Subject: Re: [PATCH BUGFIX V2] block, bfq: update wr_busy_queues if needed on
 a queue split
To: Paolo Valente <paolo.valente@linaro.org>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        ulf.hansson@linaro.org, broonie@kernel.org
References: <20170619114316.2587-1-paolo.valente@linaro.org>
 <8520D3AF-C161-439F-A7E8-A6B7202DA2D9@linaro.org>
 <e070788e-8a62-024d-7aac-8f8801722ffe@kernel.dk>
 <4AFF2E52-DCE4-4DC7-9CB0-849EEED3A9AB@linaro.org>
From: Jens Axboe <axboe@kernel.dk>
Message-ID: <3b5987e2-fa11-af94-27f4-5760612c0f22@kernel.dk>
Date: Tue, 27 Jun 2017 12:29:10 -0600
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On 06/27/2017 12:27 PM, Paolo Valente wrote:
> 
>> Il giorno 27 giu 2017, alle ore 16:41, Jens Axboe <axboe@kernel.dk> ha scritto:
>>
>> On 06/27/2017 12:09 AM, Paolo Valente wrote:
>>>
>>>> Il giorno 19 giu 2017, alle ore 13:43, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>>>
>>>> This commit fixes a bug triggered by a non-trivial sequence of
>>>> events. These events are briefly described in the next two
>>>> paragraphs. The impatiens, or those who are familiar with queue
>>>> merging and splitting, can jump directly to the last paragraph.
>>>>
>>>> On each I/O-request arrival for a shared bfq_queue, i.e., for a
>>>> bfq_queue that is the result of the merge of two or more bfq_queues,
>>>> BFQ checks whether the shared bfq_queue has become seeky (i.e., if too
>>>> many random I/O requests have arrived for the bfq_queue; if the device
>>>> is non rotational, then random requests must be also small for the
>>>> bfq_queue to be tagged as seeky). If the shared bfq_queue is actually
>>>> detected as seeky, then a split occurs: the bfq I/O context of the
>>>> process that has issued the request is redirected from the shared
>>>> bfq_queue to a new non-shared bfq_queue. As a degenerate case, if the
>>>> shared bfq_queue actually happens to be shared only by one process
>>>> (because of previous splits), then no new bfq_queue is created: the
>>>> state of the shared bfq_queue is just changed from shared to non
>>>> shared.
>>>>
>>>> Regardless of whether a brand new non-shared bfq_queue is created, or
>>>> the pre-existing shared bfq_queue is just turned into a non-shared
>>>> bfq_queue, several parameters of the non-shared bfq_queue are set
>>>> (restored) to the original values they had when the bfq_queue
>>>> associated with the bfq I/O context of the process (that has just
>>>> issued an I/O request) was merged with the shared bfq_queue. One of
>>>> these parameters is the weight-raising state.
>>>>
>>>> If, on the split of a shared bfq_queue,
>>>> 1) a pre-existing shared bfq_queue is turned into a non-shared
>>>> bfq_queue;
>>>> 2) the previously shared bfq_queue happens to be busy;
>>>> 3) the weight-raising state of the previously shared bfq_queue happens
>>>> to change;
>>>> the number of weight-raised busy queues changes. The field
>>>> wr_busy_queues must then be updated accordingly, but such an update
>>>> was missing. This commit adds the missing update.
>>>>
>>>
>>> Hi Jens,
>>> any idea of the possible fate of this fix?
>>
>> I sort of missed this one. It looks trivial enough for 4.12, or we
>> can defer until 4.13. What do you think?
>>
> 
> It should actually be something trivial, and hopefully correct,
> because a further throughput improvement (for BFQ), which depends on
> this fix, is now working properly, and we didn't see any regression so
> far.  In addition, since this improvement is virtually ready for
> submission, further steps may be probably easier if this fix gets in
> sooner (whatever the luck of the improvement will be).

OK, let's queue it up for 4.13 then.

-- 
Jens Axboe