On Thu, Dec 13, 2018 at 12:59:03PM -0700, Jens Axboe wrote: > On 12/13/18 12:52 PM, Josef Bacik wrote: > > On Thu, Dec 13, 2018 at 12:48:11PM -0700, Jens Axboe wrote: > >> 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. > >> > > > > I mean it'll work for now, but then when somebody else wants to do > > something similar *cough*io.weight*cough* it'll need a new flag. I > > kind of hate adding a new flag for every controller, but then again > > it's not like there's thousands of these things. I'm having a hard > > time coming up with a solution other than a per-tracker flag. As for > > this specific version, I still think it needs to be in iolatency > > itself, trying to make it generic just means it'll get fucked up again > > later down the line. Thanks, > > We definitely don't have that many flags, and I'd hate to add a > per-whatever flag for this. > > But do we need that? We really just need single flag for this, my main > worry was overloading ENTERED. Especially since we're adding different > clearing and setting for it. If we had a specific one, if it's set, we > would need to disallow merging for it, I guess. > > And there's already BIO_THROTTLED... > Oh well I guess we only really want to know if we saw the BIO, which should be able to be shared by all the rq_qos implementations. I think I'd rather just have a BIO_TRACKED to indicate it went through the normal rq_qos path. Thanks, Josef
