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Waiman Long c86ff8c55b clocksource: Avoid accidental unstable marking of clocksources 
Since commit db3a34e174 ("clocksource: Retry clock read if long delays
detected") and commit 2e27e793e2 ("clocksource: Reduce clocksource-skew
threshold"), it is found that tsc clocksource fallback to hpet can
sometimes happen on both Intel and AMD systems especially when they are
running stressful benchmarking workloads. Of the 23 systems tested with
a v5.14 kernel, 10 of them have switched to hpet clock source during
the test run.

The result of falling back to hpet is a drastic reduction of performance
when running benchmarks. For example, the fio performance tests can
drop up to 70% whereas the iperf3 performance can drop up to 80%.

4 hpet fallbacks happened during bootup. They were:

  [    8.749399] clocksource: timekeeping watchdog on CPU13: hpet read-back delay of 263750ns, attempt 4, marking unstable
  [   12.044610] clocksource: timekeeping watchdog on CPU19: hpet read-back delay of 186166ns, attempt 4, marking unstable
  [   17.336941] clocksource: timekeeping watchdog on CPU28: hpet read-back delay of 182291ns, attempt 4, marking unstable
  [   17.518565] clocksource: timekeeping watchdog on CPU34: hpet read-back delay of 252196ns, attempt 4, marking unstable

Other fallbacks happen when the systems were running stressful
benchmarks. For example:

  [ 2685.867873] clocksource: timekeeping watchdog on CPU117: hpet read-back delay of 57269ns, attempt 4, marking unstable
  [46215.471228] clocksource: timekeeping watchdog on CPU8: hpet read-back delay of 61460ns, attempt 4, marking unstable

Commit 2e27e793e2 ("clocksource: Reduce clocksource-skew threshold"),
changed the skew margin from 100us to 50us. I think this is too small
and can easily be exceeded when running some stressful workloads on a
thermally stressed system.  So it is switched back to 100us.

Even a maximum skew margin of 100us may be too small in for some systems
when booting up especially if those systems are under thermal stress. To
eliminate the case that the large skew is due to the system being too
busy slowing down the reading of both the watchdog and the clocksource,
an extra consecutive read of watchdog clock is being done to check this.

The consecutive watchdog read delay is compared against
WATCHDOG_MAX_SKEW/2. If the delay exceeds the limit, we assume that
the system is just too busy. A warning will be printed to the console
and the clock skew check is skipped for this round.

Fixes: db3a34e174 ("clocksource: Retry clock read if long delays detected")
Fixes: 2e27e793e2 ("clocksource: Reduce clocksource-skew threshold")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2021-11-30 17:22:29 -08:00
arch parisc architecture build-, trace-, backtrace- and page table fixes 2021-11-14 11:53:59 -08:00
block blk-mq: fix filesystem I/O request allocation 2021-11-12 09:31:13 -07:00
certs certs: Add support for using elliptic curve keys for signing modules 2021-08-23 19:55:42 +03:00
crypto Update to zstd-1.4.10 2021-11-13 15:32:30 -08:00
Documentation Devicetree fixes for v5.16, take 1: 2021-11-14 11:11:51 -08:00
drivers arch/sh updates for 5.16 2021-11-14 11:37:49 -08:00
fs Minor tweaks for 5.16: 2021-11-14 12:18:22 -08:00
include Devicetree fixes for v5.16, take 1: 2021-11-14 11:11:51 -08:00
init kconfig: Add support for -Wimplicit-fallthrough 2021-11-14 13:27:30 -08:00
ipc ipc/ipc_sysctl.c: remove fallback for !CONFIG_PROC_SYSCTL 2021-11-09 10:02:53 -08:00
kernel clocksource: Avoid accidental unstable marking of clocksources 2021-11-30 17:22:29 -08:00
lib Update to zstd-1.4.10 2021-11-13 15:32:30 -08:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
mm Revert "mm: shmem: don't truncate page if memory failure happens" 2021-11-13 12:03:03 -08:00
net One notable change here is that async creates and unlinks introduced 2021-11-13 11:31:07 -08:00
samples Add gitignore file for samples/fanotify/ subdirectory 2021-11-07 11:19:24 -08:00
scripts coccinelle patches for 5.16-rc1 2021-11-13 10:45:17 -08:00
security net,lsm,selinux: revert the security_sctp_assoc_established() hook 2021-11-12 12:07:02 -05:00
sound sound fixes for 5.16-rc1 2021-11-12 12:17:30 -08:00
tools rcutorture: Add CONFIG_PREEMPT_DYNAMIC=n to tiny scenarios 2021-11-30 17:20:58 -08:00
usr initramfs: Check timestamp to prevent broken cpio archive 2021-10-24 13:48:40 +09:00
virt KVM: generalize "bugged" VM to "dead" VM 2021-11-11 10:35:26 -05:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap Merge branch 'akpm' (patches from Andrew) 2021-11-09 10:11:53 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Daniel Drake to credits 2021-09-21 08:34:58 +03:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS - Add the model number of a new, Raptor Lake CPU, to intel-family.h 2021-11-14 09:29:03 -08:00
Makefile Linux 5.16-rc1 2021-11-14 13:56:52 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

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.