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

[KAMEZAWA Hiroyuki]

On Fri,  3 Jun 2011 09:12:16 -0700
Greg Thelen <gthelen@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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


> +		/*
> +		 * keep throttling and writing inode data so long as mem is over
> +		 * its dirty limit.
> +		 */
> +		for (shared_inodes = false; ; ) {
> +			struct writeback_control wbc = {
> +				.sync_mode	= WB_SYNC_NONE,
> +				.older_than_this = NULL,
> +				.range_cyclic	= 1,
> +				.for_cgroup	= 1,
> +				.nr_to_write	= write_chunk,
> +				.shared_inodes	= shared_inodes,
> +			};
> +
> +			/*
> +			 * if mem is under dirty limit, then break from
> +			 * throttling loop.
> +			 */
> +			mem_cgroup_dirty_info(sys_available_mem, mem, &info);
> +			nr_reclaimable = dirty_info_reclaimable(&info);
> +			over = nr_reclaimable > info.dirty_thresh;
> +			trace_mem_cgroup_consider_fg_writeback(
> +				id, bdi, nr_reclaimable, info.dirty_thresh,
> +				over);
> +			if (!over)
> +				break;
> +
> +			mem_cgroup_mark_over_bg_thresh(mem);
> +			writeback_inodes_wb(&bdi->wb, &wbc);
> +			trace_mem_cgroup_fg_writeback(write_chunk, &wbc);
> +			/* if no progress, then consider shared inodes */
> +			if ((wbc.nr_to_write == write_chunk) &&
> +			    !shared_inodes) {
> +				trace_mem_cgroup_enable_shared_writeback(id);
> +				shared_inodes = true;
> +			}
> +
> +			/*
> +			 * Sleep up to 100ms to throttle writer and wait for
> +			 * queued background I/O to complete.
> +			 */
> +			__set_current_state(TASK_UNINTERRUPTIBLE);
> +			io_schedule_timeout(pause);
> +			pause <<= 1;
> +			if (pause > HZ / 10)
> +				pause = HZ / 10;

Hmm, is this exponential back off "pause" is from mm/page-writeback.c ?
I'm happy if we can have a shared code. (But ok if it adds some messy.)



> +		}
> +
> +		/* if mem is over background limit, then queue bg writeback */
> +		over = nr_reclaimable >= info.background_thresh;
> +		trace_mem_cgroup_consider_bg_writeback(
> +			id, bdi, nr_reclaimable, info.background_thresh,
> +			over);
> +		if (over)
> +			mem_cgroup_queue_bg_writeback(mem, bdi);
> +	}
> +
> +	if (ref_mem)
> +		css_put(&ref_mem->css);
> +}
> +
> +/*
> + * Return the dirty thresholds and usage for the mem (within the ancestral chain
> + * of @mem) closest to its dirty limit or the first memcg over its limit.
> + *
> + * The check is not stable because the usage and limits can change asynchronous
> + * to this routine.
> + */
> +bool mem_cgroup_hierarchical_dirty_info(unsigned long sys_available_mem,
> +					struct mem_cgroup *mem,
> +					struct dirty_info *info)
> +{
> +	unsigned long usage;
> +	struct dirty_info uninitialized_var(cur_info);
> +
> +	if (mem_cgroup_disabled())
> +		return false;
> +
> +	info->nr_writeback = ULONG_MAX;  /* invalid initial value */
> +
> +	/* walk up hierarchy enabled parents */
> +	for (; mem_cgroup_has_dirty_limit(mem); mem = parent_mem_cgroup(mem)) {
> +		mem_cgroup_dirty_info(sys_available_mem, mem, &cur_info);
> +		usage = dirty_info_reclaimable(&cur_info) +
> +			cur_info.nr_writeback;
> +
> +		/* if over limit, stop searching */
> +		if (usage >= cur_info.dirty_thresh) {
> +			*info = cur_info;
> +			break;
> +		}
> +
> +		/*
> +		 * Save dirty usage of mem closest to its limit if either:
> +		 *     - mem is the first mem considered
> +		 *     - mem dirty margin is smaller than last recorded one
> +		 */
> +		if ((info->nr_writeback == ULONG_MAX) ||
> +		    (cur_info.dirty_thresh - usage) <
> +		    (info->dirty_thresh -
> +		     (dirty_info_reclaimable(info) + info->nr_writeback)))
> +			*info = cur_info;
> +	}
> +
> +	return info->nr_writeback != ULONG_MAX;
> +}
> +
>  static void mem_cgroup_start_move(struct mem_cgroup *mem)
>  {
>  	int cpu;
> -- 
> 1.7.3.1
> 

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