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![Feng Tang](/assets/img/avatar_default.png)
On a 8-socket server the TSC is wrongly marked as 'unstable' and disabled during boot time on about one out of 120 boot attempts: clocksource: timekeeping watchdog on CPU227: wd-tsc-wd excessive read-back delay of 153560ns vs. limit of 125000ns, wd-wd read-back delay only 11440ns, attempt 3, marking tsc unstable tsc: Marking TSC unstable due to clocksource watchdog TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'. sched_clock: Marking unstable (119294969739, 159204297)<-(125446229205, -5992055152) clocksource: Checking clocksource tsc synchronization from CPU 319 to CPUs 0,99,136,180,210,542,601,896. clocksource: Switched to clocksource hpet The reason is that for platform with a large number of CPUs, there are sporadic big or huge read latencies while reading the watchog/clocksource during boot or when system is under stress work load, and the frequency and maximum value of the latency goes up with the number of online CPUs. The cCurrent code already has logic to detect and filter such high latency case by reading the watchdog twice and checking the two deltas. Due to the randomness of the latency, there is a low probabilty that the first delta (latency) is big, but the second delta is small and looks valid. The watchdog code retries the readouts by default twice, which is not necessarily sufficient for systems with a large number of CPUs. There is a command line parameter 'max_cswd_read_retries' which allows to increase the number of retries, but that's not user friendly as it needs to be tweaked per system. As the number of required retries is proportional to the number of online CPUs, this parameter can be calculated at runtime. Scale and enlarge the number of retries according to the number of online CPUs and remove the command line parameter completely. [ tglx: Massaged change log and comments ] Signed-off-by: Feng Tang <feng.tang@intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Jin Wang <jin1.wang@intel.com> Tested-by: Paul E. McKenney <paulmck@kernel.org> Reviewed-by: Waiman Long <longman@redhat.com> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/r/20240221060859.1027450-1-feng.tang@intel.com
203 lines
5.8 KiB
C
203 lines
5.8 KiB
C
// SPDX-License-Identifier: GPL-2.0+
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/*
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* Unit test for the clocksource watchdog.
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*
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* Copyright (C) 2021 Facebook, Inc.
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*
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* Author: Paul E. McKenney <paulmck@kernel.org>
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*/
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#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
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#include <linux/device.h>
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#include <linux/clocksource.h>
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#include <linux/init.h>
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#include <linux/module.h>
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#include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
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#include <linux/tick.h>
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#include <linux/kthread.h>
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#include <linux/delay.h>
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#include <linux/prandom.h>
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#include <linux/cpu.h>
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#include "tick-internal.h"
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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("Paul E. McKenney <paulmck@kernel.org>");
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static int holdoff = IS_BUILTIN(CONFIG_TEST_CLOCKSOURCE_WATCHDOG) ? 10 : 0;
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module_param(holdoff, int, 0444);
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MODULE_PARM_DESC(holdoff, "Time to wait to start test (s).");
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/* Watchdog kthread's task_struct pointer for debug purposes. */
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static struct task_struct *wdtest_task;
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static u64 wdtest_jiffies_read(struct clocksource *cs)
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{
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return (u64)jiffies;
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}
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static struct clocksource clocksource_wdtest_jiffies = {
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.name = "wdtest-jiffies",
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.rating = 1, /* lowest valid rating*/
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.uncertainty_margin = TICK_NSEC,
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.read = wdtest_jiffies_read,
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.mask = CLOCKSOURCE_MASK(32),
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.flags = CLOCK_SOURCE_MUST_VERIFY,
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.mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
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.shift = JIFFIES_SHIFT,
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.max_cycles = 10,
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};
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static int wdtest_ktime_read_ndelays;
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static bool wdtest_ktime_read_fuzz;
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static u64 wdtest_ktime_read(struct clocksource *cs)
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{
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int wkrn = READ_ONCE(wdtest_ktime_read_ndelays);
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static int sign = 1;
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u64 ret;
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if (wkrn) {
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udelay(cs->uncertainty_margin / 250);
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WRITE_ONCE(wdtest_ktime_read_ndelays, wkrn - 1);
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}
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ret = ktime_get_real_fast_ns();
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if (READ_ONCE(wdtest_ktime_read_fuzz)) {
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sign = -sign;
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ret = ret + sign * 100 * NSEC_PER_MSEC;
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}
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return ret;
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}
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static void wdtest_ktime_cs_mark_unstable(struct clocksource *cs)
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{
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pr_info("--- Marking %s unstable due to clocksource watchdog.\n", cs->name);
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}
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#define KTIME_FLAGS (CLOCK_SOURCE_IS_CONTINUOUS | \
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CLOCK_SOURCE_VALID_FOR_HRES | \
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CLOCK_SOURCE_MUST_VERIFY | \
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CLOCK_SOURCE_VERIFY_PERCPU)
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static struct clocksource clocksource_wdtest_ktime = {
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.name = "wdtest-ktime",
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.rating = 300,
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.read = wdtest_ktime_read,
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.mask = CLOCKSOURCE_MASK(64),
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.flags = KTIME_FLAGS,
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.mark_unstable = wdtest_ktime_cs_mark_unstable,
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.list = LIST_HEAD_INIT(clocksource_wdtest_ktime.list),
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};
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/* Reset the clocksource if needed. */
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static void wdtest_ktime_clocksource_reset(void)
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{
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if (clocksource_wdtest_ktime.flags & CLOCK_SOURCE_UNSTABLE) {
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clocksource_unregister(&clocksource_wdtest_ktime);
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clocksource_wdtest_ktime.flags = KTIME_FLAGS;
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schedule_timeout_uninterruptible(HZ / 10);
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clocksource_register_khz(&clocksource_wdtest_ktime, 1000 * 1000);
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}
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}
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/* Run the specified series of watchdog tests. */
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static int wdtest_func(void *arg)
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{
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unsigned long j1, j2;
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int i, max_retries;
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char *s;
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schedule_timeout_uninterruptible(holdoff * HZ);
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/*
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* Verify that jiffies-like clocksources get the manually
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* specified uncertainty margin.
