[PATCH 2/7] watchdog/hardlockup: Make the config checks more straightforward

From: Petr Mladek
Date: Wed Jun 07 2023 - 11:27:09 EST

There are four possible variants of hardlockup detectors:

+ buddy: available when SMP is set.

+ perf: available when HAVE_HARDLOCKUP_DETECTOR_PERF is set.

+ arch-specific: available when HAVE_HARDLOCKUP_DETECTOR_ARCH is set.

+ sparc64 special variant: available when HAVE_NMI_WATCHDOG is set
and HAVE_HARDLOCKUP_DETECTOR_ARCH is not set.

The check for the sparc64 variant is more complicated because
HAVE_NMI_WATCHDOG is used to #ifdef code used by both arch-specific
and sparc64 specific variant. Therefore it is automatically
selected with HAVE_HARDLOCKUP_DETECTOR_ARCH.

This complexity is partly hidden in HAVE_HARDLOCKUP_DETECTOR_NON_ARCH.
It reduces the size of some checks but it makes them harder to follow.

Finally, the other temporary variable HARDLOCKUP_DETECTOR_NON_ARCH
is used to re-compute HARDLOCKUP_DETECTOR_PERF/BUDDY when the global
HARDLOCKUP_DETECTOR switch is enabled/disabled.

Make the logic more straightforward by the following changes:

+ Better explain the role of HAVE_HARDLOCKUP_DETECTOR_ARCH and
HAVE_NMI_WATCHDOG in comments.

+ Add HAVE_HARDLOCKUP_DETECTOR_BUDDY so that there is separate
HAVE_* for all four hardlockup detector variants.

Use it in the other conditions instead of SMP. It makes it
clear that it is about the buddy detector.

+ Open code HAVE_HARDLOCKUP_DETECTOR_NON_ARCH in HARDLOCKUP_DETECTOR
and HARDLOCKUP_DETECTOR_PREFER_BUDDY. It helps to understand
the conditions between the four hardlockup detector variants.

+ Define the exact conditions when HARDLOCKUP_DETECTOR_PERF/BUDDY
can be enabled. It explains the dependency on the other
hardlockup detector variants.

Also it allows to remove HARDLOCKUP_DETECTOR_NON_ARCH by using "imply".
It triggers re-evaluating HARDLOCKUP_DETECTOR_PERF/BUDDY when
the global HARDLOCKUP_DETECTOR switch is changed.

+ Add dependency on HARDLOCKUP_DETECTOR so that the affected variables
disappear when the hardlockup detectors are disabled.

Another nice side effect is that HARDLOCKUP_DETECTOR_PREFER_BUDDY
value is not preserved when the global switch is disabled.
The user has to make the decision when it is enabled again.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
arch/Kconfig | 22 ++++++++++++-----
lib/Kconfig.debug | 63 ++++++++++++++++++++++++++++-------------------
2 files changed, 53 insertions(+), 32 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 422f0ffa269e..13c6e596cf9e 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -404,17 +404,27 @@ config HAVE_NMI_WATCHDOG
depends on HAVE_NMI
bool
help
- The arch provides a low level NMI watchdog. It provides
- asm/nmi.h, and defines its own watchdog_hardlockup_probe() and
- arch_touch_nmi_watchdog().
+ The arch provides its own hardlockup detector implementation instead
+ of the generic perf one.
+
+ Sparc64 defines this variable without HAVE_HARDLOCKUP_DETECTOR_ARCH.
+ It does _not_ use the command line parameters and sysctl interface
+ used by generic hardlockup detectors. Instead it is enabled/disabled
+ by the top-level watchdog interface that is common for both softlockup
+ and hardlockup detectors.

config HAVE_HARDLOCKUP_DETECTOR_ARCH
bool
select HAVE_NMI_WATCHDOG
help
- The arch chooses to provide its own hardlockup detector, which is
- a superset of the HAVE_NMI_WATCHDOG. It also conforms to config
- interfaces and parameters provided by hardlockup detector subsystem.
+ The arch provides its own hardlockup detector implementation instead
+ of the generic ones.
+
+ It uses the same command line parameters, and sysctl interface,
+ as the generic hardlockup detectors.
+
+ HAVE_NMI_WATCHDOG is selected to build the code shared with
+ the sparc64 specific implementation.

config HAVE_PERF_REGS
bool
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 3e91fa33c7a0..d201f5d3876b 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1035,16 +1035,33 @@ config BOOTPARAM_SOFTLOCKUP_PANIC

Say N if unsure.

