From 671a63517cf983ad8eaa324167165cef245ab744 Mon Sep 17 00:00:00 2001 From: "Joel Fernandes (Google)" Date: Sat, 19 Jan 2019 11:14:18 -0500 Subject: rcu: Avoid unnecessary softirq when system is idle When there are no callbacks pending on an idle system, I noticed that RCU softirq is continuously firing. During this the cpu_no_qs is set to false, and core_needs_qs is set to true indefinitely. This causes rcu_process_callbacks to be repeatedly called, even though the node corresponding to the CPU has that CPU's mask bit cleared and the system is idle. I believe the race is when such mask clearing is done during idle CPU scan of the quiescent state forcing stage in the kthread instead of the softirq. Since the rnp mask is cleared, but the flags on the CPU's rdp are not cleared, the CPU thinks it still needs to report to core RCU. Cure this by clearing the core_needs_qs flag when the CPU detects that its node is already updated which will avoid the unwanted softirq raises to the benefit of real-time systems. Test: Ran rcutorture for various tree RCU configs. Signed-off-by: Joel Fernandes (Google) Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 1 + 1 file changed, 1 insertion(+) (limited to 'kernel/rcu') diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 95e3250b7b6e..2f78a115d34c 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2296,6 +2296,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_data *rdp) } mask = rdp->grpmask; if ((rnp->qsmask & mask) == 0) { + rdp->core_needs_qs = false; raw_spin_unlock_irqrestore_rcu_node(rnp, flags); } else { rdp->core_needs_qs = false; -- cgit v1.2.3-59-g8ed1b