Re: [PATCH v9 11/13] writeback: make background writeback cgroup aware

[Greg Thelen <gthelen@google.com>]

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

> On Thu, 18 Aug 2011 00:10:56 -0700
> Greg Thelen <gthelen@google.com> wrote:
>
>> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:
>> 
>> > On Wed, 17 Aug 2011 09:15:03 -0700
>> > Greg Thelen <gthelen@google.com> wrote:
>> >
>> >> When the system is under background dirty memory threshold but some
>> >> cgroups are over their background dirty memory thresholds, then only
>> >> writeback inodes associated with the over-limit cgroups.
>> >> 
>> >> In addition to checking if the system dirty memory usage is over the
>> >> system background threshold, over_bground_thresh() now checks if any
>> >> cgroups are over their respective background dirty memory thresholds.
>> >> 
>> >> If over-limit cgroups are found, then the new
>> >> wb_writeback_work.for_cgroup field is set to distinguish between system
>> >> and memcg overages.  The new wb_writeback_work.shared_inodes field is
>> >> also set.  Inodes written by multiple cgroup are marked owned by
>> >> I_MEMCG_SHARED rather than a particular cgroup.  Such shared inodes
>> >> cannot easily be attributed to a cgroup, so per-cgroup writeback
>> >> (futures version of wakeup_flusher_threads and balance_dirty_pages)
>> >> performs suboptimally in the presence of shared inodes.  Therefore,
>> >> write shared inodes when performing cgroup background writeback.
>> >> 
>> >> If performing cgroup writeback, move_expired_inodes() skips inodes that
>> >> do not contribute dirty pages to the cgroup being written back.
>> >> 
>> >> After writing some pages, wb_writeback() will call
>> >> mem_cgroup_writeback_done() to update the set of over-bg-limits memcg.
>> >> 
>> >> This change also makes wakeup_flusher_threads() memcg aware so that
>> >> per-cgroup try_to_free_pages() is able to operate more efficiently
>> >> without having to write pages of foreign containers.  This change adds a
>> >> mem_cgroup parameter to wakeup_flusher_threads() to allow callers,
>> >> especially try_to_free_pages() and foreground writeback from
>> >> balance_dirty_pages(), to specify a particular cgroup to write inodes
>> >> from.
>> >> 
>> >> Signed-off-by: Greg Thelen <gthelen@google.com>
>> >> ---
>> >> Changelog since v8:
>> >> 
>> >> - Added optional memcg parameter to __bdi_start_writeback(),
>> >>   bdi_start_writeback(), wakeup_flusher_threads(), writeback_inodes_wb().
>> >> 
>> >> - move_expired_inodes() now uses pass in struct wb_writeback_work instead of
>> >>   struct writeback_control.
>> >> 
>> >> - Added comments to over_bground_thresh().
>> >> 
>> >>  fs/buffer.c               |    2 +-
>> >>  fs/fs-writeback.c         |   96 +++++++++++++++++++++++++++++++++-----------
>> >>  fs/sync.c                 |    2 +-
>> >>  include/linux/writeback.h |    6 ++-
>> >>  mm/backing-dev.c          |    3 +-
>> >>  mm/page-writeback.c       |    3 +-
>> >>  mm/vmscan.c               |    3 +-
>> >>  7 files changed, 84 insertions(+), 31 deletions(-)
>> >> 
>> >> diff --git a/fs/buffer.c b/fs/buffer.c
>> >> index dd0220b..da1fb23 100644
>> >> --- a/fs/buffer.c
>> >> +++ b/fs/buffer.c
>> >> @@ -293,7 +293,7 @@ static void free_more_memory(void)
>> >>  	struct zone *zone;
>> >>  	int nid;
>> >>  
>> >> -	wakeup_flusher_threads(1024);
>> >> +	wakeup_flusher_threads(1024, NULL);
>> >>  	yield();
>> >>  
>> >>  	for_each_online_node(nid) {
>> >> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> >> index e91fb82..ba55336 100644
>> >> --- a/fs/fs-writeback.c
>> >> +++ b/fs/fs-writeback.c
>> >> @@ -38,10 +38,14 @@ struct wb_writeback_work {
>> >>  	struct super_block *sb;
>> >>  	unsigned long *older_than_this;
>> >>  	enum writeback_sync_modes sync_mode;
>> >> +	unsigned short memcg_id;	/* If non-zero, then writeback specified
>> >> +					 * cgroup. */
>> >>  	unsigned int tagged_writepages:1;
>> >>  	unsigned int for_kupdate:1;
>> >>  	unsigned int range_cyclic:1;
>> >>  	unsigned int for_background:1;
>> >> +	unsigned int for_cgroup:1;	/* cgroup writeback */
>> >> +	unsigned int shared_inodes:1;	/* write inodes spanning cgroups */
>> >>  
>> >>  	struct list_head list;		/* pending work list */
>> >>  	struct completion *done;	/* set if the caller waits */
>> >> @@ -114,9 +118,12 @@ static void bdi_queue_work(struct backing_dev_info *bdi,
>> >>  	spin_unlock_bh(&bdi->wb_lock);
>> >>  }
>> >>  
>> >> +/*
>> >> + * @memcg is optional.  If set, then limit writeback to the specified cgroup.
>> >> + */
>> >>  static void
>> >>  __bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
>> >> -		      bool range_cyclic)
>> >> +		      bool range_cyclic, struct mem_cgroup *memcg)
>> >>  {
>> >>  	struct wb_writeback_work *work;
>> >>  
>> >> @@ -136,6 +143,8 @@ __bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
>> >>  	work->sync_mode	= WB_SYNC_NONE;
>> >>  	work->nr_pages	= nr_pages;
>> >>  	work->range_cyclic = range_cyclic;
>> >> +	work->memcg_id = memcg ? css_id(mem_cgroup_css(memcg)) : 0;
>> >> +	work->for_cgroup = memcg != NULL;
>> >>  
>> >
>> >
>> > I couldn't find a patch for mem_cgroup_css(NULL). Is it in patch 1-10 ?
>> > Other parts seems ok to me.
>> >
>> >
>> > Thanks,
>> > -Kame
>> 
>> Mainline commit d324236b3333e87c8825b35f2104184734020d35 adds
>> mem_cgroup_css() to memcontrol.c.  The above code does not call
>> mem_cgroup_css() with a NULL parameter due to the 'memcg ? ...' check.
>> So I do not think any additional changes to mem_cgroup_css() are needed.
>> Am I missing your point?
>> 
>
> I thought you need
> ==
> struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *mem)
> {
> +	if (!mem)
> +		return NULL;
>        return &mem->css;
> }
> ==
> And
> ==
> unsigned short css_id(struct cgroup_subsys_state *css)
> {
>         struct css_id *cssid;
>
> +	if (!css)
> 		return 0;
> }
> ==
>
> Thanks,
> -Kame

I think that your changes to mem_cgroup_css() and css_id() are
unnecessary for my patches because my patches do not call
mem_cgroup_css(NULL).  The "?" check below prevents NULL from being
passed into mem_cgroup_css():

+	work->memcg_id = memcg ? css_id(mem_cgroup_css(memcg)) : 0;

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