[PATCH 1/1] rcu: use __this_cpu_read helper instead of per_cpu_ptr(p, raw_smp_processor_id())

Pranith Kumar
2014-06-19T20:12:22Z

Use __this_cpu_read() instead of per_cpu_ptr() for optimized access.

Last time when Shan Wei posted this, you wanted before/after code for ARM and x86.
(http://lkml.iu.edu//hypermail/linux/kernel/1211.2/00498.html).

There are few other location which use per_cpu_ops instead of this_cpu_ops. I
can convert them accordingly if you are accept this :)

Using gcc (Ubuntu/Linaro 4.7.3-12ubuntu1) 4.7.3, I get (trimmed to relevant assembly, from make kernel/rcu/tree.s)

ARMv7 per_cpu_ptr():

force_quiescent_state:
mov r3, sp @,
bic r1, r3, #8128 @ tmp171,,
ldr r2, .L98 @ tmp169,
bic r1, r1, #63 @ tmp170, tmp171,
ldr r3, [r0, #220] @ __ptr, rsp_6(D)->rda
ldr r1, [r1, #20] @ D.35903_68->cpu, D.35903_68->cpu
mov r6, r0 @ rsp, rsp
ldr r2, [r2, r1, asl #2] @ tmp173, __per_cpu_offset
add r3, r3, r2 @ tmp175, __ptr, tmp173
ldr r5, [r3, #12] @ rnp_old, D.29162_13->mynode

ARMv7 using __this_cpu_read():

force_quiescent_state:
ldr r3, [r0, #220] @ rsp_7(D)->rda, rsp_7(D)->rda
mov r6, r0 @ rsp, rsp
add r3, r3, #12 @ __ptr, rsp_7(D)->rda,
ldr r5, [r2, r3] @ rnp_old, *D.29176_13

Using gcc 4.8.2:

x86_64 per_cpu_ptr():

movl %gs:cpu_number,%edx # cpu_number, pscr_ret__
movslq %edx, %rdx # pscr_ret__, pscr_ret__
movq __per_cpu_offset(,%rdx,8), %rdx # __per_cpu_offset, tmp93
movq %rdi, %r13 # rsp, rsp
movq 1000(%rdi), %rax # rsp_9(D)->rda, __ptr
movq 24(%rdx,%rax), %r12 # _15->mynode, rnp_old

x86_64 __this_cpu_read():

movq %rdi, %r13 # rsp, rsp
movq 1000(%rdi), %rax # rsp_9(D)->rda, rsp_9(D)->rda
movq %gs:24(%rax),%r12 # _10->mynode, rnp_old

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Shan Wei <davidshan@tencent.com>
Acked-by: Christoph Lameter <cl@linux.com>
---
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 f1ba773..c6de285 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2404,7 +2404,7 @@ static void force_quiescent_state(struct rcu_state *rsp)
struct rcu_node *rnp_old = NULL;

/* Funnel through hierarchy to reduce memory contention. */
- rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
+ rnp = __this_cpu_read(rsp->rda->mynode);
for (; rnp != NULL; rnp = rnp->parent) {
ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
!raw_spin_trylock(&rnp->fqslock);
--
2.0.0