linux-kernel - Re: Re: [RFC PATCH] block:Fix some problems about handling plug in blk_queue

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Date:	Thu, 9 Aug 2012 16:50:10 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Jianpeng Ma <majianpeng@...il.com>
Cc:	axboe <axboe@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re: [RFC PATCH] block:Fix some problems about handling plug in blk_queue_bio().

2012/8/8 Jianpeng Ma <majianpeng@...il.com>:
> On 2012-08-08 11:06 Shaohua Li <shli@...nel.org> Wrote:
>>2012/8/8 Jianpeng Ma <majianpeng@...il.com>:
>>> I think there are three problems about handling plug in blk_queue_bio():
>>> 1:if request_count >= BLK_MAX_REQUEST_COUNT, avoid unnecessary plug->should_sort judge.
>>this makes sense, though not a big deal, nice to fix it.
> Thanks
>>
>>> 2:Only two device can trace plug.
>>I didn't get the point, can you have more details?
>
>>>if (plug) {
>>>              /*
>>>               * If this is the first request added after a plug, fire
>>>               * of a plug trace. If others have been added before, check
>>>               * if we have multiple devices in this plug. If so, make a
>>>               * note to sort the list before dispatch.
>>>               */
>>>              if (list_empty(&plug->list))
>>>                      trace_block_plug(q);
>>>              else {
>>>                      if (!plug->should_sort) {
>>>                              struct request *__rq;
>
>>>                              __rq = list_entry_rq(plug->list.prev);
>>>                              if (__rq->q != q)
>>>                                      plug->should_sort = 1;
>>>                      }
>>>                      if (request_count >= BLK_MAX_REQUEST_COUNT) {
>>>                              blk_flush_plug_list(plug, false);
>>>                              trace_block_plug(q);
> The code only trace two point;
> A:      list_empty(&plug->list)
> B:      request_count >= BLK_MAX_REQUEST_COUNT). it's the same like A which plug->list is empty.
> Suppose:
> 1;reqA-deviceA firstly come, it will call trace_block_plug because the list_empty(plug->list) is true.
> 2:reqB-deviceB comed, attempt_plug_merge will failed because not deviceB-request-queue.But it'll not to call trace_block_plug.
>
> But call blk_flush_plug_list,it will trace_block_unplug all request_queue.

ok, this is true. please send a new patch for the item 1&2 then.
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