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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>
From:   Bob Liu <bob.liu@oracle.com>
Message-ID: <52fa1d81-e585-37eb-55e5-0ed07ce7adc0@oracle.com>
Date:   Mon, 29 Jun 2020 08:11:34 +0800
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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.

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.
> 
>>  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
>>