[PATCH] [RT] avoid preemption in memory controller code

[PATCH] [RT] avoid preemption in memory controller code

[Tim Blechmann]

the lru_lock of struct mem_group_per_zone is used to avoid preemption
during the __mem_cgroup_stat_add_safe function.
therefore, a raw_spinlock_t should be used.

Signed-off-by: Tim Blechmann <tim@klingt.org>
---
 mm/memcontrol.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 517f945..8661732 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -95,9 +95,9 @@ enum mem_cgroup_zstat_index {
 
 struct mem_cgroup_per_zone {
 	/*
-	 * spin_lock to protect the per cgroup LRU
+	 * raw_spin_lock to protect the per cgroup LRU
 	 */
-	spinlock_t		lru_lock;
+	raw_spinlock_t		lru_lock;
 	struct list_head	active_list;
 	struct list_head	inactive_list;
 	unsigned long count[NR_MEM_CGROUP_ZSTAT];
-- 
1.5.6.3

Re: [PATCH] [RT] avoid preemption in memory controller code

[Steven Rostedt]

Hi Tim,

On Mon, 8 Dec 2008, Tim Blechmann wrote:

> the lru_lock of struct mem_group_per_zone is used to avoid preemption
> during the __mem_cgroup_stat_add_safe function.
> therefore, a raw_spinlock_t should be used.

What is the reason that this must avoid preemption? Is there another
way to solve this? I rather not just add a raw spinlock if we can
help it.

-- Steve

> 
> Signed-off-by: Tim Blechmann <tim@klingt.org>
> ---
>  mm/memcontrol.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 517f945..8661732 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -95,9 +95,9 @@ enum mem_cgroup_zstat_index {
>  
>  struct mem_cgroup_per_zone {
>  	/*
> -	 * spin_lock to protect the per cgroup LRU
> +	 * raw_spin_lock to protect the per cgroup LRU
>  	 */
> -	spinlock_t		lru_lock;
> +	raw_spinlock_t		lru_lock;
>  	struct list_head	active_list;
>  	struct list_head	inactive_list;
>  	unsigned long count[NR_MEM_CGROUP_ZSTAT];
> -- 
> 1.5.6.3
> 
> 
> 

Re: [PATCH] [RT] avoid preemption in memory controller code

[Tim Blechmann]

[-- Attachment #1: Type: text/plain, Size: 680 bytes --]

> > the lru_lock of struct mem_group_per_zone is used to avoid preemption
> > during the __mem_cgroup_stat_add_safe function.
> > therefore, a raw_spinlock_t should be used.
> 
> What is the reason that this must avoid preemption? 

it guards a call to smp_processor_id() in __mem_cgroup_stat_add_safe().
see http://article.gmane.org/gmane.linux.rt.user/3690

> Is there another
> way to solve this? I rather not just add a raw spinlock if we can
> help it.

not sure, maybe one can disable preemption for that specific function?

tim

--
tim@klingt.org
http://tim.klingt.org

Art is either a complaint or do something else
  John Cage quoting Jasper Johns

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 197 bytes --]

Re: [PATCH] [RT] avoid preemption in memory controller code

[Steven Rostedt]

On Mon, 8 Dec 2008, Tim Blechmann wrote:

> > > the lru_lock of struct mem_group_per_zone is used to avoid preemption
> > > during the __mem_cgroup_stat_add_safe function.
> > > therefore, a raw_spinlock_t should be used.
> > 
> > What is the reason that this must avoid preemption? 
> 
> it guards a call to smp_processor_id() in __mem_cgroup_stat_add_safe().
> see http://article.gmane.org/gmane.linux.rt.user/3690

We could simply create a 
	preempt_disable_rt();
that will only disable preemption when CONFIG_PREEMPT_RT is set.
and then we could add

	int cpu;

	preempt_disable_rt();
	cpu = smp_processor_id();
	stat->cpustat[cpu].count[idx] += val;
	preempt_enable_rt();

Or something similar.

