From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965413AbaFSVFr (ORCPT ); Thu, 19 Jun 2014 17:05:47 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:46937 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965269AbaFSVFq (ORCPT ); Thu, 19 Jun 2014 17:05:46 -0400 Date: Thu, 19 Jun 2014 14:04:19 -0700 From: "Paul E. McKenney" To: Pranith Kumar Cc: josh@joshtriplett.org, davidshan@tencent.com, cl@linux.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/1] rcu: use __this_cpu_read helper instead of per_cpu_ptr(p, raw_smp_processor_id()) Message-ID: <20140619210419.GP4904@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <53A3443E.8010102@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53A3443E.8010102@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14061921-0928-0000-0000-000002D10FD2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). That was some time back, wasn't it? Anyway, yes, this does look quite a bit more convincing. Thanx, Paul > 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 > Signed-off-by: Shan Wei > Acked-by: Christoph Lameter > --- > 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 >