[PATCH tip/core/rcu 3/5] srcu: Reduce scans of srcu_data in counter wrap check

From: Paul E. McKenney
Date: Mon Feb 26 2018 - 16:56:42 EST


From: Ildar Ismagilov <devix84@xxxxxxxxx>

Currently, given a multi-level srcu_node tree, SRCU can scan the full
set of srcu_data structures at each level when cleaning up after a grace
period. This, though harmless otherwise, represents pointless overhead.
This commit therefore eliminates this overhead by scanning the srcu_data
structures only when traversing the leaf srcu_node structures.

Signed-off-by: Ildar Ismagilov <devix84@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
kernel/rcu/srcutree.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index bb9607baad9b..d582a2352cdb 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -527,6 +527,7 @@ static void srcu_gp_end(struct srcu_struct *sp)
{
unsigned long cbdelay;
bool cbs;
+ bool last_lvl;
int cpu;
unsigned long flags;
unsigned long gpseq;
@@ -559,7 +560,8 @@ static void srcu_gp_end(struct srcu_struct *sp)
rcu_for_each_node_breadth_first(sp, snp) {
spin_lock_irq_rcu_node(snp);
cbs = false;
- if (snp >= sp->level[rcu_num_lvls - 1])
+ last_lvl = snp >= sp->level[rcu_num_lvls - 1];
+ if (last_lvl)
cbs = snp->srcu_have_cbs[idx] == gpseq;
snp->srcu_have_cbs[idx] = gpseq;
rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
@@ -572,7 +574,7 @@ static void srcu_gp_end(struct srcu_struct *sp)
srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);

/* Occasionally prevent srcu_data counter wrap. */
- if (!(gpseq & counter_wrap_check))
+ if (!(gpseq & counter_wrap_check) && last_lvl)
for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
sdp = per_cpu_ptr(sp->sda, cpu);
spin_lock_irqsave_rcu_node(sdp, flags);
--
2.5.2