Re: [PATCH 16/17] writeback: trace event writeback_single_inode

[Dave Chinner <david@fromorbit.com>]

On Fri, May 13, 2011 at 01:37:00PM +0800, Wu Fengguang wrote:
> On Fri, May 13, 2011 at 07:20:51AM +0800, Dave Chinner wrote:
> > On Thu, May 12, 2011 at 09:57:22PM +0800, Wu Fengguang wrote:
> > > It is valuable to know how the dirty inodes are iterated and their IO size.
> > > 
> > > "writeback_single_inode: bdi 8:0: ino=134246746 state=I_DIRTY_SYNC|I_SYNC age=414 index=0 to_write=1024 wrote=0"
> > > 
> > > - "state" reflects inode->i_state at the end of writeback_single_inode()
> > > - "index" reflects mapping->writeback_index after the ->writepages() call
> > > - "to_write" is the wbc->nr_to_write at entrance of writeback_single_inode()
> > > - "wrote" is the number of pages actually written
> > > 
> > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > >  fs/fs-writeback.c                |    6 ++-
> > >  include/trace/events/writeback.h |   56 +++++++++++++++++++++++++++++
> > >  2 files changed, 61 insertions(+), 1 deletion(-)
> > > 
> > > --- linux-next.orig/include/trace/events/writeback.h	2011-05-12 11:46:27.000000000 +0800
> > > +++ linux-next/include/trace/events/writeback.h	2011-05-12 11:48:55.000000000 +0800
> > > @@ -8,6 +8,19 @@
> > >  #include <linux/device.h>
> > >  #include <linux/writeback.h>
> > >  
> > > +#define show_inode_state(state)					\
> > > +	__print_flags(state, "|",				\
> > > +		{I_DIRTY_SYNC,		"I_DIRTY_SYNC"},	\
> > > +		{I_DIRTY_DATASYNC,	"I_DIRTY_DATASYNC"},	\
> > > +		{I_DIRTY_PAGES,		"I_DIRTY_PAGES"},	\
> > > +		{I_NEW,			"I_NEW"},		\
> > > +		{I_WILL_FREE,		"I_WILL_FREE"},		\
> > > +		{I_FREEING,		"I_FREEING"},		\
> > > +		{I_CLEAR,		"I_CLEAR"},		\
> > > +		{I_SYNC,		"I_SYNC"},		\
> > > +		{I_REFERENCED,		"I_REFERENCED"}		\
> > > +	)
> > > +
> > >  struct wb_writeback_work;
> > >  
> > >  DECLARE_EVENT_CLASS(writeback_work_class,
> > > @@ -201,6 +214,49 @@ DEFINE_EVENT(writeback_congest_waited_te
> > >  	TP_ARGS(usec_timeout, usec_delayed)
> > >  );
> > >  
> > > +TRACE_EVENT(writeback_single_inode,
> > > +
> > > +	TP_PROTO(struct inode *inode,
> > > +		 struct writeback_control *wbc,
> > > +		 unsigned long nr_to_write
> > > +	),
> > > +
> > > +	TP_ARGS(inode, wbc, nr_to_write),
> > > +
> > > +	TP_STRUCT__entry(
> > > +		__array(char, name, 32)
> > > +		__field(unsigned long, ino)
> > > +		__field(unsigned long, state)
> > > +		__field(unsigned long, age)
> > > +		__field(unsigned long, writeback_index)
> > > +		__field(long, nr_to_write)
> > > +		__field(unsigned long, wrote)
> > > +	),
> > > +
> > > +	TP_fast_assign(
> > > +		strncpy(__entry->name,
> > > +			dev_name(inode->i_mapping->backing_dev_info->dev), 32);
> > > +		__entry->ino		= inode->i_ino;
> > > +		__entry->state		= inode->i_state;
> > > +		__entry->age		= (jiffies - inode->dirtied_when) *
> > > +								1000 / HZ;
> > > +		__entry->writeback_index = inode->i_mapping->writeback_index;
> > > +		__entry->nr_to_write	= nr_to_write;
> > > +		__entry->wrote		= nr_to_write - wbc->nr_to_write;
> > > +	),
> > > +
> > > +	TP_printk("bdi %s: ino=%lu state=%s age=%lu "
> > > +		  "index=%lu to_write=%ld wrote=%lu",
> > > +		  __entry->name,
> > > +		  __entry->ino,
> > > +		  show_inode_state(__entry->state),
> > > +		  __entry->age,
> > > +		  __entry->writeback_index,
> > > +		  __entry->nr_to_write,
> > > +		  __entry->wrote
> > > +	)
> > > +);
> > > +
> > >  #endif /* _TRACE_WRITEBACK_H */
> > >  
> > >  /* This part must be outside protection */
> > > --- linux-next.orig/fs/fs-writeback.c	2011-05-12 11:46:27.000000000 +0800
> > > +++ linux-next/fs/fs-writeback.c	2011-05-12 11:48:55.000000000 +0800
> > > @@ -356,6 +356,7 @@ writeback_single_inode(struct inode *ino
> > >  		       struct writeback_control *wbc)
> > >  {
> > >  	struct address_space *mapping = inode->i_mapping;
> > > +	long nr_to_write = wbc->nr_to_write;
> > >  	unsigned dirty;
> > >  	int ret;
> > >  
> > > @@ -378,7 +379,8 @@ writeback_single_inode(struct inode *ino
> > >  		 */
> > >  		if (wbc->sync_mode != WB_SYNC_ALL) {
> > >  			requeue_io(inode, wb);
> > > -			return 0;
> > > +			ret = 0;
> > > +			goto out;
> > >  		}
> > >  
> > >  		/*
> > > @@ -476,6 +478,8 @@ writeback_single_inode(struct inode *ino
> > >  		}
> > >  	}
> > >  	inode_sync_complete(inode);
> > > +out:
> > > +	trace_writeback_single_inode(inode, wbc, nr_to_write);
> > >  	return ret;
> > >  }
> > 
> > I think I'd prefer two separate trace calls rather than jumping to
> > one. That is, a trace_writeback_single_inode_requeue() event and a
> > trace_writeback_single_inode_done() event so we can see the separate
> > conditions in the trace....
> 
> The requeue events are likely to disappear when IO-less
> balance_dirty_pages() are upstream. Do you still feel like adding a
> trace for it?

Requeue events are not specific to balance_dirty_pages() based
writeback. background/kupdate writeback can trigger them because a
sync write, fsync or sync is in progress. So yes, please add a
separate trace for it.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com