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

From: Paul E. McKenney
Date: Thu May 13 2021 - 00:01:33 EST

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On Thu, May 13, 2021 at 11:29:47AM +0800, Feng Tang wrote:
> 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>

Thank you! I will apply this on my next rebase.

Thanx, Paul

> 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

Next message: Tong Zhang: "[PATCH v3] misc: alcor_pci: fix null-ptr-deref when there is no PCI bridge"
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In reply to: Feng Tang: "Re: [PATCH v14 clocksource 5/6] clocksource: Provide kernel module to test clocksource watchdog"
Next in thread: Paul E. McKenney: "[PATCH v14 clocksource 4/6] clocksource: Reduce clocksource-skew threshold for TSC"
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