lkml.org 
[lkml]   [2021]   [May]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Subject[RFC PATCH v5 00/16] x86: Implement an HPET-based hardlockup detector
Date
Hi,

This is the long overdue fifth attempt to implement a hardlockup detector
that uses an HPET channel. Previous versions can be found in [1], [2],
[3], and [4]. I will soon post in the IOMMU mailing list a separate series
to deal with interrupt remapping. Thus, x86 reviewers may hold off on their
review momentarily while the IOMMU experts decide on whether what I propose
makes sense.

This RFC series has been reviewed by Tony Luck <tony.luck@intel.com>

== Introduction ==

In CPU architectures that do not have an NMI watchdog, one can be
constructed using a counter of the Performance Monitoring Unit (PMU).
Counters in the PMU have high granularity and high visibility of the CPU.
These capabilities and their limited number make these counters precious
resources. Unfortunately, the perf-based hardlockup detector permanently
consumes one of these counters per CPU. These counters could be freed
if the hardlockup detector were driven by another timer.

The hardlockup detector runs relatively infrequently and does not require
visibility of the CPU activity (in addition to detect locked-up CPUs). A
timer that is external to the CPU (e.g., in the chipset) can be used to
drive the detector.

A key requirement is that the timer needs to be capable of issuing a
non-maskable interrupt to the CPU. In most cases, this can be achieved
by tweaking the delivery mode of the interrupt. It is especially
straightforward for MSI interrupts.

This implementation uses an HPET timer to deliver an NMI interrupt via
an MSI message.

== Details of this implementation

Unlike the perf-based hardlockup detector, this implementation is
driven by a single timer. Driving the detector with a single timer brings
certain complexities to the implementation: accessing the HPET timer is
slow, and the frequency and the affinity of the timer interrupt needs to be
adjusted periodically. These operations need to happen in addition to
servicing the NMI. In order to address these complexities, this design
meets the following goals:

* Minimize updates to the affinity of the HPET timer interrupt and do
it outside of NMI context.

* Avoid races with System Management Mode that may lead to see the HPET
count stall or run backwards.

* Minimize the number of reads and writes to the HPET registers.

Also, as per feedback from Thomas Gleixner,

* Do not implement an IRQF_NMI to request an NMI interrupt for x86 due
to the difficulty of identifying its source.

In order to meet the goals above, I implemented what Thomas suggested [5]:
a detector that mixes inter-processor interrupts (IPIs) and updating the
affinity of the HPET interrupt in a round-robin fashion.

The CPUs that the hardlockup detector monitors are partitioned into groups.
A CPU from such group, the handling CPU, handles the NMI interrupt from the
HPET timer and then issues an NMI IPI to the rest of the CPUs in the group.
Each of the CPUs looks for hardlockups upon reception of the NMI. To
minimize IPIs among packages, a group constitutes all the CPUs in a
package.

Since the monitored CPUs now have been partitioned in groups, the HPET
timer needs to target the handling CPU of each group. Hence, the affinity
of the HPET timer is updated in round-robin manner to sequentially target
each group of CPUs. Each group of CPUs will be monitored every 1 second or
less frequently. This is the frequency of the HPET timer interrupt. In the
unlikely case of having more packages than the value watch_thresh, several
packages will be grouped together to keep the HPET timer interrupt at 1
second.

In order to avoid reading HPET registers in every NMI, the time-stamp
counter is used to determine whether the HPET caused the interrupt. At
every timer expiration, we compute the value the time-stamp counter is
expected to have the next time the timer expires. I have found
experimentally that expected TSC value consistently has an error of less
than 1%.

== Parts of this series ==

For clarity, patches are grouped as follows:

1) HPET updates. Patches 1-3 prepare the HPET code to accommodate the
new detector: rework periodic programming, reserve and configure a
timer for the detector and expose a few existing functions.

3) NMI watchdog. Patches 4-6 updates the existing hardlockup detector
to uncouple it from perf, and introduces a new NMI handler category
intended to run after the NMI_LOCAL handlers.

4) New HPET-based hardlockup detector. Patches 7-11 includes changes to
probe the hardware resources, configure the interrupt, and rotate the
destination of the interrupts among all monitored CPUs.

5) Hardlockup detector management. Patches 12-16 is a collection of
miscellaneous patches to determine when to use the HPET hardlockup
detector and stop it if necessary. It also includes an x86-specific
shim hardlockup detector that selects between the perf- and hpet-based
implementations. It also switches back to the perf implementation if
the TSC becomes unstable.

== Testing ==

I tested this series in both client and server systems, single-package and
dual-package systems. I let the system run normally and observed the NMI
interrupts keep coming by inspecting the file /proc/interrupts.

I also implemented a test module to hog a CPU and observed the detector
complain about soft and hardlockups.

