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

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Date:   Sun, 28 Jun 2020 23:54:38 +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 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.

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
>