linux-kernel - [PATCH tip/core/rcu 03/12] rcu: Restrict default jiffies_till_first

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Date:   Wed, 12 Aug 2020 15:57:23 -0700
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        elver@...gle.com, dvyukov@...gle.com,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 03/12] rcu: Restrict default jiffies_till_first_fqs for strict RCU GPs

From: "Paul E. McKenney" <paulmck@...nel.org>

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@...gle.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 65e1b5e..d333f1b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -471,7 +471,7 @@ 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;
-- 
2.9.5