Re: [dm-devel] Re: [RFC PATCH 1/8] rqbased-dm: allow blk_get_request() to be called from interrupt context

[Mike Christie <michaelc@cs.wisc.edu>]

Mike Christie wrote:
> Jens Axboe wrote:
>> On Wed, Dec 20 2006, Kiyoshi Ueda wrote:
>>> Hi Jens,
>>>
>>> On Wed, 20 Dec 2006 19:49:17 +0100, Jens Axboe <jens.axboe@oracle.com> wrote:
>>>>>> Big NACK on this - it's not only really ugly, it's also buggy to pass
>>>>>> interrupt flags as function arguments. As you also mention in the 0/1
>>>>>> mail, this also breaks CFQ.
>>>>>>
>>>>>> Why do you need in-interrupt request allocation?
>>>>>  
>>>>> Because I'd like to use blk_get_request() in q->request_fn()
>>>>> which can be called from interrupt context like below:
>>>>>   scsi_io_completion -> scsi_end_request -> scsi_next_command
>>>>>   -> scsi_run_queue -> blk_run_queue -> q->request_fn
>>>>>
>>>>> Generally, device-mapper (dm) clones an original I/O and dispatches
>>>>> the clones to underlying destination devices.
>>>>> In the request-based dm patch, the clone creation and the dispatch
>>>>> are done in q->request_fn().  To create the clone, blk_get_request()
>>>>> is used to get a request from underlying destination device's queue.
>>>>> By doing that in q->request_fn(), dm can deal with struct request
>>>>> after bios are merged by __make_request().
>>>>>
>>>>> Do you think creating another function like blk_get_request_nowait()
>>>>> is acceptable?
>>>>> Or request should not be allocated in q->request_fn() anyway?
>>>> You should not be allocating requests from that path, for a number of
>>>> reasons.
>>> Could I hear the reasons for my further work if possible?
>>> Because of breaking current CFQ?  And is there any reason?
>> Mainly I just don't like the design, there are better ways to achieve
>> what you need. The block layer has certain assumptions on the context
>> from which rq allocation happens, and this breaks it. As I also
>> mentioned, you cannot pass flags around as arguments. So the patch is
>> even broken as-is.
>>
> 
> 
> I was thinking that since this was going to be hooked into dm which has
> the make_request hook in code, could we just allocate the cloned request
> when from dm's make_request callout. The dm queue would call
> __make_request, and if it detected that the bio started a new request it
> would just allocate a second request which would be used as a clone or
> maybe the block layer could allocate the clone request for us. On the
> request_fn callout side, DM could then setup the cloned rq based on the
> original fields and pass it down to the dm-multipath request_fn. The
> dm-mutlipath request_fn then just decides which path to use based on the
> path-selector modules and then we send it off.
> 

Or the block layer code could set up the clone too. elv_next_request
could prep a clone based on the orignal request for the driver then dm
would not have to worry about that part.
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