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From: paulmck@kernel.org
To: rcu@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com,
	mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
	fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
	elver@google.com, dvyukov@google.com,
	"Paul E. McKenney" <paulmck@kernel.org>
Subject: [PATCH tip/core/rcu 02/12] rcu: Reduce leaf fanout for strict RCU grace periods
Date: Wed, 12 Aug 2020 15:57:22 -0700	[thread overview]
Message-ID: <20200812225732.20068-2-paulmck@kernel.org> (raw)
In-Reply-To: <20200812225632.GA19759@paulmck-ThinkPad-P72>

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

Because strict RCU grace periods will complete more quickly, they will
experience greater lock contention on each leaf rcu_node structure's
->lock.  This commit therefore reduces the leaf fanout in order to reduce
this lock contention.

Note that this also has the effect of reducing the number of CPUs
supported to 16 in the case of CONFIG_RCU_FANOUT_LEAF=2 or 81 in the
case of CONFIG_RCU_FANOUT_LEAF=3.  However, greater numbers of CPUs are
probably a bad idea when using CONFIG_RCU_STRICT_GRACE_PERIOD=y.  Those
wishing to live dangerously are free to edit their kernel/rcu/Kconfig
files accordingly.

Reported-by Jann Horn <jannh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/Kconfig | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 0ebe15a..b71e21f 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -135,10 +135,12 @@ config RCU_FANOUT
 
 config RCU_FANOUT_LEAF
 	int "Tree-based hierarchical RCU leaf-level fanout value"
-	range 2 64 if 64BIT
-	range 2 32 if !64BIT
+	range 2 64 if 64BIT && !RCU_STRICT_GRACE_PERIOD
+	range 2 32 if !64BIT && !RCU_STRICT_GRACE_PERIOD
+	range 2 3 if RCU_STRICT_GRACE_PERIOD
 	depends on TREE_RCU && RCU_EXPERT
-	default 16
+	default 16 if !RCU_STRICT_GRACE_PERIOD
+	default 2 if RCU_STRICT_GRACE_PERIOD
 	help
 	  This option controls the leaf-level fanout of hierarchical
 	  implementations of RCU, and allows trading off cache misses
-- 
2.9.5


  parent reply	other threads:[~2020-08-12 22:58 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-12 22:56 [PATCH tip/core/rcu 0/12] Add strict short-grace-period Kconfig option Paul E. McKenney
2020-08-12 22:57 ` [PATCH tip/core/rcu 01/12] rcu: Add Kconfig option for strict RCU grace periods paulmck
2020-08-12 22:57 ` paulmck [this message]
2020-08-12 22:57 ` [PATCH tip/core/rcu 03/12] rcu: Restrict default jiffies_till_first_fqs for strict RCU GPs paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 04/12] rcu: Force DEFAULT_RCU_BLIMIT to 1000 " paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 05/12] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 06/12] rcu: Do full report for .need_qs " paulmck
2020-08-13  0:50   ` Jann Horn
2020-08-13  3:29     ` Paul E. McKenney
2020-08-12 22:57 ` [PATCH tip/core/rcu 07/12] rcu: Attempt QS when CPU discovers GP " paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 08/12] rcu: IPI all CPUs at GP start " paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 09/12] rcu: IPI all CPUs at GP end " paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 10/12] rcu: Provide optional RCU-reader exit delay " paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 11/12] rcu: Execute RCU reader shortly after rcu_core " paulmck
2020-08-12 22:57 ` [PATCH tip/core/rcu 12/12] rcu: Report QS for outermost PREEMPT=n rcu_read_unlock() " paulmck

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