Re: [PATCH 07/10] memcg: add dirty limits to mem_cgroup

[Greg Thelen <gthelen@google.com>]

KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:

> On Tue, 05 Oct 2010 12:00:17 -0700
> Greg Thelen <gthelen@google.com> wrote:
>
>> Andrea Righi <arighi@develer.com> writes:
>> 
>> > On Sun, Oct 03, 2010 at 11:58:02PM -0700, Greg Thelen wrote:
>> >> Extend mem_cgroup to contain dirty page limits.  Also add routines
>> >> allowing the kernel to query the dirty usage of a memcg.
>> >> 
>> >> These interfaces not used by the kernel yet.  A subsequent commit
>> >> will add kernel calls to utilize these new routines.
>> >
>> > A small note below.
>> >
>> >> 
>> >> Signed-off-by: Greg Thelen <gthelen@google.com>
>> >> Signed-off-by: Andrea Righi <arighi@develer.com>
>> >> ---
>> >>  include/linux/memcontrol.h |   44 +++++++++++
>> >>  mm/memcontrol.c            |  180 +++++++++++++++++++++++++++++++++++++++++++-
>> >>  2 files changed, 223 insertions(+), 1 deletions(-)
>> >> 
>> >> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> >> index 6303da1..dc8952d 100644
>> >> --- a/include/linux/memcontrol.h
>> >> +++ b/include/linux/memcontrol.h
>> >> @@ -19,6 +19,7 @@
>> >>  
>> >>  #ifndef _LINUX_MEMCONTROL_H
>> >>  #define _LINUX_MEMCONTROL_H
>> >> +#include <linux/writeback.h>
>> >>  #include <linux/cgroup.h>
>> >>  struct mem_cgroup;
>> >>  struct page_cgroup;
>> >> @@ -33,6 +34,30 @@ enum mem_cgroup_write_page_stat_item {
>> >>  	MEMCG_NR_FILE_UNSTABLE_NFS, /* # of NFS unstable pages */
>> >>  };
>> >>  
>> >> +/* Cgroup memory statistics items exported to the kernel */
>> >> +enum mem_cgroup_read_page_stat_item {
>> >> +	MEMCG_NR_DIRTYABLE_PAGES,
>> >> +	MEMCG_NR_RECLAIM_PAGES,
>> >> +	MEMCG_NR_WRITEBACK,
>> >> +	MEMCG_NR_DIRTY_WRITEBACK_PAGES,
>> >> +};
>> >> +
>> >> +/* Dirty memory parameters */
>> >> +struct vm_dirty_param {
>> >> +	int dirty_ratio;
>> >> +	int dirty_background_ratio;
>> >> +	unsigned long dirty_bytes;
>> >> +	unsigned long dirty_background_bytes;
>> >> +};
>> >> +
>> >> +static inline void get_global_vm_dirty_param(struct vm_dirty_param *param)
>> >> +{
>> >> +	param->dirty_ratio = vm_dirty_ratio;
>> >> +	param->dirty_bytes = vm_dirty_bytes;
>> >> +	param->dirty_background_ratio = dirty_background_ratio;
>> >> +	param->dirty_background_bytes = dirty_background_bytes;
>> >> +}
>> >> +
>> >>  extern unsigned long mem_cgroup_isolate_pages(unsigned long nr_to_scan,
>> >>  					struct list_head *dst,
>> >>  					unsigned long *scanned, int order,
>> >> @@ -145,6 +170,10 @@ static inline void mem_cgroup_dec_page_stat(struct page *page,
>> >>  	mem_cgroup_update_page_stat(page, idx, -1);
>> >>  }
>> >>  
>> >> +bool mem_cgroup_has_dirty_limit(void);
>> >> +void get_vm_dirty_param(struct vm_dirty_param *param);
>> >> +s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item);
>> >> +
>> >>  unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
>> >>  						gfp_t gfp_mask);
>> >>  u64 mem_cgroup_get_limit(struct mem_cgroup *mem);
>> >> @@ -326,6 +355,21 @@ static inline void mem_cgroup_dec_page_stat(struct page *page,
>> >>  {
>> >>  }
>> >>  
>> >> +static inline bool mem_cgroup_has_dirty_limit(void)
>> >> +{
>> >> +	return false;
>> >> +}
>> >> +
>> >> +static inline void get_vm_dirty_param(struct vm_dirty_param *param)
>> >> +{
>> >> +	get_global_vm_dirty_param(param);
>> >> +}
>> >> +
>> >> +static inline s64 mem_cgroup_page_stat(enum mem_cgroup_read_page_stat_item item)
>> >> +{
>> >> +	return -ENOSYS;
>> >> +}
>> >> +
>> >>  static inline
>> >>  unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
>> >>  					    gfp_t gfp_mask)
>> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> >> index f40839f..6ec2625 100644
>> >> --- a/mm/memcontrol.c
>> >> +++ b/mm/memcontrol.c
>> >> @@ -233,6 +233,10 @@ struct mem_cgroup {
>> >>  	atomic_t	refcnt;
>> >>  
>> >>  	unsigned int	swappiness;
>> >> +
>> >> +	/* control memory cgroup dirty pages */
>> >> +	struct vm_dirty_param dirty_param;
>> >> +