Lastly, I wrote a script to keep only one CPU online at a time in a round-
robin manner by doing 500 back-to-back CPU hotplug online/offline 500
times. During the test, I set /proc/sys/kernel/watchdog_thresh to 1 second.
The HPET-based hardlockup detector is still functional after the test. I
used the latest master branch from the tip tree.

The tests above were performed with interrupt remapping both enabled and
disabled.

Thanks and BR,
Ricardo

Changes since v4:
* Added commentary on the tests performed on this feature. (Andi)
* Added a stub version of hardlockup_detector_switch_to_perf() for
!CONFIG_HPET_TIMER. (lkp)
* Use switch to select the type of x86 hardlockup detector. (Andi)
* Renamed a local variable in update_ticks_per_group(). (Andi)
* Made this hardlockup detector available to X86_32.
* Reworked logic to kick the HPET timer to remove a local variable.
(Andi)
* Added a comment on what type of timer channel will be assigned to the
detector. (Andi)
* Reworded help comment in the X86_HARDLOCKUP_DETECTOR_HPET Kconfig
option. (Andi)
* Removed unnecessary switch to level interrupt mode when disabling the
timer. (Andi)
* Disabled the HPET timer to avoid a race between an incoming interrupt
and an update of the MSI destination ID. (Ashok)
* Renamed hpet_hardlockup_detector_get_timer() as hpet_hld_get_timer()
* Added commentary on an undocumented udelay() when programming an
HPET channel in periodic mode. (Ashok)
* Reworked code to use new enumeration apic_delivery_modes and reworked
MSI message composition fields [6].
* Partitioned monitored CPUs into groups. Each CPU in the group is
inspected for hardlockups using an IPI.
* Use a round-robin mechanism to update the affinity of the HPET timer.
Affinity is updated every watchdog_thresh seconds to target the
handling CPU of the group.
* Moved update of the HPET interrupt affinity to an irq_work. (Thomas
Gleixner).
* Updated expiration of the HPET timer and the expected value of the
TSC based on the number of groups of monitored CPUs.
* Renamed hpet_set_comparator() to hpet_set_comparator_periodic() to
remove decision logic for periodic case. (Thomas Gleixner)
* Reworked timer reservation to use Thomas' rework on HPET channel
management[7].
* Removed hard-coded channel number for the hardlockup detector.
* Provided more details on the sequence of HPET channel reservation.
(Thomas Gleixner)
* Only reserve a channel for the hardlockup detector if enabled via
kernel command line. The function reserving the channel is called from
hardlockup detector. (Thomas Gleixner)
* Dropped hpet_hld_data::enabled_cpus and instead use cpumask_weight().
* Renamed hpet_hld_data::cpu_monitored_mask to
hld_data_data.cpu_monitored_mask and converted it to cpumask_var_t.
* Flushed out any outstanding interrupt before enabling the HPET channel.
* Removed unnecessary MSI_DATA_LEVEL_ASSERT from the MSI message.
* Added comments in hardlockup_detector_nmi_handler() to explain how
CPUs are targeted for an IPI.
* Updated code to only issue an IPI when needed (i.e., there are CPUs in
the group other than the handling CPU).
* Reworked hardlockup_detector_hpet_init() for readability.
* Now reserve the cpumasks in the hardlockup detector code and not in the
generic HPET code.
* Handle the case of watchdog_thresh = 0 when disabling the detector.

Change since v3:
* Fixed yet another bug in periodic programming of the HPET timer that
prevented the system from booting.
* Fixed computation of HPET frequency to use hpet_readl() only.
* Added a missing #include in the watchdog_hld_hpet.c
* Fixed various typos and grammar errors (Randy Dunlap)

Changes since v2:
* Added functionality to switch to the perf-based hardlockup
detector if the TSC becomes unstable (Thomas Gleixner).
* Brought back the round-robin mechanism proposed in v1 (this time not
using the interrupt subsystem). This also requires computing
expiration times as in v1 (Andi Kleen, Stephane Eranian).
* Fixed a bug in which using a periodic timer was not working(thanks
to Suravee Suthikulpanit!).
* In this version, I incorporate support for interrupt remapping in the
last 4 patches so that they can be reviewed separately if needed.
* Removed redundant documentation of functions (Thomas Gleixner).
* Added a new category of NMI handler, NMI_WATCHDOG, which executes after
NMI_LOCAL handlers (Andi Kleen).
* Updated handling of "nmi_watchdog" to support comma-separated
arguments.
* Undid split of the generic hardlockup detector into a separate file
(Thomas Gleixner).
* Added a new intermediate symbol CONFIG_HARDLOCKUP_DETECTOR_CORE to
select generic parts of the detector (Paul E. McKenney,
Thomas Gleixner).
* Removed use of struct cpumask in favor of a variable length array in
conjunction with kzalloc (Peter Zijlstra).
* Added CPU as argument hardlockup_detector_hpet_enable()/disable()
(Thomas Gleixner).
* Remove unnecessary export of function declarations, flags, and bit
fields (Thomas Gleixner).
* Removed unnecessary check for FSB support when reserving timer for the
detector (Thomas Gleixner).
* Separated TSC code from HPET code in kick_timer() (Thomas Gleixner).
* Reworked condition to check if the expected TSC value is within the
error margin to avoid conditional (Peter Zijlstra).
* Removed TSC error margin from struct hld_data; use global variable
instead (Peter Zijlstra).
* Removed previously introduced watchdog_get_allowed_cpumask*() and
reworked hardlockup_detector_hpet_enable()/disable() to not need
access to watchdog_allowed_mask (Thomas Gleixner).

