linux-kernel - RE: [PATCH] rcu: Add rnp->expmask output to show_rcu_gp

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Date:   Wed, 25 May 2022 14:42:41 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     "paulmck@...nel.org" <paulmck@...nel.org>,
        Mukesh Ojha <quic_mojha@...cinc.com>
CC:     "frederic@...nel.org" <frederic@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu: Add rnp->expmask output to show_rcu_gp_kthreads()


On Wed, May 25, 2022 at 10:29:23AM +0530, Mukesh Ojha wrote:
> 
> 
> On 5/25/2022 10:15 AM, Zqiang wrote:
> > Currently, for expedited rcu we only check 'rnp->exp_tasks', however 
> > the 'rnp->exp_tasks' are not always exist for expedited grace period 
> > that not end. this commit add 'rnp->expmask' condition.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> 
> Thanks !! for the patch.
> Commit log could require wordsmith from Paul :-)

>For Paul to wordsmith this commit log, he must first understand why this change is useful, keeping in mind that show_rcu_gp_kthreads() is invoked for normal grace periods, but not for expedited ones.
>
>The point of ->exp_mask for debugging forward-progress issues for normal grace periods is to understand why tasks blocking the current normal grace period are not yet being boosted (because the expedited grace period's boosting takes precedence).  How do these additions help you?

Hi Paul

Oh sorry, I misunderstood, my previous understanding was only to get the status of the current grace period.
thank you for the explanation.

Thanks
Zqiang 

>
>(Not saying that they cannot possibly help you.  I am instead saying that you must tell me how they have helped you guys.)
>
>							Thanx, Paul

> Reviewed-by: quic_mojha@...cinc.com
> 
> -Mukesh
> 
> > ---
> >   kernel/rcu/tree_stall.h | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index 
> > 2464b0eccfd0..5c4df855f2ac 100644
> > --- a/kernel/rcu/tree_stall.h
> > +++ b/kernel/rcu/tree_stall.h
> > @@ -865,13 +865,15 @@ void show_rcu_gp_kthreads(void)
> >   	rcu_for_each_node_breadth_first(rnp) {
> >   		if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
> >   		    !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
> > -		    !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
> > +		    !data_race(READ_ONCE(rnp->expmask)) && !data_race(READ_ONCE(rnp->exp_tasks)) &&
> > +			!data_race(READ_ONCE(rnp->gp_tasks)))
> >   			continue;
> > -		pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
> > +		pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld 
> > +->qsmask %#lx ->expmask %#lx %c%c%c%c ->n_boosts %ld\n",
> >   			rnp->grplo, rnp->grphi,
> >   			(long)data_race(READ_ONCE(rnp->gp_seq)),
> >   			(long)data_race(READ_ONCE(rnp->gp_seq_needed)),
> >   			data_race(READ_ONCE(rnp->qsmask)),
> > +			data_race(READ_ONCE(rnp->expmask)),
> >   			".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
> >   			".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
> >   			".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],