linux-kernel - Re: [RFC][PATCH v3 6/6] rcu/tree: Use irq_work_queue

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Message-ID: <20201029091053.GG2628@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Oct 2020 10:10:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, will@...nel.org,
        hch@....de, axboe@...nel.dk, chris@...is-wilson.co.uk,
        davem@...emloft.net, kuba@...nel.org, fweisbec@...il.com,
        oleg@...hat.com, vincent.guittot@...aro.org
Subject: Re: [RFC][PATCH v3 6/6] rcu/tree: Use irq_work_queue_remote()

On Wed, Oct 28, 2020 at 01:15:54PM -0700, Paul E. McKenney wrote:
> On Wed, Oct 28, 2020 at 09:02:43PM +0100, Peter Zijlstra wrote:

> > Subject: rcu/tree: Use irq_work_queue_remote()
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Wed Oct 28 11:53:40 CET 2020
> > 
> > All sites that consume rcu_iw_gp_seq seem to have rcu_node lock held,
> > so setting it probably should too. Also the effect of self-IPI here
> > would be setting rcu_iw_gp_seq to the value we just set it to
> > (pointless) and clearing rcu_iw_pending, which we just set, so don't
> > set it.
> > 
> > Passes TREE01.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  kernel/rcu/tree.c |   10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1308,14 +1308,16 @@ static int rcu_implicit_dynticks_qs(stru
> >  			resched_cpu(rdp->cpu);
> >  			WRITE_ONCE(rdp->last_fqs_resched, jiffies);
> >  		}
> > -#ifdef CONFIG_IRQ_WORK
> > +		raw_spin_lock_rcu_node(rnp);
> 
> The caller of rcu_implicit_dynticks_qs() already holds this lock.
> Please see the force_qs_rnp() function and its second call site,
> to which rcu_implicit_dynticks_qs() is passed as an argument.
> 
> But other than that, this does look plausible.  And getting rid of
> that #ifdef is worth something.  ;-)

Dang, clearly TREE01 didn't actually hit any of this code :/ Is there
another test I should be running?