[Patch v2] rcu: simplify the calculation of rcu_state.ncpus

From: Wei Yang <richard.weiyang@gmail.com>
To: paulmck@kernel.org
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	Wei Yang <richard.weiyang@gmail.com>
Subject: [Patch v2] rcu: simplify the calculation of rcu_state.ncpus
Date: Sun, 19 Apr 2020 21:57:15 +0000	[thread overview]
Message-ID: <20200419215715.21071-1-richard.weiyang@gmail.com> (raw)

There is only 1 bit set in mask, which means the difference between
oldmask and the new one would be at the position where the bit is set in
mask.

Based on this knowledge, rcu_state.ncpus could be calculated by checking
whether mask is already set in rnp->expmaskinitnext.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>

---
v2:
  * trivial adjust based on Paul's suggestion.
---
 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 f288477ee1c2..6d39485f7f51 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3732,10 +3732,9 @@ 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;
@@ -3747,12 +3746,10 @@ 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);
-- 
2.23.0

