tree 71685e79fabea4499032719f71671420dec26a6b
parent 76534da749ceb3b4c13ec09b173bcf6d6c2e36d4
author Stefan Hajnoczi <stefanha@redhat.com> 1365154339 +0200
committer Kevin Wolf <kwolf@redhat.com> 1365181085 +0200

block: fix I/O throttling accounting blind spot

I/O throttling relies on bdrv_acct_done() which is called when a request
completes.  This leaves a blind spot since we only charge for completed
requests, not submitted requests.

For example, if there is 1 operation remaining in this time slice the
guest could submit 3 operations and they will all be submitted
successfully since they don't actually get accounted for until they
complete.

Originally we probably thought this is okay since the requests will be
accounted when the time slice is extended.  In practice it causes
fluctuations since the guest can exceed its I/O limit and it will be
punished for this later on.

Account for I/O upon submission so that I/O limits are enforced
properly.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-By: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>