linux-kernel - Re: [PATCH tip/core/rcu 4/8] rcu: Ensure that ->rcu_urgent

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Message-ID: <20191003134501.GP2689@paulmck-ThinkPad-P72>
Date:   Thu, 3 Oct 2019 06:45:01 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 4/8] rcu: Ensure that ->rcu_urgent_qs is set
 before resched IPI

On Thu, Oct 03, 2019 at 07:43:19AM -0400, Steven Rostedt wrote:
> On Wed,  2 Oct 2019 18:33:01 -0700
> paulmck@...nel.org wrote:
> 
> > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > 
> > The RCU-specific resched_cpu() function sends a resched IPI to the
> > specified CPU, which can be used to force the tick on for a given
> > nohz_full CPU.  This is needed when this nohz_full CPU is looping in the
> > kernel while blocking the current grace period.  However, for the tick
> > to actually be forced on in all cases, that CPU's rcu_data structure's
> > ->rcu_urgent_qs flag must be set beforehand.  This commit therefore  
> > causes rcu_implicit_dynticks_qs() to set this flag prior to invoking
> > resched_cpu() on a holdout nohz_full CPU.
> 
> Should this be marked for stable?

Not unless and until people are actually running into this.  NO_HZ_FULL
has left the tick off for in-kernel loops on nohz_full CPUs for almost
ten years now, and as far as I know, without complaint.

So from what I am seeing, the risk of backporting far exceeds the benefit.

							Thanx, Paul

> -- Steve
> 
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> > ---
> >  kernel/rcu/tree.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 8110514..0d83b19 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1073,6 +1073,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
> >  	if (tick_nohz_full_cpu(rdp->cpu) &&
> >  		   time_after(jiffies,
> >  			      READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) {
> > +		WRITE_ONCE(*ruqp, true);
> >  		resched_cpu(rdp->cpu);
> >  		WRITE_ONCE(rdp->last_fqs_resched, jiffies);
> >  	}
>