This patch reimplements elevator switch. This patch assumes generic
dispatch queue patchset is applied.
* Each request is tagged with REQ_ELVPRIV flag if it has its elevator
private data set.
* Requests which doesn't have REQ_ELVPRIV flag set never enter
iosched. They are always directly back inserted to dispatch queue.
Of course, elevator_put_req_fn is called only for requests which
have its REQ_ELVPRIV set.
* Request queue maintains the current number of requests which have
its elevator data set (elevator_set_req_fn called) in
q->rq->elvpriv.
* If a request queue has QUEUE_FLAG_BYPASS set, elevator private data
is not allocated for new requests.
To switch to another iosched, we set QUEUE_FLAG_BYPASS and wait until
elvpriv goes to zero; then, we attach the new iosched and clears
QUEUE_FLAG_BYPASS. New implementation is much simpler and main code
paths are less cluttered, IMHO.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>
cb98fc8bb9