On 22/03/2016 16:33, Kevin Wolf wrote: > This removes the last part of I/O throttling from block/io.c and moves > it to the BlockBackend. > > When draining the queue of a BlockDriverState, we must make sure that no > new requests can come in for it. Request sources from outside the block > layer are disabled with aio_disable_external(), but the throttling queue > must be handled separately.
I have looked at the strategy we talked about today to implement request cancellation (so that e.g. system reset doesn't take ages because of throttled requests). While that may be a worthwhile addition anyway, I think throttling bdrv_drain() may impose an excessive cost for cases such as live migration. The risk of the guest using bdrv_drain() to game throttling is low enough that we can keep on disabling throttling during bdrv_drain(). So for now I think we can merge the two series just fine. The strategy I used in my patch, adding bdrv_no_throttling_begin and bdrv_no_throttling_end around the bdrv_drain loop, can be adapted just as use BdrvChildRole callbacks ->drained_begin and ->drained_end. I will post v3 of my series tomorrow, adopting your patch 1/12 of this series and removing the recursion on bdrv_no_throttling_begin and bdrv_no_throttling_end, which is unnecessary. Paolo
