[PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Shan Wei]

From: Shan Wei <davidshan@tencent.com>

smp_processor_id is defined as raw_smp_processor_id.
replace per_cpu_ptr(p, raw_smp_processor_id()) is also ok.

Signed-off-by: Shan Wei <davidshan@tencent.com>
---
 kernel/rcutree.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 74df86b..3a21fcf 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1960,7 +1960,7 @@ 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_ptr(rsp->rda)->mynode;
 	for (; rnp != NULL; rnp = rnp->parent) {
 		ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
 		      !raw_spin_trylock(&rnp->fqslock);
-- 
1.7.1

Re: [PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Paul E. McKenney]

On Wed, Oct 31, 2012 at 07:23:03PM +0800, Shan Wei wrote:
> From: Shan Wei <davidshan@tencent.com>
> 
> smp_processor_id is defined as raw_smp_processor_id.
> replace per_cpu_ptr(p, raw_smp_processor_id()) is also ok.
> 
> Signed-off-by: Shan Wei <davidshan@tencent.com>

Hello, Shan Wei,

There are several definitions of this_cpu_ptr():

0 percpu.h 63 #define this_cpu_ptr(ptr) SHIFT_PERCPU_PTR(ptr, my_cpu_offset)
1 percpu.h 65 #define this_cpu_ptr(ptr) __this_cpu_ptr(ptr)
2 percpu.h 85 #define this_cpu_ptr(ptr) per_cpu_ptr(ptr, 0)

The first uses my_cpu_offset, which is further defined in two ways:

0 percpu.h 33 #define my_cpu_offset per_cpu_offset(smp_processor_id())
1 percpu.h 35 #define my_cpu_offset __my_cpu_offset

The first uses smp_processor_id(), which will complain if
force_quiescent_state() is called with preemption disabled, which it
sometimes is.

So what am I missing here?

							Thanx, Paul

> ---
>  kernel/rcutree.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 74df86b..3a21fcf 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1960,7 +1960,7 @@ 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_ptr(rsp->rda)->mynode;
>  	for (; rnp != NULL; rnp = rnp->parent) {
>  		ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
>  		      !raw_spin_trylock(&rnp->fqslock);
> -- 
> 1.7.1
> 

Re: [PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Shan Wei]

Paul E. McKenney said, at 2012/10/31 19:51:
> 
> The first uses smp_processor_id(), which will complain if
> force_quiescent_state() is called with preemption disabled, which it
> sometimes is.
> 
> So what am I missing here?

Hi Paul

this patch is not right for CONFIG_DEBUG_PREEMPT case.
__this_cpu_ptr is ok which do not check for preemption context.


> 
> 							Thanx, Paul

Re: [PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Paul E. McKenney]

On Wed, Oct 31, 2012 at 09:20:19PM +0800, Shan Wei wrote:
> Paul E. McKenney said, at 2012/10/31 19:51:
> > 
> > The first uses smp_processor_id(), which will complain if
> > force_quiescent_state() is called with preemption disabled, which it
> > sometimes is.
> > 
> > So what am I missing here?
> 
> Hi Paul
> 
> this patch is not right for CONFIG_DEBUG_PREEMPT case.
> __this_cpu_ptr is ok which do not check for preemption context.

Ah, got it.  Please feel free to submit an updated patch.

							Thanx, Paul

Re: [PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Christoph Lameter]

On Wed, 31 Oct 2012, Shan Wei wrote:

> Signed-off-by: Shan Wei <davidshan@tencent.com>
> ---
>  kernel/rcutree.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 74df86b..3a21fcf 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1960,7 +1960,7 @@ 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_ptr(rsp->rda)->mynode;

Do

	rnp = this_cpu_read(rsp->rda->mynode);

instad.

Re: [PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Paul E. McKenney]

On Wed, Oct 31, 2012 at 05:47:04PM +0000, Christoph Lameter wrote:
> On Wed, 31 Oct 2012, Shan Wei wrote:
> 
> > Signed-off-by: Shan Wei <davidshan@tencent.com>
> > ---
> >  kernel/rcutree.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 74df86b..3a21fcf 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1960,7 +1960,7 @@ 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_ptr(rsp->rda)->mynode;
> 
> Do
> 
> 	rnp = this_cpu_read(rsp->rda->mynode);
> 
> instad.

One thing to keep in mind -- the only purpose of this is to diffuse
memory contention.  So there is no need to disable preemption.

							Thanx, Paul

Re: [PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Shan Wei]

Paul E. McKenney said, at 2012/11/1 4:08:
> 
> One thing to keep in mind -- the only purpose of this is to diffuse
> memory contention.  So there is no need to disable preemption.

same question to me.
Christoph, maybe __this_cpu_read is a better choice which don't disable preemption.

> 
> 							Thanx, Paul
> 
> 

Re: [PATCH 7/9] rcu: use this_cpu_ptr per-cpu helper instead of per_cpu_ptr(p, raw_smp_processor_id())

[Christoph Lameter]

On Thu, 1 Nov 2012, Shan Wei wrote:

> Paul E. McKenney said, at 2012/11/1 4:08:
> >
> > One thing to keep in mind -- the only purpose of this is to diffuse
> > memory contention.  So there is no need to disable preemption.
>
> same question to me.
> Christoph, maybe __this_cpu_read is a better choice which don't disable preemption.

Correct. Use __this_cpu_read to reduce overhead on processor that require
emulation.