[PATCH] workqueue: Remove rcu_read_lock/unlock() in workqueue_congested()

[PATCH] workqueue: Remove rcu_read_lock/unlock() in workqueue_congested()

[qiang.zhang]

From: Zqiang <qiang.zhang@windriver.com>

The RCU read critical area already by preempt_disable/enable()
(equivalent to rcu_read_lock_sched/unlock_sched()) mark, so remove
rcu_read_lock/unlock().

Signed-off-by: Zqiang <qiang.zhang@windriver.com>
---
 kernel/workqueue.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 0d150da252e8..c599835ad6c3 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4540,7 +4540,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
 	struct pool_workqueue *pwq;
 	bool ret;
 
-	rcu_read_lock();
 	preempt_disable();
 
 	if (cpu == WORK_CPU_UNBOUND)
@@ -4553,7 +4552,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
 
 	ret = !list_empty(&pwq->delayed_works);
 	preempt_enable();
-	rcu_read_unlock();
 
 	return ret;
 }
-- 
2.25.1

Re: [PATCH] workqueue: Remove rcu_read_lock/unlock() in workqueue_congested()

[Lai Jiangshan]

+CC Paul


On Wed, Feb 17, 2021 at 7:58 PM <qiang.zhang@windriver.com> wrote:
>
> From: Zqiang <qiang.zhang@windriver.com>
>
> The RCU read critical area already by preempt_disable/enable()
> (equivalent to rcu_read_lock_sched/unlock_sched()) mark, so remove
> rcu_read_lock/unlock().

I think we can leave it which acks like document, especially
workqueue_congested() is not performance crucial.  Either way
is Ok for me.

If it needs to be changed, please also do the same for
rcu_read_lock() in wq_watchdog_timer_fn().

And __queue_work() and try_to_grab_pending() also use local_irq_save()
and rcu_read_lock() at the same time, but I don't know will these
local_irq_save() be changed to raw_local_irq_save() in PREEMPT_RT.


>
> Signed-off-by: Zqiang <qiang.zhang@windriver.com>
> ---
>  kernel/workqueue.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 0d150da252e8..c599835ad6c3 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -4540,7 +4540,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
>         struct pool_workqueue *pwq;
>         bool ret;
>
> -       rcu_read_lock();
>         preempt_disable();
>
>         if (cpu == WORK_CPU_UNBOUND)
> @@ -4553,7 +4552,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
>
>         ret = !list_empty(&pwq->delayed_works);
>         preempt_enable();
> -       rcu_read_unlock();
>
>         return ret;
>  }
> --
> 2.25.1
>

Re: [PATCH] workqueue: Remove rcu_read_lock/unlock() in workqueue_congested()

[Paul E. McKenney]

On Thu, Feb 18, 2021 at 11:04:00AM +0800, Lai Jiangshan wrote:
> +CC Paul
> 
> 
> On Wed, Feb 17, 2021 at 7:58 PM <qiang.zhang@windriver.com> wrote:
> >
> > From: Zqiang <qiang.zhang@windriver.com>
> >
> > The RCU read critical area already by preempt_disable/enable()
> > (equivalent to rcu_read_lock_sched/unlock_sched()) mark, so remove
> > rcu_read_lock/unlock().
> 
> I think we can leave it which acks like document, especially
> workqueue_congested() is not performance crucial.  Either way
> is Ok for me.

If the rcu_read_lock() is removed, should there be a comment saying that
it interacts with synchronize_rcu()?  Just in case one of the real-time
guys figures out a way to get the job done without disabling preemption...

							Thanx, Paul

> If it needs to be changed, please also do the same for
> rcu_read_lock() in wq_watchdog_timer_fn().
> 
> And __queue_work() and try_to_grab_pending() also use local_irq_save()
> and rcu_read_lock() at the same time, but I don't know will these
> local_irq_save() be changed to raw_local_irq_save() in PREEMPT_RT.
> 
> 
> >
> > Signed-off-by: Zqiang <qiang.zhang@windriver.com>
> > ---
> >  kernel/workqueue.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 0d150da252e8..c599835ad6c3 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -4540,7 +4540,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
> >         struct pool_workqueue *pwq;
> >         bool ret;
> >
> > -       rcu_read_lock();
> >         preempt_disable();
> >
> >         if (cpu == WORK_CPU_UNBOUND)
> > @@ -4553,7 +4552,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
> >
> >         ret = !list_empty(&pwq->delayed_works);
> >         preempt_enable();
> > -       rcu_read_unlock();
> >
> >         return ret;
> >  }
> > --
> > 2.25.1
> >

回复: [PATCH] workqueue: Remove rcu_read_lock/unlock() in workqueue_congested()

[Zhang, Qiang]

________________________________________
发件人: Paul E. McKenney <paulmck@kernel.org>
发送时间: 2021年2月18日 23:17
收件人: Lai Jiangshan
抄送: Zhang, Qiang; Tejun Heo; Tejun Heo; LKML
主题: Re: [PATCH] workqueue: Remove rcu_read_lock/unlock() in workqueue_congested()

[Please note: This e-mail is from an EXTERNAL e-mail address]

On Thu, Feb 18, 2021 at 11:04:00AM +0800, Lai Jiangshan wrote:
> +CC Paul
>
>
> On Wed, Feb 17, 2021 at 7:58 PM <qiang.zhang@windriver.com> wrote:
> >
> > From: Zqiang <qiang.zhang@windriver.com>
> >
> > The RCU read critical area already by preempt_disable/enable()
> > (equivalent to rcu_read_lock_sched/unlock_sched()) mark, so remove
> > rcu_read_lock/unlock().
>
> I think we can leave it which acks like document, especially
> workqueue_congested() is not performance crucial.  Either way
> is Ok for me.
>
>If the rcu_read_lock() is removed, should there be a comment saying >that
>it interacts with synchronize_rcu()?  Just in case one of the real-time
>guys figures out a way to get the job done without disabling >preemption...
>
>                                                        Thanx, Paul
>
> If it needs to be changed, please also do the same for
> rcu_read_lock() in wq_watchdog_timer_fn().
>
>
> And __queue_work() and try_to_grab_pending() also use local_irq_save()
> and rcu_read_lock() at the same time, but I don't know will these
> local_irq_save() be changed to raw_local_irq_save() in PREEMPT_RT.

The local_irq_save function is not change  in PREEMPT_RT system.

Thanks
Qiang

>
>
> >
> > Signed-off-by: Zqiang <qiang.zhang@windriver.com>
> > ---
> >  kernel/workqueue.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 0d150da252e8..c599835ad6c3 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -4540,7 +4540,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
> >         struct pool_workqueue *pwq;
> >         bool ret;
> >
> > -       rcu_read_lock();
> >         preempt_disable();
> >
> >         if (cpu == WORK_CPU_UNBOUND)
> > @@ -4553,7 +4552,6 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
> >
> >         ret = !list_empty(&pwq->delayed_works);
> >         preempt_enable();
> > -       rcu_read_unlock();
> >
> >         return ret;
> >  }
> > --
> > 2.25.1
> >