Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use call_rcu_flush()

[Paul E. McKenney]

On Sun, Oct 23, 2022 at 08:36:00PM -0400, Joel Fernandes wrote:
> Hello,
> 
> On Wed, Oct 19, 2022 at 6:51 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > From: Uladzislau Rezki <urezki@xxxxxxxxx>
> >
> > call_rcu() changes to save power will slow down RCU workqueue items
> > queued via queue_rcu_work(). This may not be an issue, however we cannot
> > assume that workqueue users are OK with long delays. Use
> > call_rcu_flush() API instead which reverts to the old behavio
> 
> On ChromeOS, I can see that queue_rcu_work() is pretty noisy and the
> batching is much better if we can just keep it as call_rcu() instead
> of call_rcu_flush().
> 
> Is there really any reason to keep it as call_rcu_flush() ?  If I
> recall, the real reason Vlad's system was slowing down was because of
> scsi and the queue_rcu_work() conversion was really a red herring.

There are less than 20 invocations of queue_rcu_work(), so it should
be possible look through each.  The low-risk approach is of course to
have queue_rcu_work() use call_rcu_flush().

The next approach might be to have a Kconfig option and/or kernel
boot parameter that allowed a per-system choice.

But it would not hurt to double-check on Android.

							Thanx, Paul

> Vlad, any thoughts?
> 
> thanks,
> 
>  - Joel
> 
> .
> >
> > Signed-off-by: Uladzislau Rezki <urezki@xxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> >  kernel/workqueue.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 7cd5f5e7e0a1b..b4b0e828b529e 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -1771,7 +1771,7 @@ bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork)
> >
> >         if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
> >                 rwork->wq = wq;
> > -               call_rcu(&rwork->rcu, rcu_work_rcufn);
> > +               call_rcu_flush(&rwork->rcu, rcu_work_rcufn);
> >                 return true;
> >         }
> >
> > --
> > 2.31.1.189.g2e36527f23
> >