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

Paul E. McKenney
2019-10-05T16:42:17Z

On Thu, Oct 03, 2019 at 04:10:52PM +0200, Frederic Weisbecker wrote:
> On Wed, Oct 02, 2019 at 06:38:54PM -0700, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@linux.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 can be preempted, this forcing is withdrawn on each context switch.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@linux.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
> >