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>
From: Jens Axboe <axboe@kernel.dk>
Message-ID: <e070788e-8a62-024d-7aac-8f8801722ffe@kernel.dk>
Date: Tue, 27 Jun 2017 08:41:15 -0600
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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?

-- 
Jens Axboe