On 12/13/18 1:03 PM, Josef Bacik wrote: > 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, Agree, so how about renaming BIO_THROTTLED to BIO_TRACKED and using that? -- Jens Axboe
