On 12/11/18 4:01 PM, 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, to tag all bios w/ blkgs in [1] changed the timing > incorrectly as well. First, the iolatency controller was tagging bios > and using that information if it should process it in rq_qos_done_bio(). > However, now that all bios are tagged, this caused the atomic_t for the > struct rq_wait inflight count to underflow resulting in a stall. Second, > now the timing was using the duration a bio from generic_make_request() > rather than the timing mentioned above. > > This patch fixes these issues by reusing the BLK_QUEUE_ENTERED flag to > determine if a bio has entered the request layer and is responsible for > starting a request. Stacked drivers don't recurse through > blk_mq_make_request(), so the overhead of using time between > generic_make_request() and the blk_mq_get_request() should be minimal. > blk-iolatency now checks if this flag is set to determine if it should > process the bio in rq_qos_done_bio(). I'm having a hard time convincing myself that this is correct... Maybe we should just add a new flag for this specific use case? Or feel free to convince me otherwise. -- Jens Axboe
