Date:   Wed, 28 Jun 2017 07:39:05 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org,
        Linux-Kernal <linux-kernel@...r.kernel.org>,
        ulf.hansson@...aro.org, broonie@...nel.org
Subject: Re: [PATCH BUGFIX V2] block, bfq: update wr_busy_queues if needed on a queue split


> Il giorno 27 giu 2017, alle ore 20:29, Jens Axboe <axboe@...nel.dk> ha scritto:
> 
> On 06/27/2017 12:27 PM, Paolo Valente wrote:
>> 
>>> Il giorno 27 giu 2017, alle ore 16:41, Jens Axboe <axboe@...nel.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@...aro.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.
> 

My arguments was in favor of 4.12 actually.  Maybe you did mean 4.12
here?

Thanks,
Paolo

> -- 
> Jens Axboe

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