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Paolo Valente 1a2d99218d block, bfq: fix wrong init of saved start time for weight raising 
commit 4baa8bb13f upstream.

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>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2018-08-17 21:01:11 +02:00
arch crypto: x86/sha256-mb - fix digest copy in sha256_mb_mgr_get_comp_job_avx2() 2018-08-17 21:01:10 +02:00
block block, bfq: fix wrong init of saved start time for weight raising 2018-08-17 21:01:11 +02:00
certs License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
crypto crypto: skcipher - fix crash flushing dcache in error path 2018-08-17 21:01:10 +02:00
Documentation kbuild: verify that $DEPMOD is installed 2018-08-17 21:01:10 +02:00
drivers clk: sunxi-ng: Fix missing CLK_SET_RATE_PARENT in ccu-sun4i-a10.c 2018-08-17 21:01:11 +02:00
firmware License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
fs fix __legitimize_mnt()/mntput() race 2018-08-15 18:12:48 +02:00
include sched/debug: Fix task state recording/printout 2018-08-17 21:01:11 +02:00
init init: rename and re-order boot_cpu_state_init() 2018-08-15 18:12:48 +02:00
ipc ipc/shm: fix shmat() nil address after round-down when remapping 2018-05-30 07:51:49 +02:00
kernel cpu/hotplug: Non-SMP machines do not make use of booted_once 2018-08-15 18:13:01 +02:00
lib lib/rhashtable: consider param->min_size when setting initial table size 2018-07-25 11:25:09 +02:00
mm x86/speculation/l1tf: Limit swap file size to MAX_PA/2 2018-08-15 18:12:51 +02:00
net netlink: Don't shift on 64 for ngroups 2018-08-09 12:16:38 +02:00
samples samples/bpf: Partially fixes the bpf.o build 2018-04-26 11:02:12 +02:00
scripts kbuild: verify that $DEPMOD is installed 2018-08-17 21:01:10 +02:00
security ima: based on policy verify firmware signatures (pre-allocated buffer) 2018-08-03 07:50:31 +02:00
sound ASoC: rsnd: fix ADG flags 2018-08-17 21:01:11 +02:00
tools tools headers: Synchronise x86 cpufeatures.h for L1TF additions 2018-08-15 18:13:01 +02:00
usr initramfs: fix initramfs rebuilds w/ compression after disabling 2017-11-03 07:39:19 -07:00
virt KVM/Eventfd: Avoid crash when assign and deassign specific eventfd in parallel. 2018-07-25 11:25:07 +02:00
.cocciconfig
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore kbuild: rpm-pkg: keep spec file until make mrproper 2018-02-13 10:19:46 +01:00
.mailmap .mailmap: Add Maciej W. Rozycki's Imagination e-mail address 2017-11-10 12:16:15 -08:00
COPYING
CREDITS MAINTAINERS: update TPM driver infrastructure changes 2017-11-09 17:58:40 -08:00
Kbuild License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
Kconfig License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
MAINTAINERS dt-bindings: Document mti,mips-cpc binding 2018-03-15 10:54:35 +01:00
Makefile Linux 4.14.63 2018-08-15 18:13:02 +02:00
README README: add a new README file, pointing to the Documentation/ 2016-10-24 08:12:35 -02:00

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.