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

[Kiyoshi Ueda]

Hi Jens,

On Wed, 20 Dec 2006 19:49:17 +0100, Jens Axboe <jens.axboe@xxxxxxxxxx> 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?


> The design isn't very nice either.
> 
> The easy way out would be to punt to a workqueue to handle the requests.
> 
> An alternative way would be to set aside some requests that you can get
> at without allocation (maintain a little freelist of manually allocated
> requests), and retrieve a free one from there when inside request_fn. If
> you run out, just bail out of request_fn and make sure to reinvoke it
> when some of your previously issued requests complete and are added back
> to that freelist.

Thank you for the suggestions.
OK, I'll think other designs based on your suggestions.
 
Thanks,
Kiyoshi Ueda

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