Messages in this thread | | | Date | Sat, 27 Aug 2011 08:36:52 +0200 | From | Jens Axboe <> | Subject | Re: [patch]block: document blk_plug |
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On 2011-08-27 00:25, Andrew Morton wrote: > 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?
The pointer in the task_struct is to be able to auto-flush the plug if the task inadvertently ends up scheduling with requests plugged. For instance, if we end up doing a wait for a page that is already plugged, then we must ensure that everything up to that point has been flushed out.
-- Jens Axboe
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