linux-kernel - Re: [PATCH tip/core/rcu 03/12] rcu: Force on tick when invoking lots of callbacks

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Date:   Sat, 5 Oct 2019 09:42:14 -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 03/12] rcu: Force on tick when invoking lots
 of callbacks

On Thu, Oct 03, 2019 at 04:10:52PM +0200, Frederic Weisbecker wrote:
> On Wed, Oct 02, 2019 at 06:38:54PM -0700, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> > 
> > Callback invocation can run for a significant time period, and within
> > CONFIG_NO_HZ_FULL=y kernels, this period will be devoid of scheduler-clock
> > interrupts.  In-kernel execution without such interrupts can cause all
> > manner of malfunction, with RCU CPU stall warnings being but one result.
> > 
> > This commit therefore forces scheduling-clock interrupts on whenever more
> > than a few RCU callbacks are invoked.  Because offloaded callback invocation
> > can be preempted, this forcing is withdrawn on each context switch.  This
> > in turn requires that the loop invoking RCU callbacks reiterate the forcing
> > periodically.
> > 
> > [ 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 | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 8110514..db673ae 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2151,6 +2151,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> >  	rcu_nocb_unlock_irqrestore(rdp, flags);
> >  
> >  	/* Invoke callbacks. */
> > +	if (IS_ENABLED(CONFIG_NO_HZ_FULL))
> 
> No need for the IS_ENABLED(), the API takes care of that.
> 
> > +		tick_dep_set_task(current, TICK_DEP_BIT_RCU);
> >  	rhp = rcu_cblist_dequeue(&rcl);
> >  	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
> >  		debug_rcu_head_unqueue(rhp);
> > @@ -2217,6 +2219,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> >  	/* Re-invoke RCU core processing if there are callbacks remaining. */
> >  	if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist))
> >  		invoke_rcu_core();
> > +	if (IS_ENABLED(CONFIG_NO_HZ_FULL))
> 
> Same here.

Good catches!  Applied, thank you!

							Thanx, Paul

> Thanks.
> 
> > +		tick_dep_clear_task(current, TICK_DEP_BIT_RCU);
> >  }
> >  
> >  /*
> > -- 
> > 2.9.5
> >