linux-kernel - Re: [PATCH 1/1] rcu: use __this_cpu_read helper instead of per_cpu_ptr(p, raw_smp_processor

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Date:	Thu, 19 Jun 2014 14:04:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	josh@...htriplett.org, davidshan@...cent.com, cl@...ux.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] rcu: use __this_cpu_read helper instead of
 per_cpu_ptr(p, raw_smp_processor_id())

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 <bobby.prani@...il.com>
> Signed-off-by: Shan Wei <davidshan@...cent.com>
> Acked-by: Christoph Lameter <cl@...ux.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
> 

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