Messages in this thread | | | Date | Tue, 21 Jun 2022 12:33:12 +0530 | Subject | Re: [PATCH rcu 08/12] rcu: Cleanup RCU urgency state for offline CPU | From | Neeraj Upadhyay <> |
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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/ ?
> 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>
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); > }
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