Commit aecd34b9 authored Aug 05, 2020 by Paul E. McKenney

rcu: Restrict default jiffies_till_first_fqs for strict RCU GPs

If there are idle CPUs, RCU's grace-period kthread will wait several
jiffies before even thinking about polling them. This promotes
efficiency, which is normally a good thing, but when the kernel
has been built with CONFIG_RCU_STRICT_GRACE_PERIOD=y, we care more
about short grace periods. This commit therefore restricts the
default jiffies_till_first_fqs value to zero in kernels built with
CONFIG_RCU_STRICT_GRACE_PERIOD=y, which causes RCU's grace-period kthread
to poll for idle CPUs immediately after starting a grace period.

Reported-by Jann Horn <jannh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

parent dc126918

kernel/rcu/tree.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -485,7 +485,7 @@ module_param(qhimark, long, 0444);
		module_param(qlowmark, long, 0444);
		module_param(qovld, long, 0444);

		static ulong jiffies_till_first_fqs = ULONG_MAX;
		static ulong jiffies_till_first_fqs = IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ? 0 : ULONG_MAX;
		static ulong jiffies_till_next_fqs = ULONG_MAX;
		static bool rcu_kick_kthreads;
		static int rcu_divisor = 7;

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