Re: [PATCH v8 10/12] memcg: create support routines for page-writeback

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 07 Jun 2011 08:58:16 -0700
Greg Thelen <gthelen@google.com> wrote:

> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:
> 
> > On Fri,  3 Jun 2011 09:12:16 -0700
> > Greg Thelen <gthelen@google.com> wrote:
> >
> >> Introduce memcg routines to assist in per-memcg dirty page management:
> >> 
> >> - mem_cgroup_balance_dirty_pages() walks a memcg hierarchy comparing
> >>   dirty memory usage against memcg foreground and background thresholds.
> >>   If an over-background-threshold memcg is found, then per-memcg
> >>   background writeback is queued.  Per-memcg writeback differs from
> >>   classic, non-memcg, per bdi writeback by setting the new
> >>   writeback_control.for_cgroup bit.
> >> 
> >>   If an over-foreground-threshold memcg is found, then foreground
> >>   writeout occurs.  When performing foreground writeout, first consider
> >>   inodes exclusive to the memcg.  If unable to make enough progress,
> >>   then consider inodes shared between memcg.  Such cross-memcg inode
> >>   sharing likely to be rare in situations that use per-cgroup memory
> >>   isolation.  The approach tries to handle the common (non-shared)
> >>   case well without punishing well behaved (non-sharing) cgroups.
> >>   As a last resort writeback shared inodes.
> >> 
> >>   This routine is used by balance_dirty_pages() in a later change.
> >> 
> >> - mem_cgroup_hierarchical_dirty_info() returns the dirty memory usage
> >>   and limits of the memcg closest to (or over) its dirty limit.  This
> >>   will be used by throttle_vm_writeout() in a latter change.
> >> 
> >> Signed-off-by: Greg Thelen <gthelen@google.com>
> >> ---
> >> Changelog since v7:
> >> - Add more detail to commit description.
> >> 
> >> - Declare the new writeback_control for_cgroup bit in this change, the
> >>   first patch that uses the new field is first used.  In -v7 the field
> >>   was declared in a separate patch.
> >> 
> >>  include/linux/memcontrol.h        |   18 +++++
> >>  include/linux/writeback.h         |    1 +
> >>  include/trace/events/memcontrol.h |   83 ++++++++++++++++++++
> >>  mm/memcontrol.c                   |  150 +++++++++++++++++++++++++++++++++++++
> >>  4 files changed, 252 insertions(+), 0 deletions(-)
> >> 
> >> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> >> index 3d72e09..0d0363e 100644
> >> --- a/include/linux/memcontrol.h
> >> +++ b/include/linux/memcontrol.h
> >> @@ -167,6 +167,11 @@ bool should_writeback_mem_cgroup_inode(struct inode *inode,
> >>  				       struct writeback_control *wbc);
> >>  bool mem_cgroups_over_bground_dirty_thresh(void);
> >>  void mem_cgroup_writeback_done(void);
> >> +bool mem_cgroup_hierarchical_dirty_info(unsigned long sys_available_mem,
> >> +					struct mem_cgroup *mem,
> >> +					struct dirty_info *info);
> >> +void mem_cgroup_balance_dirty_pages(struct address_space *mapping,
> >> +				    unsigned long write_chunk);
> >>  
> >>  unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
> >>  						gfp_t gfp_mask,
> >> @@ -383,6 +388,19 @@ static inline void mem_cgroup_writeback_done(void)
> >>  {
> >>  }
> >>  
> >> +static inline void mem_cgroup_balance_dirty_pages(struct address_space *mapping,
> >> +						  unsigned long write_chunk)
> >> +{
> >> +}
> >> +
> >> +static inline bool
> >> +mem_cgroup_hierarchical_dirty_info(unsigned long sys_available_mem,
> >> +				   struct mem_cgroup *mem,
