forked from Minki/linux
mainlining shenanigans
4baa8bb13f
This commit fixes a bug that causes bfq to fail to guarantee a high responsiveness on some drives, if there is heavy random read+write I/O in the background. More precisely, such a failure allowed this bug to be found [1], but the bug may well cause other yet unreported anomalies. BFQ raises the weight of the bfq_queues associated with soft real-time applications, to privilege the I/O, and thus reduce latency, for these applications. This mechanism is named soft-real-time weight raising in BFQ. A soft real-time period may happen to be nested into an interactive weight raising period, i.e., it may happen that, when a bfq_queue switches to a soft real-time weight-raised state, the bfq_queue is already being weight-raised because deemed interactive too. In this case, BFQ saves in a special variable wr_start_at_switch_to_srt, the time instant when the interactive weight-raising period started for the bfq_queue, i.e., the time instant when BFQ started to deem the bfq_queue interactive. This value is then used to check whether the interactive weight-raising period would still be in progress when the soft real-time weight-raising period ends. If so, interactive weight raising is restored for the bfq_queue. This restore is useful, in particular, because it prevents bfq_queues from losing their interactive weight raising prematurely, as a consequence of spurious, short-lived soft real-time weight-raising periods caused by wrong detections as soft real-time. If, instead, a bfq_queue switches to soft-real-time weight raising while it *is not* already in an interactive weight-raising period, then the variable wr_start_at_switch_to_srt has no meaning during the following soft real-time weight-raising period. Unfortunately the handling of this case is wrong in BFQ: not only the variable is not flagged somehow as meaningless, but it is also set to the time when the switch to soft real-time weight-raising occurs. This may cause an interactive weight-raising period to be considered mistakenly as still in progress, and thus a spurious interactive weight-raising period to start for the bfq_queue, at the end of the soft-real-time weight-raising period. In particular the spurious interactive weight-raising period will be considered as still in progress, if the soft-real-time weight-raising period does not last very long. The bfq_queue will then be wrongly privileged and, if I/O bound, will unjustly steal bandwidth to truly interactive or soft real-time bfq_queues, harming responsiveness and low latency. This commit fixes this issue by just setting wr_start_at_switch_to_srt to minus infinity (farthest past time instant according to jiffies macros): when the soft-real-time weight-raising period ends, certainly no interactive weight-raising period will be considered as still in progress. [1] Background I/O Type: Random - Background I/O mix: Reads and writes - Application to start: LibreOffice Writer in http://www.phoronix.com/scan.php?page=news_item&px=Linux-4.13-IO-Laptop Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Tested-by: Lee Tibbert <lee.tibbert@gmail.com> Tested-by: Mirko Montanari <mirkomontanari91@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk> |
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| firmware | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.