rcu: IPI all CPUs at GP start for strict GPs

Currently, each CPU discovers the beginning of a given grace period
on its own time, which is again good for efficiency but bad for fast
grace periods.  This commit therefore uses on_each_cpu() to IPI each
CPU after grace-period initialization in order to inform each CPU of
the new grace period in a timely manner, but only in kernels build with
CONFIG_RCU_STRICT_GRACE_PERIOD=y.

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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 36a860c..88f4fa6 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1695,6 +1695,15 @@ static void rcu_gp_torture_wait(void)
        }
 }
 
+/*
+ * Handler for on_each_cpu() to invoke the target CPU's RCU core
+ * processing.
+ */
+static void rcu_strict_gp_boundary(void *unused)
+{
+       invoke_rcu_core();
+}
+
 /*
  * Initialize a new grace period.  Return false if no grace period required.
  */
@@ -1823,6 +1832,10 @@ static bool rcu_gp_init(void)
                WRITE_ONCE(rcu_state.gp_activity, jiffies);
        }
 
+       // If strict, make all CPUs aware of new grace period.
+       if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
+               on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
+
        return true;
 }