> >> +				   struct dirty_info *info)
> >> +{
> >> +	return false;
> >> +}
> >> +
> >>  static inline
> >>  unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
> >>  					    gfp_t gfp_mask,
> >> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> >> index 66ec339..4f5c0d2 100644
> >> --- a/include/linux/writeback.h
> >> +++ b/include/linux/writeback.h
> >> @@ -47,6 +47,7 @@ struct writeback_control {
> >>  	unsigned for_reclaim:1;		/* Invoked from the page allocator */
> >>  	unsigned range_cyclic:1;	/* range_start is cyclic */
> >>  	unsigned more_io:1;		/* more io to be dispatched */
> >> +	unsigned for_cgroup:1;		/* enable cgroup writeback */
> >>  	unsigned shared_inodes:1;	/* write inodes spanning cgroups */
> >>  };
> >>  
> >> diff --git a/include/trace/events/memcontrol.h b/include/trace/events/memcontrol.h
> >> index 326a66b..b42dae1 100644
> >> --- a/include/trace/events/memcontrol.h
> >> +++ b/include/trace/events/memcontrol.h
> >> @@ -109,6 +109,89 @@ TRACE_EVENT(mem_cgroups_over_bground_dirty_thresh,
> >>  		  __entry->first_id)
> >>  )
> >>  
> >> +DECLARE_EVENT_CLASS(mem_cgroup_consider_writeback,
> >> +	TP_PROTO(unsigned short css_id,
> >> +		 struct backing_dev_info *bdi,
> >> +		 unsigned long nr_reclaimable,
> >> +		 unsigned long thresh,
> >> +		 bool over_limit),
> >> +
> >> +	TP_ARGS(css_id, bdi, nr_reclaimable, thresh, over_limit),
> >> +
> >> +	TP_STRUCT__entry(
> >> +		__field(unsigned short, css_id)
> >> +		__field(struct backing_dev_info *, bdi)
> >> +		__field(unsigned long, nr_reclaimable)
> >> +		__field(unsigned long, thresh)
> >> +		__field(bool, over_limit)
> >> +	),
> >> +
> >> +	TP_fast_assign(
> >> +		__entry->css_id = css_id;
> >> +		__entry->bdi = bdi;
> >> +		__entry->nr_reclaimable = nr_reclaimable;
> >> +		__entry->thresh = thresh;
> >> +		__entry->over_limit = over_limit;
> >> +	),
> >> +
> >> +	TP_printk("css_id=%d bdi=%p nr_reclaimable=%ld thresh=%ld "
> >> +		  "over_limit=%d", __entry->css_id, __entry->bdi,
> >> +		  __entry->nr_reclaimable, __entry->thresh, __entry->over_limit)
> >> +)
> >> +
> >> +#define DEFINE_MEM_CGROUP_CONSIDER_WRITEBACK_EVENT(name) \
> >> +DEFINE_EVENT(mem_cgroup_consider_writeback, name, \
> >> +	TP_PROTO(unsigned short id, \
> >> +		 struct backing_dev_info *bdi, \
> >> +		 unsigned long nr_reclaimable, \
> >> +		 unsigned long thresh, \
> >> +		 bool over_limit), \
> >> +	TP_ARGS(id, bdi, nr_reclaimable, thresh, over_limit) \
> >> +)
> >> +
> >> +DEFINE_MEM_CGROUP_CONSIDER_WRITEBACK_EVENT(mem_cgroup_consider_bg_writeback);
> >> +DEFINE_MEM_CGROUP_CONSIDER_WRITEBACK_EVENT(mem_cgroup_consider_fg_writeback);
> >> +
> >> +TRACE_EVENT(mem_cgroup_fg_writeback,
> >> +	TP_PROTO(unsigned long write_chunk,
> >> +		 struct writeback_control *wbc),
> >> +
> >> +	TP_ARGS(write_chunk, wbc),
> >> +
> >> +	TP_STRUCT__entry(
> >> +		__field(unsigned long, write_chunk)
> >> +		__field(long, wbc_to_write)
> >> +		__field(bool, shared_inodes)
> >> +	),
> >> +
> >> +	TP_fast_assign(
> >> +		__entry->write_chunk = write_chunk;
> >> +		__entry->wbc_to_write = wbc->nr_to_write;
> >> +		__entry->shared_inodes = wbc->shared_inodes;
> >> +	),
> >> +
> >> +	TP_printk("write_chunk=%ld nr_to_write=%ld shared_inodes=%d",
> >> +		  __entry->write_chunk,
> >> +		  __entry->wbc_to_write,
> >> +		  __entry->shared_inodes)
> >> +)
> >> +
> >> +TRACE_EVENT(mem_cgroup_enable_shared_writeback,
> >> +	TP_PROTO(unsigned short css_id),
> >> +
> >> +	TP_ARGS(css_id),
> >> +
> >> +	TP_STRUCT__entry(
