On 12/17/18 9:03 AM, Dennis Zhou wrote: > The blk-iolatency controller measures the time from rq_qos_throttle() to > rq_qos_done_bio() and attributes this time to the first bio that needs > to create the request. This means if a bio is plug-mergeable or > bio-mergeable, it gets to bypass the blk-iolatency controller. > > The recent series [1], to tag all bios w/ blkgs undermined how iolatency > was determining which bios it was charging and should process in > rq_qos_done_bio(). Because all bios are being tagged, this caused the > atomic_t for the struct rq_wait inflight count to underflow and result > in a stall. > > This patch adds a new flag BIO_TRACKED to let controllers know that a > bio is going through the rq_qos path. blk-iolatency now checks if this > flag is set to see if it should process the bio in rq_qos_done_bio(). > > Overloading BLK_QUEUE_ENTERED works, but makes the flag rules confusing. > BIO_THROTTLED was another candidate, but the flag is set for all bios > that have gone through blk-throttle code. Overloading a flag comes with > the burden of making sure that when either implementation changes, a > change in setting rules for one doesn't cause a bug in the other. So > here, we unfortunately opt for adding a new flag. I think this is better than (ab)using QUEUE_ENTERED, but this is an area that needs some love and cleanup in the future. -- Jens Axboe
