On Thu, Jun 19, 2014 at 04:12:46PM -0400, Pranith Kumar wrote:
> 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 :)
Please do.
> 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.pr...@gmail.com>
> Signed-off-by: Shan Wei <davids...@tencent.com>
> Acked-by: Christoph Lameter <c...@linux.com>
Reviewed-by: Josh Triplett <j...@joshtriplett.org>
> ---
> 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
>
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