linux-kernel - Re: [PATCH 1/2] workqueue: don't always set __WQ

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Date:   Mon, 29 Jun 2020 08:37:50 +0800
From:   Lai Jiangshan <jiangshanlai+lkml@...il.com>
To:     Bob Liu <bob.liu@...cle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
        open-iscsi@...glegroups.com, lduncan@...e.com,
        michael.christie@...cle.com
Subject: Re: [PATCH 1/2] workqueue: don't always set __WQ_ORDERED implicitly

On Mon, Jun 29, 2020 at 8:13 AM Bob Liu <bob.liu@...cle.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@...cle.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@...cle.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).

>
> 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
> >>
>