linux-kernel - [tip: core/rcu] rcu: Simplify the calculation of rcu

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Message-ID: <159618741942.4006.16696150175049470222.tip-bot2@tip-bot2>
Date:   Fri, 31 Jul 2020 09:23:39 -0000
From:   "tip-bot2 for Wei Yang" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Wei Yang <richard.weiyang@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: core/rcu] rcu: Simplify the calculation of rcu_state.ncpus

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     abfce0414814149f716e1d30da1fb3140d1b3473
Gitweb:        https://git.kernel.org/tip/abfce0414814149f716e1d30da1fb3140d1b3473
Author:        Wei Yang <richard.weiyang@...il.com>
AuthorDate:    Sun, 19 Apr 2020 21:57:15 
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 29 Jun 2020 11:58:49 -07:00

rcu: Simplify the calculation of rcu_state.ncpus

There is only 1 bit set in mask, which means that the only difference
between oldmask and the new one will be at the position where the bit is
set in mask.  This commit therefore updates rcu_state.ncpus by checking
whether the bit in mask is already set in rnp->expmaskinitnext.

Signed-off-by: Wei Yang <richard.weiyang@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c |  9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6c6569e..bef1dc9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3842,10 +3842,9 @@ void rcu_cpu_starting(unsigned int cpu)
 {
 	unsigned long flags;
 	unsigned long mask;
-	int nbits;
-	unsigned long oldmask;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
+	bool newcpu;
 
 	if (per_cpu(rcu_cpu_started, cpu))
 		return;
@@ -3857,12 +3856,10 @@ void rcu_cpu_starting(unsigned int cpu)
 	mask = rdp->grpmask;
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
-	oldmask = rnp->expmaskinitnext;
+	newcpu = !(rnp->expmaskinitnext & mask);
 	rnp->expmaskinitnext |= mask;
-	oldmask ^= rnp->expmaskinitnext;
-	nbits = bitmap_weight(&oldmask, BITS_PER_LONG);
 	/* Allow lockless access for expedited grace periods. */
-	smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + nbits); /* ^^^ */
+	smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + newcpu); /* ^^^ */
 	ASSERT_EXCLUSIVE_WRITER(rcu_state.ncpus);
 	rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
 	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);