2021-02-17 12:02:22

by Zhang, Qiang

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

From: Zqiang <[email protected]>

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 <[email protected]>
---
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

2021-02-18 03:05:44

by Lai Jiangshan

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

+CC Paul


On Wed, Feb 17, 2021 at 7:58 PM <[email protected]> wrote:
>
> From: Zqiang <[email protected]>
>
> 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 <[email protected]>
> ---
> 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
>

2021-02-18 18:02:38

by Paul E. McKenney

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

On Thu, Feb 18, 2021 at 11:04:00AM +0800, Lai Jiangshan wrote:
> +CC Paul
>
>
> On Wed, Feb 17, 2021 at 7:58 PM <[email protected]> wrote:
> >
> > From: Zqiang <[email protected]>
> >
> > 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 <[email protected]>
> > ---
> > 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
> >

2021-02-22 09:19:04

by Zhang, Qiang

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

________________________________________
??????: Paul E. McKenney <[email protected]>
????ʱ??: 2021??2??18?? 23:17
?ռ???: Lai Jiangshan
????: Zhang, Qiang; Tejun Heo; Tejun Heo; LKML
????: 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 <[email protected]> wrote:
> >
> > From: Zqiang <[email protected]>
> >
> > 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 <[email protected]>
> > ---
> > 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
> >