-- Steve

> 
> > Is there another
> > way to solve this? I rather not just add a raw spinlock if we can
> > help it.
> 
> not sure, maybe one can disable preemption for that specific function?
> 
> tim
> 
> --
> tim@klingt.org
> http://tim.klingt.org
> 
> Art is either a complaint or do something else
>   John Cage quoting Jasper Johns
> 

Re: [PATCH] [RT] avoid preemption in memory controller code

[Li Zefan]

CC: Balbir Singh <balbir@linux.vnet.ibm.com>
CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Tim Blechmann wrote:
> the lru_lock of struct mem_group_per_zone is used to avoid preemption
> during the __mem_cgroup_stat_add_safe function.
> therefore, a raw_spinlock_t should be used.
> 

FYI, this lru_lock no longer exists in -mm tree. The following patch
removes that lock:

	http://marc.info/?l=linux-mm&m=122665814801979&w=2

> Signed-off-by: Tim Blechmann <tim@klingt.org>
> ---
>  mm/memcontrol.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 517f945..8661732 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -95,9 +95,9 @@ enum mem_cgroup_zstat_index {
>  
>  struct mem_cgroup_per_zone {
>  	/*
> -	 * spin_lock to protect the per cgroup LRU
> +	 * raw_spin_lock to protect the per cgroup LRU
>  	 */
> -	spinlock_t		lru_lock;
> +	raw_spinlock_t		lru_lock;
>  	struct list_head	active_list;
>  	struct list_head	inactive_list;
>  	unsigned long count[NR_MEM_CGROUP_ZSTAT];

Re: [PATCH] [RT] avoid preemption in memory controller code

[KAMEZAWA Hiroyuki]

On Tue, 09 Dec 2008 09:13:12 +0800
Li Zefan <lizf@cn.fujitsu.com> wrote:

> CC: Balbir Singh <balbir@linux.vnet.ibm.com>
> CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Tim Blechmann wrote:
> > the lru_lock of struct mem_group_per_zone is used to avoid preemption
> > during the __mem_cgroup_stat_add_safe function.
> > therefore, a raw_spinlock_t should be used.
> > 
> 
> FYI, this lru_lock no longer exists in -mm tree. The following patch
> removes that lock:
> 
> 	http://marc.info/?l=linux-mm&m=122665814801979&w=2
> 
> > Signed-off-by: Tim Blechmann <tim@klingt.org>
> > ---
> >  mm/memcontrol.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 517f945..8661732 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -95,9 +95,9 @@ enum mem_cgroup_zstat_index {
> >  
> >  struct mem_cgroup_per_zone {
> >  	/*
> > -	 * spin_lock to protect the per cgroup LRU
> > +	 * raw_spin_lock to protect the per cgroup LRU
> >  	 */
> > -	spinlock_t		lru_lock;
> > +	raw_spinlock_t		lru_lock;
> >  	struct list_head	active_list;
> >  	struct list_head	inactive_list;
> >  	unsigned long count[NR_MEM_CGROUP_ZSTAT];

yes..I remvoed that. Please see mmotm and subscribe linux-mm, 
if you have interests  memory resource controller.

Anyway, thanks.

-Kame

[PATCH] [RT] preempt_disable_rt for CONFIG_PREEMPT_RT

[Tim Blechmann]

in some cases, spinlocks are used to avoid preemption. does not work
correctly, when CONFIG_PREEMPT_RT is enabled.

therefore two new macros are introduced:
preempt_disable_rt/preempt_disable_rt behave like their equivalents, if
CONFIG_PREEMPT_RT is enabled, and as noops otherwise.

Signed-off-by: Tim Blechmann <tim@klingt.org>
---
 include/linux/preempt.h |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 72b1a10..2526170 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -93,6 +93,16 @@ do { \
 
 #endif
 
+#ifdef CONFIG_PREEMPT_RT
+#define preempt_disable_rt	preempt_disable
+#define preempt_enable_rt	preempt_enable
+#else
+#define
+#define preempt_disable_rt	do { } while (0)
+#define preempt_enable_rt	do { } while (0)
+#endif
+
+
 #ifdef CONFIG_PREEMPT_NOTIFIERS
 
 struct preempt_notifier;
-- 
1.5.6.3