Re: [PATCH v14 clocksource 5/6] clocksource: Provide kernel module to test clocksource watchdog

[Feng Tang]

On Tue, May 11, 2021 at 04:34:54PM -0700, Paul E. McKenney wrote:
> When the clocksource watchdog marks a clock as unstable, this might
> be due to that clock being unstable or it might be due to delays that
> happen to occur between the reads of the two clocks.  It would be good
> to have a way of testing the clocksource watchdog's ability to
> distinguish between these two causes of clock skew and instability.
> 
> Therefore, provide a new clocksource-wdtest module selected by a new
> TEST_CLOCKSOURCE_WATCHDOG Kconfig option.  This module has a single module
> parameter named "holdoff" that provides the number of seconds of delay
> before testing should start, which defaults to zero when built as a module
> and to 10 seconds when built directly into the kernel.  Very large systems
> that boot slowly may need to increase the value of this module parameter.
> 
> This module uses hand-crafted clocksource structures to do its testing,
> thus avoiding messing up timing for the rest of the kernel and for user
> applications.  This module first verifies that the ->uncertainty_margin
> field of the clocksource structures are set sanely.  It then tests the
> delay-detection capability of the clocksource watchdog, increasing the
> number of consecutive delays injected, first provoking console messages
> complaining about the delays and finally forcing a clock-skew event.
> Unexpected test results cause at least one WARN_ON_ONCE() console splat.
> If there are no splats, the test has passed.  Finally, it fuzzes the
> value returned from a clocksource to test the clocksource watchdog's
> ability to detect time skew.
> 
> This module checks the state of its clocksource after each test, and
> uses WARN_ON_ONCE() to emit a console splat if there are any failures.
> This should enable all types of test frameworks to detect any such
> failures.
> 
> This facility is intended for diagnostic use only, and should be avoided
> on production systems.

I tried the kvm rcutorture way:
 "tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 45s
 --configs TREE03 --kconfig "CONFIG_TEST_CLOCKSOURCE_WATCHDOG=y"
 --bootargs "clo
 +cksource.max_cswd_read_retries=1" --trust-make > /tmp/kvm.sh.out 2>&1"

Also I run it through a baremetal laptop with and without "nohpet" in
kernel cmdline. The debug messages from this module all look as expected, 
so