Changes since v1:

* Removed reads to HPET registers at every NMI. Instead use the time-stamp
counter to infer the interrupt source (Thomas Gleixner, Andi Kleen).
* Do not target CPUs in a round-robin manner. Instead, the HPET timer
always targets the same CPU; other CPUs are monitored via an
interprocessor interrupt.
* Removed use of generic irq code to set interrupt affinity and NMI
delivery. Instead, configure the interrupt directly in HPET registers
(Thomas Gleixner).
* Removed the proposed ops structure for NMI watchdogs. Instead, split
the existing implementation into a generic library and perf-specific
infrastructure (Thomas Gleixner, Nicholas Piggin).
* Added an x86-specific shim hardlockup detector that selects between
HPET and perf infrastructures as needed (Nicholas Piggin).
* Removed locks taken in NMI and !NMI context. This was wrong and is no
longer needed (Thomas Gleixner).
* Fixed unconditional return NMI_HANDLED when the HPET timer is programmed
for FSB/MSI delivery (Peter Zijlstra).

[1]. https://lore.kernel.org/lkml/1528851463-21140-1-git-send-email-ricardo.neri-calderon@linux.intel.com/
[2]. https://lore.kernel.org/lkml/1551283518-18922-1-git-send-email-ricardo.neri-calderon@linux.intel.com/
[3]. https://lore.kernel.org/lkml/1557842534-4266-1-git-send-email-ricardo.neri-calderon@linux.intel.com/
[4]. https://lore.kernel.org/lkml/1558660583-28561-1-git-send-email-ricardo.neri-calderon@linux.intel.com/
[5]. https://lore.kernel.org/lkml/alpine.DEB.2.21.1906172343120.1963@nanos.tec.linutronix.de/
[6]. https://lore.kernel.org/r/20201024213535.443185-6-dwmw2@infradead.org
[7]. https://lore.kernel.org/lkml/20190623132340.463097504@linutronix.de/

Ricardo Neri (16):
x86/hpet: Expose hpet_writel() in header
x86/hpet: Add helper function hpet_set_comparator_periodic()
x86/hpet: Reserve an HPET channel for the hardlockup detector
watchdog/hardlockup: Define a generic function to detect hardlockups
watchdog/hardlockup: Decouple the hardlockup detector from perf
x86/nmi: Add an NMI_WATCHDOG NMI handler category
x86/watchdog/hardlockup: Add an HPET-based hardlockup detector
x86/watchdog/hardlockup/hpet: Introduce a target_cpumask
watchdog/hardlockup/hpet: Group packages receiving IPIs when needed
watchdog/hardlockup/hpet: Adjust timer expiration on the number of
monitored groups
x86/watchdog/hardlockup/hpet: Determine if HPET timer caused NMI
watchdog/hardlockup: Use parse_option_str() to handle "nmi_watchdog"
watchdog/hardlockup/hpet: Only enable the HPET watchdog via a boot
parameter
x86/watchdog: Add a shim hardlockup detector
watchdog: Expose lockup_detector_reconfigure()
x86/tsc: Switch to perf-based hardlockup detector if TSC become
unstable

.../admin-guide/kernel-parameters.txt | 8 +-
arch/x86/Kconfig.debug | 14 +
arch/x86/include/asm/hpet.h | 67 ++
arch/x86/include/asm/nmi.h | 1 +
arch/x86/kernel/Makefile | 2 +
arch/x86/kernel/hpet.c | 124 +++-
arch/x86/kernel/nmi.c | 10 +
arch/x86/kernel/tsc.c | 2 +
arch/x86/kernel/watchdog_hld.c | 86 +++
arch/x86/kernel/watchdog_hld_hpet.c | 633 ++++++++++++++++++
include/linux/nmi.h | 8 +-
kernel/Makefile | 2 +-
kernel/watchdog.c | 12 +-
kernel/watchdog_hld.c | 50 +-
lib/Kconfig.debug | 4 +
15 files changed, 982 insertions(+), 41 deletions(-)
create mode 100644 arch/x86/kernel/watchdog_hld.c
create mode 100644 arch/x86/kernel/watchdog_hld_hpet.c

--
2.17.1

\
 
 \ /
  Last update: 2021-05-04 21:39    [W:0.085 / U:0.280 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site