linux-kernel - Re: [PATCH tip/core/rcu 08/12] rcu: Force tick on for nohz

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Message-ID: <20191005172129.GL2689@paulmck-ThinkPad-P72>
Date:   Sat, 5 Oct 2019 10:21:29 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.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,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 08/12] rcu: Force tick on for nohz_full CPUs
 not reaching quiescent states

On Thu, Oct 03, 2019 at 04:50:55PM +0200, Frederic Weisbecker wrote:
> On Wed, Oct 02, 2019 at 06:38:59PM -0700, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> > 
> > CPUs running for long time periods in the kernel in nohz_full mode
> > might leave the scheduling-clock interrupt disabled for then full
> > duration of their in-kernel execution.  This can (among other things)
> > delay grace periods.  This commit therefore forces the tick back on
> > for any nohz_full CPU that is failing to pass through a quiescent state
> > upon return from interrupt, which the resched_cpu() will induce.
> > 
> > Reported-by: Joel Fernandes <joel@...lfernandes.org>
> > [ paulmck: Clear ->rcu_forced_tick as reported by Joel Fernandes testing. ]
> > [ paulmck: Apply Joel Fernandes TICK_DEP_MASK_RCU->TICK_DEP_BIT_RCU fix. ]
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> > ---
> >  kernel/rcu/tree.c | 38 +++++++++++++++++++++++++++++++-------
> >  kernel/rcu/tree.h |  1 +
> >  2 files changed, 32 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 74bf5c65..621cc06 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -650,6 +650,12 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
> >  	 */
> >  	if (rdp->dynticks_nmi_nesting != 1) {
> >  		trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
> > +		if (tick_nohz_full_cpu(rdp->cpu) &&
> > +		    rdp->dynticks_nmi_nesting == 2 &&
> > +		    rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
> > +			rdp->rcu_forced_tick = true;
> > +			tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
> 
> I understand rdp->cpu is always smp_processor_id() here, right? Because calling
> tick_dep_set_cpu() to a remote CPU while in NMI wouldn't be safe. It would warn anyway.

Yes, this is always invoked on the CPU whose ID is rdp->cpu, but thank
you for checking!

							Thanx, Paul