	Tested-by: Feng Tang <feng.tang@xxxxxxxxx>

Thanks,
Feng


> Link: https://lore.kernel.org/lkml/202104291438.PuHsxRkl-lkp@xxxxxxxxx/
> Link: https://lore.kernel.org/lkml/20210429140440.GT975577@paulmck-ThinkPad-P17-Gen-1
> Link: https://lore.kernel.org/lkml/20210425224540.GA1312438@paulmck-ThinkPad-P17-Gen-1/
> Link: https://lore.kernel.org/lkml/20210420064934.GE31773@xsang-OptiPlex-9020/
> Link: https://lore.kernel.org/lkml/20210106004013.GA11179@paulmck-ThinkPad-P72/
> Link: https://lore.kernel.org/lkml/20210414043435.GA2812539@paulmck-ThinkPad-P17-Gen-1/
> Link: https://lore.kernel.org/lkml/20210419045155.GA596058@paulmck-ThinkPad-P17-Gen-1/
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Mark Rutland <Mark.Rutland@xxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Feng Tang <feng.tang@xxxxxxxxx>
> Cc: Xing Zhengjun <zhengjun.xing@xxxxxxxxxxxxxxx>
> Reported-by: Chris Mason <clm@xxxxxx>
> [ paulmck: Export clocksource_verify_percpu per kernel test robot and Stephen Rothwell. ]
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>  .../admin-guide/kernel-parameters.txt         |   6 +
>  include/linux/clocksource.h                   |   3 +
>  kernel/time/Makefile                          |   1 +
>  kernel/time/clocksource-wdtest.c              | 202 ++++++++++++++++++
>  kernel/time/clocksource.c                     |  12 +-
>  lib/Kconfig.debug                             |  12 ++
>  6 files changed, 231 insertions(+), 5 deletions(-)
>  create mode 100644 kernel/time/clocksource-wdtest.c
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 0ef0ee743f65..9da285a98aa6 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -597,6 +597,12 @@
>  			The actual CPUs are chosen randomly, with
>  			no replacement if the same CPU is chosen twice.
>  
> +	clocksource-wdtest.holdoff= [KNL]
> +			Set the time in seconds that the clocksource
> +			watchdog test waits before commencing its tests.
> +			Defaults to zero when built as a module and to
> +			10 seconds when built into the kernel.
> +
>  	clearcpuid=BITNUM[,BITNUM...] [X86]
>  			Disable CPUID feature X for the kernel. See
>  			arch/x86/include/asm/cpufeatures.h for the valid bit
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index 895203727cb5..1d42d4b17327 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -291,4 +291,7 @@ static inline void timer_probe(void) {}
>  #define TIMER_ACPI_DECLARE(name, table_id, fn)		\
>  	ACPI_DECLARE_PROBE_ENTRY(timer, name, table_id, 0, NULL, 0, fn)
>  
> +extern ulong max_cswd_read_retries;
> +void clocksource_verify_percpu(struct clocksource *cs);
> +
>  #endif /* _LINUX_CLOCKSOURCE_H */
> diff --git a/kernel/time/Makefile b/kernel/time/Makefile
> index 1fb1c1ef6a19..1ed85b25b096 100644
> --- a/kernel/time/Makefile
> +++ b/kernel/time/Makefile
> @@ -21,3 +21,4 @@ obj-$(CONFIG_HAVE_GENERIC_VDSO)			+= vsyscall.o
>  obj-$(CONFIG_DEBUG_FS)				+= timekeeping_debug.o
>  obj-$(CONFIG_TEST_UDELAY)			+= test_udelay.o
>  obj-$(CONFIG_TIME_NS)				+= namespace.o
> +obj-$(CONFIG_TEST_CLOCKSOURCE_WATCHDOG)		+= clocksource-wdtest.o
> diff --git a/kernel/time/clocksource-wdtest.c b/kernel/time/clocksource-wdtest.c
> new file mode 100644
> index 000000000000..01df12395c0e
> --- /dev/null
> +++ b/kernel/time/clocksource-wdtest.c
> @@ -0,0 +1,202 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Unit test for the clocksource watchdog.
> + *
> + * Copyright (C) 2021 Facebook, Inc.
> + *
> + * Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> + */
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <linux/device.h>
> +#include <linux/clocksource.h>
> +#include <linux/init.h>
> +#include <linux/module.h>
> +#include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
> +#include <linux/tick.h>
> +#include <linux/kthread.h>
> +#include <linux/delay.h>
> +#include <linux/prandom.h>
> +#include <linux/cpu.h>
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Paul E. McKenney <paulmck@xxxxxxxxxx>");
> +
> +static int holdoff = IS_BUILTIN(CONFIG_TEST_CLOCKSOURCE_WATCHDOG) ? 10 : 0;
> +module_param(holdoff, int, 0444);
> +MODULE_PARM_DESC(holdoff, "Time to wait to start test (s).");
> +
> +/* Watchdog kthread's task_struct pointer for debug purposes. */
> +static struct task_struct *wdtest_task;
