rcu: Avoid having just-onlined CPU resched itself when RCU is idle (06ae115a) · Commits · 戴 / test

CPUs set rdp->qs_pending when coming online to resolve races with grace-period start. However, this means that if RCU is idle, the just-onlined CPU might needlessly send itself resched IPIs. Adjust the online-CPU initialization to avoid this, and also to correctly cause the CPU to respond to the current grace period if needed. Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by:

Josh Boyer <jwboyer@redhat.com> Tested-by:

Christian Hoffmann <email@christianhoffmann.info>

kernel/rcutree.c

+8 −3

Original line number	Diff line number	Diff line
		@@ -1911,8 +1911,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)

		/* Set up local state, ensuring consistent view of global state. */
		raw_spin_lock_irqsave(&rnp->lock, flags);
		rdp->passed_quiesce = 0; /* We could be racing with new GP, */
		rdp->qs_pending = 1; /* so set up to respond to current GP. */
		rdp->beenonline = 1; /* We have now been online. */
		rdp->preemptible = preemptible;
		rdp->qlen_last_fqs_check = 0;
		@@ -1937,8 +1935,15 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
		rnp->qsmaskinit \|= mask;
		mask = rnp->grpmask;
		if (rnp == rdp->mynode) {
		rdp->gpnum = rnp->completed; /* if GP in progress... */
		/*
		* If there is a grace period in progress, we will
		* set up to wait for it next time we run the
		* RCU core code.
		*/
		rdp->gpnum = rnp->completed;
		rdp->completed = rnp->completed;
		rdp->passed_quiesce = 0;
		rdp->qs_pending = 0;
		rdp->passed_quiesce_gpnum = rnp->gpnum - 1;
		trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuonl");
		}

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