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

[Zhang, Qiang]

________________________________________
发件人: Paul E. McKenney <paulmck@xxxxxxxxxx>
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()

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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@xxxxxxxxxxxxx> wrote:
> >
> > From: Zqiang <qiang.zhang@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > ---
> >  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
> >