>> >>  	/* OOM-Killer disable */
>> >>  	int		oom_kill_disable;
>> >>  
>> >> @@ -1132,6 +1136,172 @@ static unsigned int get_swappiness(struct mem_cgroup *memcg)
>> >>  	return swappiness;
>> >>  }
>> >>  
>> >> +/*
>> >> + * Returns a snapshot of the current dirty limits which is not synchronized with
>> >> + * the routines that change the dirty limits.  If this routine races with an
>> >> + * update to the dirty bytes/ratio value, then the caller must handle the case
>> >> + * where both dirty_[background_]_ratio and _bytes are set.
>> >> + */
>> >> +static void __mem_cgroup_get_dirty_param(struct vm_dirty_param *param,
>> >> +					 struct mem_cgroup *mem)
>> >> +{
>> >> +	if (mem && !mem_cgroup_is_root(mem)) {
>> >> +		param->dirty_ratio = mem->dirty_param.dirty_ratio;
>> >> +		param->dirty_bytes = mem->dirty_param.dirty_bytes;
>> >> +		param->dirty_background_ratio =
>> >> +			mem->dirty_param.dirty_background_ratio;
>> >> +		param->dirty_background_bytes =
>> >> +			mem->dirty_param.dirty_background_bytes;
>> >> +	} else {
>> >> +		get_global_vm_dirty_param(param);
>> >> +	}
>> >> +}
>> >> +
>> >> +/*
>> >> + * Get dirty memory parameters of the current memcg or global values (if memory
>> >> + * cgroups are disabled or querying the root cgroup).
>> >> + */
>> >> +void get_vm_dirty_param(struct vm_dirty_param *param)
>> >> +{
>> >> +	struct mem_cgroup *memcg;
>> >> +
>> >> +	if (mem_cgroup_disabled()) {
>> >> +		get_global_vm_dirty_param(param);
>> >> +		return;
>> >> +	}
>> >> +
>> >> +	/*
>> >> +	 * It's possible that "current" may be moved to other cgroup while we
>> >> +	 * access cgroup. But precise check is meaningless because the task can
>> >> +	 * be moved after our access and writeback tends to take long time.  At
>> >> +	 * least, "memcg" will not be freed under rcu_read_lock().
>> >> +	 */
>> >> +	rcu_read_lock();
>> >> +	memcg = mem_cgroup_from_task(current);
>> >> +	__mem_cgroup_get_dirty_param(param, memcg);
>> >> +	rcu_read_unlock();
>> >> +}
>> >> +
>> >> +/*
>> >> + * Check if current memcg has local dirty limits.  Return true if the current
>> >> + * memory cgroup has local dirty memory settings.
>> >> + */
>> >> +bool mem_cgroup_has_dirty_limit(void)
>> >> +{
>> >> +	struct mem_cgroup *mem;
>> >> +
>> >> +	if (mem_cgroup_disabled())
>> >> +		return false;
>> >> +
>> >> +	mem = mem_cgroup_from_task(current);
>> >> +	return mem && !mem_cgroup_is_root(mem);
>> >> +}
>> >
>> > We only check the pointer without dereferencing it, so this is probably
>> > ok, but maybe this is safer:
>> >
>> > bool mem_cgroup_has_dirty_limit(void)
>> > {
>> > 	struct mem_cgroup *mem;
>> > 	bool ret;
>> >
>> > 	if (mem_cgroup_disabled())
>> > 		return false;
>> >
>> > 	rcu_read_lock();
>> > 	mem = mem_cgroup_from_task(current);
>> > 	ret = mem && !mem_cgroup_is_root(mem);
>> > 	rcu_read_unlock();
>> >
>> > 	return ret;
>> > }
>> >
>> > rcu_read_lock() should be held in mem_cgroup_from_task(), otherwise
>> > lockdep could detect this as an error.
>> >
>> > Thanks,
>> > -Andrea
>> 
>> Good suggestion.  I agree that lockdep might catch this.  There are some
>> unrelated debug_locks failures (even without my patches) that I worked
>> around to get lockdep to complain about this one.  I applied your
>> suggested fix and lockdep was happy.  I will incorporate this fix into
>> the next revision of the patch series.
>> 
>
> Hmm, considering other parts, shouldn't we define mem_cgroup_from_task
> as macro ?
>
> Thanks,
> -Kame

Is your motivation to increase performance with the same functionality?
If so, then would a 'static inline' be performance equivalent to a
preprocessor macro yet be safer to use?

Maybe it makes more sense to find a way to perform this check in
mem_cgroup_has_dirty_limit() without needing to grab the rcu lock.  I
think this lock grab is unneeded.  I am still collecting performance
data, but suspect that this may be making the code slower than it needs
to be.

--
Greg

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