On 06/27/2017 12:27 PM, Paolo Valente wrote: > >> Il giorno 27 giu 2017, alle ore 16:41, Jens Axboe <axboe@xxxxxxxxx> 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@xxxxxxxxxx> 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
