Re: [PATCH rcu 08/12] rcu: Cleanup RCU urgency state for offline CPU

Paul E. McKenney
2022-06-21T22:24:56Z

On Tue, Jun 21, 2022 at 12:33:12PM +0530, Neeraj Upadhyay wrote:
>
>
> On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
> > From: Zqiang <qiang1.zhang@intel.com>
> >
> > When a CPU is slow to provide a quiescent state for a given grace
> > period, RCU takes steps to encourage that CPU to get with the
> > quiescent-state program in a more timely fashion. These steps
> > include these flags in the rcu_data structure:
> >
> > 1. ->rcu_urgent_qs, which causes the scheduling-clock interrupt to
> > request an otherwise pointless context switch from the scheduler.
> >
> > 2. ->rcu_need_heavy_qs, which causes both cond_resched() and RCU's
> > context-switch hook to do an immediate momentary quiscent state.
> >
> > 3. ->rcu_need_heavy_qs, which causes the scheduler-clock tick to
>
> nit: s/->rcu_need_heavy_qs/->rcu_forced_tick/ ?

This one got lost in the shuffle, so I will apply it on my next rebase.

> > be enabled even on nohz_full CPUs with only one runnable task.
> >
> > These flags are of course cleared once the corresponding CPU has passed
> > through a quiescent state. Unless that quiescent state is the CPU
> > going offline, which means that when the CPU comes back online, it will
> > needlessly consume additional CPU time and incur additional latency,
> > which constitutes a minor but very real performance bug.
> >
> > This commit therefore adds the call to rcu_disable_urgency_upon_qs()
> > that clears these flags to the CPU-hotplug offlining code path.
> >
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
>
>
> Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>

And again, thank you!

Thanx, Paul

> Thanks
> Neeraj
>
> > kernel/rcu/tree.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index f4a37f2032664..5445b19b48408 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -4446,6 +4446,7 @@ void rcu_report_dead(unsigned int cpu)
> > rdp->rcu_ofl_gp_flags = READ_ONCE(rcu_state.gp_flags);
> > if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
> > /* Report quiescent state -before- changing ->qsmaskinitnext! */
> > + rcu_disable_urgency_upon_qs(rdp);
> > rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
> > raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > }