply cpumask if only __WQ_ORDERED is set.

5280 ctx = apply_wqattrs_prepare(wq, wq->unbound_attrs);

}

Thanks for your review.
Bob

> Just use alloc_workqueue() with __WQ_ORDERED and max_active=1. It can
> be wrapped as a new function or macro, but I don't think> create_singlethread_workqueue_noorder() is a good name for it.

[Bob Liu]

On 6/29/20 8:37 AM, Lai Jiangshan wrote:

> wq_unbound_cpumask_store() is for changing the cpumask of
> *all* workqueues.

Isn't '/sys/bus/workqueue/devices/xxxx/cpumask' using the same function to change cpumask of
specific workqueue?
Am I missing something..

> I don't think it can be used to make
> scsi and iscsi workqueues bound to different cpu.
>

The idea is to register scsi/iscsi workqueues with WQ_SYSFS, and then they can be bounded to different
cpu by writing cpu number to "/sys/bus/workqueue/devices/xxxx/cpumask".

> apply_workqueue_attrs() is for changing the cpumask of the specific
> workqueue, which can change the cpumask of __WQ_ORDERED workqueue
> (but without __WQ_ORDERED_EXPLICIT).

[Lai Jiangshan]

On Thu, Jun 11, 2020 at 6:29 PM Bob Liu <bob...@oracle.com> wrote:
>

> Current code always set 'Unbound && max_active == 1' workqueues to ordered
> implicitly, while this may be not an expected behaviour for some use cases.
>
> E.g some scsi and iscsi workqueues(unbound && max_active = 1) want to be bind
> to different cpu so as to get better isolation, but their cpumask can't be
> changed because WQ_ORDERED is set implicitly.

Hello

If I read the code correctly, the reason why their cpumask can't
be changed is because __WQ_ORDERED_EXPLICIT, not __WQ_ORDERED.

>

> This patch adds a flag __WQ_ORDERED_DISABLE and also
> create_singlethread_workqueue_noorder() to offer an new option.
>
> Signed-off-by: Bob Liu <bob...@oracle.com>
> ---
> include/linux/workqueue.h | 4 ++++
> kernel/workqueue.c | 4 +++-
> 2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> index e48554e..4c86913 100644
> --- a/include/linux/workqueue.h
> +++ b/include/linux/workqueue.h
> @@ -344,6 +344,7 @@ enum {
> __WQ_ORDERED = 1 << 17, /* internal: workqueue is ordered */
> __WQ_LEGACY = 1 << 18, /* internal: create*_workqueue() */
> __WQ_ORDERED_EXPLICIT = 1 << 19, /* internal: alloc_ordered_workqueue() */
> + __WQ_ORDERED_DISABLE = 1 << 20, /* internal: don't set __WQ_ORDERED implicitly */
>
> WQ_MAX_ACTIVE = 512, /* I like 512, better ideas? */
> WQ_MAX_UNBOUND_PER_CPU = 4, /* 4 * #cpus for unbound wq */
> @@ -433,6 +434,9 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
> #define create_singlethread_workqueue(name) \
> alloc_ordered_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, name)
>
> +#define create_singlethread_workqueue_noorder(name) \
> + alloc_workqueue("%s", WQ_SYSFS | __WQ_LEGACY | WQ_MEM_RECLAIM | \
> + WQ_UNBOUND | __WQ_ORDERED_DISABLE, 1, (name))

I think using __WQ_ORDERED without __WQ_ORDERED_EXPLICIT is what you
need, in which case cpumask is allowed to be changed.

Just use alloc_workqueue() with __WQ_ORDERED and max_active=1. It can
be wrapped as a new function or macro, but I don't think
create_singlethread_workqueue_noorder() is a good name for it.

[Lai Jiangshan]

On Mon, Jun 29, 2020 at 8:13 AM Bob Liu <bob...@oracle.com> wrote:
>

wq_unbound_cpumask_store() is for changing the cpumask of

*all* workqueues. I don't think it can be used to make

scsi and iscsi workqueues bound to different cpu.

apply_workqueue_attrs() is for changing the cpumask of the specific
workqueue, which can change the cpumask of __WQ_ORDERED workqueue
(but without __WQ_ORDERED_EXPLICIT).

>

[Bob Liu]

On 6/29/20 8:37 AM, Lai Jiangshan wrote:

Yes, you are right. I made a mistake.
Sorry for the noise.

Regards,
Bob