什么使blk_execute_rq_nowait排队的请求出队

Question

I'm working on increasing a timeout in the SCSI mid-layer driver in Linux. At least, that's the quest. I'm familiarizing myself with the driver. This is turning out to be a formidable task. The Linux Documentation Project seems to be woefully out of date (the tour of the kernel is based on v 1.0.9 ... really?). I also found this from kernel.org. I'm not sure how up-to-date that is either.

A description of the problem is that we send SCSI commands through sg. Any timeout specified in sg_io_hdr_t seems to be ignored if it's longer than 30 seconds. I haven't seen anything in the sg driver code which seems to trump with 30 if the timeout requested is larger. Normally, we submit commands using the write/poll/read method through sg. I've traced through the sg code and I believe calling write(2) takes the following path:

sg_write()
   sg_common_write()
      blk_execute_rq_nowait()

By no means am I 100% positive of this, but it does seem plausible. My question to kernel developers here is, what call should I grep for which would dequeue this request? I haven't found anything in the references I do have which state this.

Ultimately, I'm looking for where, in the mid-layer, requests like this are dequeued for transmission to the lower layer. My premise is that, if I know what calls dequeues requests from the queue used in blk_execute_rq_nowait(), then I can grep through the appropriate source files looking for that and move from there. (If someone would be kind enough to tell me if all of the files listed in the first link are the correct list of files for the SCSI mid-layer in Linux, I thank you in advance. My kernel version: 2.6.32.)

Do I have things incorrect? Are requests like this just taken by the lower layer? I assume "no" because this seems like what the mid-layer is supposed to do: route these things to the proper place.

Answer 1

blk_execute_rq() - this call inserts a request at the back of the I/O scheduler queue. So you should be looking into the I/O scheduler code which dequeues the requests. You may want to start of looking at what I/O scheduler your system is running under,cat /sys/block/sda/queue/scheduler and settings under

ls /sys/block/sda/queue/scheduler 

(should be something like noop [deadline] cfq), and thereafter look into the scheduler code.