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Subject: Re: [PATCH 1/2] workqueue: don't always set __WQ_ORDERED implicitly
To:     Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Tejun Heo <tj@kernel.org>,
        martin.petersen@oracle.com, linux-scsi@vger.kernel.org,
        open-iscsi@googlegroups.com, lduncan@suse.com,
        michael.christie@oracle.com
References: <20200611100717.27506-1-bob.liu@oracle.com>
 <CAJhGHyDQLuoCkjwnze_6ZOLwXPtbNxnjxOr=fqqqsR_yxB9xtA@mail.gmail.com>
 <52fa1d81-e585-37eb-55e5-0ed07ce7adc0@oracle.com>
 <CAJhGHyBPrCr3+iu-dMe69J3+tj99ea8crCGBuXc4yoStD+dEFA@mail.gmail.com>
From:   Bob Liu <bob.liu@oracle.com>
Message-ID: <bdf25dd0-7ac3-52b5-855b-14955443c52b@oracle.com>
Date:   Wed, 1 Jul 2020 11:06:41 +0800
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On 6/29/20 8:37 AM, Lai Jiangshan wrote:
> On Mon, Jun 29, 2020 at 8:13 AM Bob Liu <bob.liu@oracle.com> wrote:
>>
>> On 6/28/20 11:54 PM, Lai Jiangshan wrote:
>>> On Thu, Jun 11, 2020 at 6:29 PM Bob Liu <bob.liu@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.liu@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.
> 
> 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).
> 

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

Regards,
Bob

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