> >> +		__field(unsigned short, css_id)
> >> +		),
> >> +
> >> +	TP_fast_assign(
> >> +		__entry->css_id = css_id;
> >> +		),
> >> +
> >> +	TP_printk("enabling shared writeback for memcg %d", __entry->css_id)
> >> +)
> >> +
> >>  #endif /* _TRACE_MEMCONTROL_H */
> >>  
> >>  /* This part must be outside protection */
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index a5b1794..17cb888 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -1622,6 +1622,156 @@ void mem_cgroup_writeback_done(void)
> >>  	}
> >>  }
> >>  
> >> +/*
> >> + * This routine must be called by processes which are generating dirty pages.
> >> + * It considers the dirty pages usage and thresholds of the current cgroup and
> >> + * (depending if hierarchical accounting is enabled) ancestral memcg.  If any of
> >> + * the considered memcg are over their background dirty limit, then background
> >> + * writeback is queued.  If any are over the foreground dirty limit then
> >> + * throttle the dirtying task while writing dirty data.  The per-memcg dirty
> >> + * limits check by this routine are distinct from either the per-system,
> >> + * per-bdi, or per-task limits considered by balance_dirty_pages().
> >> + */
> >> +void mem_cgroup_balance_dirty_pages(struct address_space *mapping,
> >> +				    unsigned long write_chunk)
> >> +{
> >> +	struct backing_dev_info *bdi = mapping->backing_dev_info;
> >> +	struct mem_cgroup *mem;
> >> +	struct mem_cgroup *ref_mem;
> >> +	struct dirty_info info;
> >> +	unsigned long nr_reclaimable;
> >> +	unsigned long sys_available_mem;
> >> +	unsigned long pause = 1;
> >> +	unsigned short id;
> >> +	bool over;
> >> +	bool shared_inodes;
> >> +
> >> +	if (mem_cgroup_disabled())
> >> +		return;
> >> +
> >> +	sys_available_mem = determine_dirtyable_memory();
> >> +
> >> +	/* reference the memcg so it is not deleted during this routine */
> >> +	rcu_read_lock();
> >> +	mem = mem_cgroup_from_task(current);
> >> +	if (mem && mem_cgroup_is_root(mem))
> >> +		mem = NULL;
> >> +	if (mem)
> >> +		css_get(&mem->css);
> >> +	rcu_read_unlock();
> >> +	ref_mem = mem;
> >> +
> >> +	/* balance entire ancestry of current's mem. */
> >> +	for (; mem_cgroup_has_dirty_limit(mem); mem = parent_mem_cgroup(mem)) {
> >> +		id = css_id(&mem->css);
> >> +
> >
> > Hmm, this sounds natural...but...don't we need to restart checking from ref_mem's
> > dirty_ratio once we find an ancestor is over dirty_ratio and we slept ?
> >
> > Even if parent's dirty ratio comes to be clean state, children's may not.
> > So, I think some "restart loop" jump after io_schedule_timeout().
> >
> > Thanks,
> > -Kame
> 
> I do not think that we need to restart, but maybe you have a case in
> mind that I am not considering.
> 
> Example hierarchy:
>               root
>          A            B
>      A1      A2
>   A11 A12  A21 A22
> 
> Assume that mem_cgroup_balance_dirty_pages(A11), so ref_mem=A11.
> 
> We start at A11 and walk up towards the root.  If A11 is over limit,
> then write A11 until under limit.  Next check A1, if over limit then
> write A1,A11,A12.  Then check A.  If A is over A limit, then we invoke
> writeback on A* until A is under A limit.  Are you concerned that while
> performing writeback on A* that other tasks may push A1 over the A1
> limit?  Such other task writers would also be calling
> mem_cgroup_balance_dirty_pages() later.
> 

Hm, ok. Could you add comments to explain the algorithm ?

Thanks,
-Kame


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