Re: [PATCH Linux 2.6.12-rc6-mm1 00/06] blk: generic dispatch queue (for review)

[Tejun Heo <htejun@gmail.com>]

Tejun Heo wrote:
>  Hello, Jens.
> 
>  This patchset implements generic dispatch queue I've talked about in
> the last ordered reimplementation patchset.  The patches are against
> 2.6.12-rc6-mm1 + ordered patchset + 3 last blk fix patches.  As I
> haven't posted ordered patchset against 2.6.12-rc6-mm1 (still waiting
> for your comments), to apply this patchset, you'll have to apply the
> ordered patchset against 2.6.12-rc5-mm2 to 2.6.12-rc6-mm1, and then
> apply these patches.  libata changes will fail but it wouldn't matter
> for review purpose.  (if you want ordered patchset against
> 2.6.12-rc6-mm1, I can send it to you, just tell me.)
> 
>  This patchset updates only noop and cfq io schedulers.  as and
> deadline wouldn't compile w/ this patchset applied.  I'll update as
> and deadline once some consensus regarding the general direction of
> this patchset is gained.
> 
>  This patchset is composed of two large parts.
> 
>  * Implementation of generic dispatch queue & updating individual
>    elevators.
>  * Move last_merge handling into generic elevator.
> 
>  Currently, each specific iosched maintains its own dispatch queue to
> handle ordering, requeueing, cluster dispatching, etc...  This causes
> the following problems.
> 
>  * duplicated codes
>  * difficult to enforce semantics over dispatch queue (request
>    ordering, requeueing, ...)
>  * specific ioscheds have to deal with non-fs or ordered requests
>    directly.
> 
>  With generic dispatch queue, specific ioscheds are guaranteed to be
> handed only non-barrier fs requests, such that ioscheds only have to
> implement ordering logic of normal fs requests.  Also, callback
> invocation is stricter now.  Each fs request follows one of the
> following paths.
> 
>  * add_req_fn -> dispatch_fn -> activate_fn (-> deactivate_fn ->
>    activate_fn)* -> completed_req_fn
>  * add_req_fn -> merged_req_fn


  Oops, sorry.  I was being delusional.  The following special case path 
doesn't exist.  It never reaches specific ioscheds, so it's just above 
two paths.

>  * add_req_fn -> dispatch_fn (This path is special case for barrier
>    request.  This can be easily removed by activating at the start of
>    ordered sequence, and completing at the end.  Would removing this
>    path be better?)

-- 
tejun