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

[Minchan Kim]

Hi Greg,

On Fri, Jun 03, 2011 at 09:12:16AM -0700, Greg Thelen 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);
> +
> +		/*
> +		 * 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);

We are over the fg_thresh.
Then, why do you mark bg_thresh, too?


> +			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;

I am not sure this is really right condition to punish shared inodes.
We requested wbc with async. If bdi was congested, isn't it possible that
we can't write anyting in this turn?

If shared inodes are on different bdi, it would be effective but they are on
same bdi?

If you assume that shared inode case is rare in memcg configuration and it's
right assumption, I don't have a concern about it. But I have no idea.

> +			}
> +
> +			/*
> +			 * 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;
> +		}
> +
> +		/* 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.

dirty_info has return value 'bool'.
What's meaning? Of course, we can guess it by look the code.
But let's make user painful.
Please write down the menaing of return value, too.

> + *
> + * 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
> 

-- 
Kind regards
Minchan Kim
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