> +
> +static u64 wdtest_jiffies_read(struct clocksource *cs)
> +{
> +	return (u64)jiffies;
> +}
> +
> +/* Assume HZ > 100. */
> +#define JIFFIES_SHIFT	8
> +
> +static struct clocksource clocksource_wdtest_jiffies = {
> +	.name			= "wdtest-jiffies",
> +	.rating			= 1, /* lowest valid rating*/
> +	.uncertainty_margin	= TICK_NSEC,
> +	.read			= wdtest_jiffies_read,
> +	.mask			= CLOCKSOURCE_MASK(32),
> +	.flags			= CLOCK_SOURCE_MUST_VERIFY,
> +	.mult			= TICK_NSEC << JIFFIES_SHIFT, /* details above */
> +	.shift			= JIFFIES_SHIFT,
> +	.max_cycles		= 10,
> +};
> +
> +static int wdtest_ktime_read_ndelays;
> +static bool wdtest_ktime_read_fuzz;
> +
> +static u64 wdtest_ktime_read(struct clocksource *cs)
> +{
> +	int wkrn = READ_ONCE(wdtest_ktime_read_ndelays);
> +	static int sign = 1;
> +	u64 ret;
> +
> +	if (wkrn) {
> +		udelay(cs->uncertainty_margin / 250);
> +		WRITE_ONCE(wdtest_ktime_read_ndelays, wkrn - 1);
> +	}
> +	ret = ktime_get_real_fast_ns();
> +	if (READ_ONCE(wdtest_ktime_read_fuzz)) {
> +		sign = -sign;
> +		ret = ret + sign * 100 * NSEC_PER_MSEC;
> +	}
> +	return ret;
> +}
> +
> +static void wdtest_ktime_cs_mark_unstable(struct clocksource *cs)
> +{
> +	pr_info("--- Marking %s unstable due to clocksource watchdog.\n", cs->name);
> +}
> +
> +#define KTIME_FLAGS (CLOCK_SOURCE_IS_CONTINUOUS | \
> +		     CLOCK_SOURCE_VALID_FOR_HRES | \
> +		     CLOCK_SOURCE_MUST_VERIFY | \
> +		     CLOCK_SOURCE_VERIFY_PERCPU)
> +
> +static struct clocksource clocksource_wdtest_ktime = {
> +	.name			= "wdtest-ktime",
> +	.rating			= 300,
> +	.read			= wdtest_ktime_read,
> +	.mask			= CLOCKSOURCE_MASK(64),
> +	.flags			= KTIME_FLAGS,
> +	.mark_unstable		= wdtest_ktime_cs_mark_unstable,
> +	.list			= LIST_HEAD_INIT(clocksource_wdtest_ktime.list),
> +};
> +
> +/* Reset the clocksource if needed. */
> +static void wdtest_ktime_clocksource_reset(void)
> +{
> +	if (clocksource_wdtest_ktime.flags & CLOCK_SOURCE_UNSTABLE) {
> +		clocksource_unregister(&clocksource_wdtest_ktime);
> +		clocksource_wdtest_ktime.flags = KTIME_FLAGS;
> +		schedule_timeout_uninterruptible(HZ / 10);
> +		clocksource_register_khz(&clocksource_wdtest_ktime, 1000 * 1000);
> +	}
> +}
> +
> +/* Run the specified series of watchdog tests. */
> +static int wdtest_func(void *arg)
> +{
> +	unsigned long j1, j2;
> +	char *s;
> +	int i;
> +
> +	schedule_timeout_uninterruptible(holdoff * HZ);
> +
> +	/*
> +	 * Verify that jiffies-like clocksources get the manually
> +	 * specified uncertainty margin.
> +	 */
> +	pr_info("--- Verify jiffies-like uncertainty margin.\n");
> +	__clocksource_register(&clocksource_wdtest_jiffies);
> +	WARN_ON_ONCE(clocksource_wdtest_jiffies.uncertainty_margin != TICK_NSEC);
> +
> +	j1 = clocksource_wdtest_jiffies.read(&clocksource_wdtest_jiffies);
> +	schedule_timeout_uninterruptible(HZ);
> +	j2 = clocksource_wdtest_jiffies.read(&clocksource_wdtest_jiffies);
> +	WARN_ON_ONCE(j1 == j2);
> +
> +	clocksource_unregister(&clocksource_wdtest_jiffies);
> +
> +	/*
> +	 * Verify that tsc-like clocksources are assigned a reasonable
> +	 * uncertainty margin.
> +	 */
> +	pr_info("--- Verify tsc-like uncertainty margin.\n");
> +	clocksource_register_khz(&clocksource_wdtest_ktime, 1000 * 1000);
> +	WARN_ON_ONCE(clocksource_wdtest_ktime.uncertainty_margin < NSEC_PER_USEC);
> +
> +	j1 = clocksource_wdtest_ktime.read(&clocksource_wdtest_ktime);
> +	udelay(1);
> +	j2 = clocksource_wdtest_ktime.read(&clocksource_wdtest_ktime);
> +	pr_info("--- tsc-like times: %lu - %lu = %lu.\n", j2, j1, j2 - j1);
> +	WARN_ON_ONCE(time_before(j2, j1 + NSEC_PER_USEC));
> +
> +	/* Verify tsc-like stability with various numbers of errors injected. */
> +	for (i = 0; i <= max_cswd_read_retries + 1; i++) {
> +		if (i <= 1 && i < max_cswd_read_retries)
> +			s = "";
> +		else if (i <= max_cswd_read_retries)
> +			s = ", expect message";
> +		else
> +			s = ", expect clock skew";
> +		pr_info("--- Watchdog with %dx error injection, %lu retries%s.\n", i, max_cswd_read_retries, s);
> +		WRITE_ONCE(wdtest_ktime_read_ndelays, i);
> +		schedule_timeout_uninterruptible(2 * HZ);
> +		WARN_ON_ONCE(READ_ONCE(wdtest_ktime_read_ndelays));
> +		WARN_ON_ONCE((i <= max_cswd_read_retries) !=
> +			     !(clocksource_wdtest_ktime.flags & CLOCK_SOURCE_UNSTABLE));