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*/
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pr_info("--- Verify jiffies-like uncertainty margin.\n");
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__clocksource_register(&clocksource_wdtest_jiffies);
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WARN_ON_ONCE(clocksource_wdtest_jiffies.uncertainty_margin != TICK_NSEC);
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j1 = clocksource_wdtest_jiffies.read(&clocksource_wdtest_jiffies);
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schedule_timeout_uninterruptible(HZ);
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j2 = clocksource_wdtest_jiffies.read(&clocksource_wdtest_jiffies);
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WARN_ON_ONCE(j1 == j2);
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clocksource_unregister(&clocksource_wdtest_jiffies);
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/*
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* Verify that tsc-like clocksources are assigned a reasonable
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* uncertainty margin.
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*/
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pr_info("--- Verify tsc-like uncertainty margin.\n");
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clocksource_register_khz(&clocksource_wdtest_ktime, 1000 * 1000);
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WARN_ON_ONCE(clocksource_wdtest_ktime.uncertainty_margin < NSEC_PER_USEC);
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j1 = clocksource_wdtest_ktime.read(&clocksource_wdtest_ktime);
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udelay(1);
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j2 = clocksource_wdtest_ktime.read(&clocksource_wdtest_ktime);
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pr_info("--- tsc-like times: %lu - %lu = %lu.\n", j2, j1, j2 - j1);
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WARN_ON_ONCE(time_before(j2, j1 + NSEC_PER_USEC));
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/* Verify tsc-like stability with various numbers of errors injected. */
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max_retries = clocksource_get_max_watchdog_retry();
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for (i = 0; i <= max_retries + 1; i++) {
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if (i <= 1 && i < max_retries)
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s = "";
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else if (i <= max_retries)
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s = ", expect message";
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else
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s = ", expect clock skew";
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pr_info("--- Watchdog with %dx error injection, %d retries%s.\n", i, max_retries, s);
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WRITE_ONCE(wdtest_ktime_read_ndelays, i);
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schedule_timeout_uninterruptible(2 * HZ);
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WARN_ON_ONCE(READ_ONCE(wdtest_ktime_read_ndelays));
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WARN_ON_ONCE((i <= max_retries) !=
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!(clocksource_wdtest_ktime.flags & CLOCK_SOURCE_UNSTABLE));
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wdtest_ktime_clocksource_reset();
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}
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/* Verify tsc-like stability with clock-value-fuzz error injection. */
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pr_info("--- Watchdog clock-value-fuzz error injection, expect clock skew and per-CPU mismatches.\n");
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WRITE_ONCE(wdtest_ktime_read_fuzz, true);
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schedule_timeout_uninterruptible(2 * HZ);
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WARN_ON_ONCE(!(clocksource_wdtest_ktime.flags & CLOCK_SOURCE_UNSTABLE));
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clocksource_verify_percpu(&clocksource_wdtest_ktime);
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WRITE_ONCE(wdtest_ktime_read_fuzz, false);
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clocksource_unregister(&clocksource_wdtest_ktime);
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pr_info("--- Done with test.\n");
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return 0;
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}
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static void wdtest_print_module_parms(void)
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{
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pr_alert("--- holdoff=%d\n", holdoff);
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}
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/* Cleanup function. */
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static void clocksource_wdtest_cleanup(void)
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{
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}
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static int __init clocksource_wdtest_init(void)
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{
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int ret = 0;
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wdtest_print_module_parms();
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/* Create watchdog-test task. */
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wdtest_task = kthread_run(wdtest_func, NULL, "wdtest");
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if (IS_ERR(wdtest_task)) {
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ret = PTR_ERR(wdtest_task);
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pr_warn("%s: Failed to create wdtest kthread.\n", __func__);
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wdtest_task = NULL;
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return ret;
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}
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return 0;
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}
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module_init(clocksource_wdtest_init);
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module_exit(clocksource_wdtest_cleanup);
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