linux-kernel - Re: [PATCH v2] block: fix iolat timestamp and restore accounting semantics

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Date:   Thu, 13 Dec 2018 13:10:07 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        kernel-team@...com, linux-block@...r.kernel.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] block: fix iolat timestamp and restore accounting
 semantics

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