Re: [patch]block: document blk_plug

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 02 Aug 2011 08:46:10 +0800
Shaohua Li <shaohua.li@intel.com> wrote:

> On Fri, 2011-07-29 at 20:14 +0800, Suresh Jayaraman wrote:
> > On 07/29/2011 08:43 AM, Shaohua Li wrote:
> > > Andrew Morton is asking to document blk_plug, so here is my attempt.
> > > 
> > > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> > > ---
> > >  include/linux/blkdev.h |   11 +++++++++++
> > >  1 file changed, 11 insertions(+)
> > > 
> > > Index: linux/include/linux/blkdev.h
> > > ===================================================================
> > > --- linux.orig/include/linux/blkdev.h	2011-07-29 10:51:29.000000000 +0800
> > > +++ linux/include/linux/blkdev.h	2011-07-29 11:07:49.000000000 +0800
> > > @@ -858,6 +858,17 @@ struct request_queue *blk_alloc_queue_no
> > >  extern void blk_put_queue(struct request_queue *);
> > >  
> > >  /*
> > > + * blk_plug gives each task a request list. Since blk_start_plug() called,
> > > + * requests from the task will be added to the per-task list and then moved
> > > + * to global request_queue in a batch way at appropriate time(either
> > > + * blk_finish_plug() is called or task goes to sleep). blk_plug has some
> > > + * advantages:
> > > + * 1. Better request merge. The assumption here is requests from a task have
> > > + *    better chances to be merged.
> > > + * 2. Better scalability. Requests are moved from per-task list to global
> > > + *    request_queue in a batch way, so the total times grabing global
> > > + *    request_queue lock are reduced.
> > > + *
> > 
> > Hi Shaohua,
> > 
> > This seems too brief atleast for someone like me who has not spent much
> > time with the code and also is not in kerneldoc format. Here's my attempt:
> Hi Suresh,
> I like the blk_start_plug part below. The blk_plug part needs more
> description to explain why we need it.
> 

I'm getting all excited about getting some blk_plug documentation!

> > From: Suresh Jayaraman <sjayaraman@suse.de>
> > Subject: [PATCH] block: document blk-plug
> > 
> > Thus spake Andrew Morton:
> > 
> > "And I have the usual maintainability whine.  If someone comes up to
> > vmscan.c and sees it calling blk_start_plug(), how are they supposed to
> > work out why that call is there?  They go look at the blk_start_plug()
> > definition and it is undocumented.  I think we can do better than this?"
> > 
> > Shaohua Li attempted to document it. But, I think it was too brief and
> > was not in kerneldoc format. Here's my attempt to document it. 
> > 
> > Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
> > ---
> > 
> >  block/blk-core.c       |   10 ++++++++++
> >  include/linux/blkdev.h |   13 ++++++++-----
> >  2 files changed, 18 insertions(+), 5 deletions(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index b850bed..355aa2c 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -2620,6 +2620,16 @@ EXPORT_SYMBOL(kblockd_schedule_delayed_work);
> >  
> >  #define PLUG_MAGIC	0x91827364
> >  
> > +/**
> > + * blk_start_plug - initialize blk_plug and track it inside the task_struct
> > + * @plug:	The &struct blk_plug that needs to be initialized
> > + *
> > + * Description:
> > + *   Tracking blk_plug inside the task_struct will help with flushing the
> > + *   pending I/O should the task end up blocking between blk_start_plug() and
> > + *   blk_finish_plug() and is important for deadlock avoidance and for the
> > + *   performance.
> > + */
> I'm not aware blk_plug is to avoid deadlock. It's most for performance
> to me. Jens, any idea?

Only we can't document it because we don't understand it.  Great.

c'mon Jens.   Talk to us?