On 11/13/17 1:37 PM, Shaohua Li wrote:
If a bio is throttled and splitted after throttling, the bio could be resubmited and enters the throttling again. This will cause part of the bio is charged multiple times. If the cgroup has an IO limit, the double charge will significantly harm the performance. The bio split becomes quite common after arbitrary bio size change.
To fix this, we always set the BIO_THROTTLED flag if a bio is throttled. If the bio is cloned/slitted, we copy the flag to new bio too to avoid double charge. However cloned bio could be directed to a new disk, keeping the flag will have problem. The observation is we always set new disk for the bio in this case, so we can clear the flag in bio_set_dev().
This issue exists a long time, arbitrary bio size change makes it worse, so this should go into stable at least since v4.2.
V1-> V2: Not add extra field in bio based on discussion with Tejun
Applied, thanks Shaohua.