> +		wdtest_ktime_clocksource_reset();
> +	}
> +
> +	/* Verify tsc-like stability with clock-value-fuzz error injection. */
> +	pr_info("--- Watchdog clock-value-fuzz error injection, expect clock skew and per-CPU mismatches.\n");
> +	WRITE_ONCE(wdtest_ktime_read_fuzz, true);
> +	schedule_timeout_uninterruptible(2 * HZ);
> +	WARN_ON_ONCE(!(clocksource_wdtest_ktime.flags & CLOCK_SOURCE_UNSTABLE));
> +	clocksource_verify_percpu(&clocksource_wdtest_ktime);
> +	WRITE_ONCE(wdtest_ktime_read_fuzz, false);
> +
> +	clocksource_unregister(&clocksource_wdtest_ktime);
> +
> +	pr_info("--- Done with test.\n");
> +	return 0;
> +}
> +
> +static void wdtest_print_module_parms(void)
> +{
> +	pr_alert("--- holdoff=%d\n", holdoff);
> +}
> +
> +/* Cleanup function. */
> +static void clocksource_wdtest_cleanup(void)
> +{
> +}
> +
> +static int __init clocksource_wdtest_init(void)
> +{
> +	int ret = 0;
> +
> +	wdtest_print_module_parms();
> +
> +	/* Create watchdog-test task. */
> +	wdtest_task = kthread_run(wdtest_func, NULL, "wdtest");
> +	if (IS_ERR(wdtest_task)) {
> +		ret = PTR_ERR(wdtest_task);
> +		pr_warn("%s: Failed to create wdtest kthread.\n", __func__);
> +		wdtest_task = NULL;
> +		return ret;
> +	}
> +
> +	return 0;
> +}
> +
> +module_init(clocksource_wdtest_init);
> +module_exit(clocksource_wdtest_cleanup);
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 9ebf9931f3d6..bbe1bcf44ffa 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -199,8 +199,9 @@ void clocksource_mark_unstable(struct clocksource *cs)
>  	spin_unlock_irqrestore(&watchdog_lock, flags);
>  }
>  
> -static ulong max_read_retries = 3;
> -module_param(max_read_retries, ulong, 0644);
> +ulong max_cswd_read_retries = 3;
> +module_param(max_cswd_read_retries, ulong, 0644);
> +EXPORT_SYMBOL_GPL(max_cswd_read_retries);
>  static int verify_n_cpus = 8;
>  module_param(verify_n_cpus, int, 0644);
>  
> @@ -210,7 +211,7 @@ static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
>  	u64 wd_end, wd_delta;
>  	int64_t wd_delay;
>  
> -	for (nretries = 0; nretries <= max_read_retries; nretries++) {
> +	for (nretries = 0; nretries <= max_cswd_read_retries; nretries++) {
>  		local_irq_disable();
>  		*wdnow = watchdog->read(watchdog);
>  		*csnow = cs->read(cs);
> @@ -220,7 +221,7 @@ static bool cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)
>  		wd_delta = clocksource_delta(wd_end, *wdnow, watchdog->mask);
>  		wd_delay = clocksource_cyc2ns(wd_delta, watchdog->mult, watchdog->shift);
>  		if (wd_delay <= WATCHDOG_MAX_SKEW) {
> -			if (nretries > 1 || nretries >= max_read_retries) {
> +			if (nretries > 1 || nretries >= max_cswd_read_retries) {
>  				pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n",
>  					smp_processor_id(), watchdog->name, nretries);
>  			}
> @@ -293,7 +294,7 @@ static void clocksource_verify_one_cpu(void *csin)
>  	csnow_mid = cs->read(cs);
>  }
>  
> -static void clocksource_verify_percpu(struct clocksource *cs)
> +void clocksource_verify_percpu(struct clocksource *cs)
>  {
>  	int64_t cs_nsec, cs_nsec_max = 0, cs_nsec_min = LLONG_MAX;
>  	u64 csnow_begin, csnow_end;
> @@ -346,6 +347,7 @@ static void clocksource_verify_percpu(struct clocksource *cs)
>  		pr_warn("        CPU %d check durations %lldns - %lldns for clocksource %s.\n",
>  			testcpu, cs_nsec_min, cs_nsec_max, cs->name);
>  }
> +EXPORT_SYMBOL_GPL(clocksource_verify_percpu);
>  
>  static void clocksource_watchdog(struct timer_list *unused)
>  {
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 678c13967580..0a5a70c742e6 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2571,6 +2571,18 @@ config TEST_FPU
>  
>  	  If unsure, say N.
>  
> +config TEST_CLOCKSOURCE_WATCHDOG
> +	tristate "Test clocksource watchdog in kernel space"
> +	depends on CLOCKSOURCE_WATCHDOG
> +	help
> +	  Enable this option to create a kernel module that will trigger
> +	  a test of the clocksource watchdog.  This module may be loaded
> +	  via modprobe or insmod in which case it will run upon being
> +	  loaded, or it may be built in, in which case it will run
> +	  shortly after boot.
> +
> +	  If unsure, say N.
> +
>  endif # RUNTIME_TESTING_MENU
>  
>  config ARCH_USE_MEMTEST
> -- 
> 2.31.1.189.g2e36527f23