Re: [patch]block: document blk_plug

[Shaohua Li <shaohua.li@intel.com>]

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.


> 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?

Thanks,
Shaohua