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Since commit430a67f9d6("block, bfq: merge bursts of newly-created queues"), BFQ may schedule a merge between a newly created sync bfq_queue and the last sync bfq_queue created. Such a merging is not performed immediately, because BFQ needs first to find out whether the newly created queue actually reaches a higher throughput if not merged at all (and in that case BFQ will not perform any stable merging). To check that, a little time must be waited after the creation of the new queue, so that some I/O can flow in the queue, and statistics on such I/O can be computed. Yet, to evaluate the above waiting time, the last split time is considered as start time, instead of the creation time of the queue. This is a mistake, because considering the split time is correct only in the following scenario. The queue undergoes a non-stable merges on the arrival of its very first I/O request, due to close I/O with some other queue. While the queue is merged for close I/O, stable merging is not considered. Yet the queue may then happen to be split, if the close I/O finishes (or happens to be a false positive). From this time on, the queue can again be considered for stable merging. But, again, a little time must elapse, to let some new I/O flow in the queue and to get updated statistics. To wait for this time, the split time is to be taken into account. Yet, if the queue does not undergo a non-stable merge on the arrival of its very first request, then BFQ immediately checks whether the stable merge is to be performed. It happens because the split time for a queue is initialized to minus infinity when the queue is created. This commit fixes this mistake by adding the missing condition. Now the check for delayed stable-merge is performed after a little time is elapsed not only from the last queue split time, but also from the creation time of the queue. Fixes:430a67f9d6("block, bfq: merge bursts of newly-created queues") Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Link: https://lore.kernel.org/r/20210619140948.98712-4-paolo.valente@linaro.org Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.