blk-iocost: revamp donation amount determination

iocost has various safety nets to combat inuse adjustment calculation
inaccuracies. With Andy's method implemented in transfer_surpluses(), inuse
adjustment calculations are now accurate and we can make donation amount
determinations accurate too.

* Stop keeping track of past usage history and using the maximum. Act on the
  immediate usage information.

* Remove donation constraints defined by SURPLUS_* constants. Donate
  whatever isn't used.

* Determine the donation amount so that the iocg will end up with
  MARGIN_TARGET_PCT budget at the end of the coming period assuming the same
  usage as the previous period. TARGET is set at 50% of period, which is the
  previous maximum. This provides smooth convergence for most repetitive IO
  patterns.

* Apply donation logic early at 20% budget. There's no risk in doing so as
  the calculation is based on the delta between the current budget and the
  target budget at the end of the coming period.

* Remove preemptive iocg activation for zero cost IOs. As donation can reach
  near zero now, the mere activation doesn't provide any protection anymore.
  In the unlikely case that this becomes a problem, the right solution is
  assigning appropriate costs for such IOs.

This significantly improves the donation determination logic while also
simplifying it. Now all donations are immediate, exact and smooth.

Signed-off-by: Tejun Heo <[email protected]>
Cc: Andy Newell <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

0 files changed, 0 insertions, 0 deletions