[RFC PATCH 0/2] elevator: restore old io scheduler on failure in elevator_switch

[Jinlong Chen <nickyc975@zju.edu.cn>]

Hi!

These two patches bring back the fallback feature in elevator_switch if
switching to the new io scheduler failed.

elevator_switch contains the fallback logic in sq era, but it was removed
when moving to mq (commit: a1ce35fa49852db60fc6e268038530be533c5b15),
leaving the document mismatched with the behavior. As far as I can see,
restoring the old io scheduler is more reasonable than just leaving the
scheduler none, hence there is the series.

However, now it's hard to keep the old io scheduler untouched. We can only
re-initialize the old scheduler if we want to restore it, and the
statistics the old scheduler collected would be lost. Besides, the
restoration itself might fail too. I have no idea whether the two problems
matter. Any comments are welcomed.

Jinlong Chen (2):
  elevator: add a helper for applying scheduler to request_queue
  elevator: restore the old io scheduler if failed to switch to the new
    one

 block/elevator.c | 49 +++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 40 insertions(+), 9 deletions(-)

-- 
2.31.1