+config HAVE_HARDLOCKUP_DETECTOR_BUDDY
+ bool
+ depends on SMP
+ default y
+
#
-# arch/ can define HAVE_HARDLOCKUP_DETECTOR_ARCH to provide their own hard
-# lockup detector rather than the perf based detector.
+# Global switch whether to build a hardlockup detector at all. It is available
+# only when the architecture supports at least one implementation. There are
+# two exceptions. The hardlockup detector is newer enabled on:
+#
+# s390: it reported many false positives there
+#
+# sparc64: has a custom implementation which is not using the common
+# hardlockup command line options and sysctl interface.
+#
+# Note that HAVE_NMI_WATCHDOG is used to distinguish the sparc64 specific
+# implementaion. It is automatically enabled also for other arch-specific
+# variants which set HAVE_HARDLOCKUP_DETECTOR_ARCH. It makes the check
+# of avaialable and supported variants quite tricky.
#
config HARDLOCKUP_DETECTOR
bool "Detect Hard Lockups"
depends on DEBUG_KERNEL && !S390
- depends on HAVE_HARDLOCKUP_DETECTOR_NON_ARCH || HAVE_HARDLOCKUP_DETECTOR_ARCH
+ depends on ((HAVE_HARDLOCKUP_DETECTOR_PERF || HAVE_HARDLOCKUP_DETECTOR_BUDDY) && !HAVE_NMI_WATCHDOG) || HAVE_HARDLOCKUP_DETECTOR_ARCH
+ imply HARDLOCKUP_DETECTOR_PERF
+ imply HARDLOCKUP_DETECTOR_BUDDY
select LOCKUP_DETECTOR
- select HARDLOCKUP_DETECTOR_NON_ARCH if HAVE_HARDLOCKUP_DETECTOR_NON_ARCH

help
Say Y here to enable the kernel to act as a watchdog to detect
@@ -1055,9 +1072,15 @@ config HARDLOCKUP_DETECTOR
chance to run. The current stack trace is displayed upon detection
and the system will stay locked up.

+#
+# Note that arch-specific variants are always preferred.
+#
config HARDLOCKUP_DETECTOR_PREFER_BUDDY
bool "Prefer the buddy CPU hardlockup detector"
- depends on HAVE_HARDLOCKUP_DETECTOR_PERF && SMP
+ depends on HARDLOCKUP_DETECTOR
+ depends on HAVE_HARDLOCKUP_DETECTOR_PERF && HAVE_HARDLOCKUP_DETECTOR_BUDDY
+ depends on !HAVE_NMI_WATCHDOG
+ default n
help
Say Y here to prefer the buddy hardlockup detector over the perf one.

@@ -1071,39 +1094,27 @@ config HARDLOCKUP_DETECTOR_PREFER_BUDDY

config HARDLOCKUP_DETECTOR_PERF
bool
- depends on HAVE_HARDLOCKUP_DETECTOR_PERF
+ depends on HARDLOCKUP_DETECTOR
+ depends on HAVE_HARDLOCKUP_DETECTOR_PERF && !HARDLOCKUP_DETECTOR_PREFER_BUDDY
+ depends on !HAVE_NMI_WATCHDOG
select HARDLOCKUP_DETECTOR_COUNTS_HRTIMER

config HARDLOCKUP_DETECTOR_BUDDY
bool
- depends on SMP
+ depends on HARDLOCKUP_DETECTOR
+ depends on HAVE_HARDLOCKUP_DETECTOR_BUDDY
+ depends on !HAVE_HARDLOCKUP_DETECTOR_PERF || HARDLOCKUP_DETECTOR_PREFER_BUDDY
+ depends on !HAVE_NMI_WATCHDOG
select HARDLOCKUP_DETECTOR_COUNTS_HRTIMER

+#
# Both the "perf" and "buddy" hardlockup detectors count hrtimer
# interrupts. This config enables functions managing this common code.
+#
config HARDLOCKUP_DETECTOR_COUNTS_HRTIMER
bool
select SOFTLOCKUP_DETECTOR

-# For hardlockup detectors you can have one directly provided by the arch
-# or use a "non-arch" one. If you're using a "non-arch" one that is
-# further divided the perf hardlockup detector (which, confusingly, needs
-# arch-provided perf support) and the buddy hardlockup detector (which just
-# needs SMP). In either case, using the "non-arch" code conflicts with
-# the NMI watchdog code (which is sometimes used directly and sometimes used
-# by the arch-provided hardlockup detector).
-config HAVE_HARDLOCKUP_DETECTOR_NON_ARCH
- bool
- depends on (HAVE_HARDLOCKUP_DETECTOR_PERF || SMP) && !HAVE_NMI_WATCHDOG
- default y
-
-# This will select the appropriate non-arch hardlockdup detector
-config HARDLOCKUP_DETECTOR_NON_ARCH
- bool
- depends on HAVE_HARDLOCKUP_DETECTOR_NON_ARCH
- select HARDLOCKUP_DETECTOR_BUDDY if !HAVE_HARDLOCKUP_DETECTOR_PERF || HARDLOCKUP_DETECTOR_PREFER_BUDDY
- select HARDLOCKUP_DETECTOR_PERF if HAVE_HARDLOCKUP_DETECTOR_PERF && !HARDLOCKUP_DETECTOR_PREFER_BUDDY
-
#
# Enables a timestamp based low pass filter to compensate for perf based
# hard lockup detection which runs too fast due to turbo modes.
--
2.35.3