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Date: Tue, 20 Feb 2024 20:42:29 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Feng Tang <feng.tang@...el.com>
Cc: John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
Stephen Boyd <sboyd@...nel.org>, Jonathan Corbet <corbet@....net>,
Peter Zijlstra <peterz@...radead.org>,
Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
Jin Wang <jin1.wang@...el.com>
Subject: Re: [PATCH v5] clocksource: Scale the max retry number of watchdog
read according to CPU numbers
On Wed, Feb 21, 2024 at 11:04:07AM +0800, Feng Tang wrote:
> There was a bug on one 8-socket server that the TSC is wrongly marked
> as 'unstable' and disabled during boot time (reproduce rate is about
> every 120 rounds of reboot tests), with log:
>
> 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 for platform with lots of CPU, there are sporadic big or
> huge read latency of read 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 increasing of CPU numbers. Current code already
> has logic to detect and filter such high latency case by reading 3 times
> of watchdog, and check the 2 deltas. Due to the randomness of the
> latency, there is a low possibility situation that the first delta
> (latency) is big, but the second delta is small and looks valid, which
> can escape from the check, and there is a 'max_cswd_read_retries' for
> retrying that check covering this case, whose default value is only 2
> and may be not enough for machines with huge number of CPUs.
>
> So scale and enlarge the max retry number according to CPU number to
> better filter those latency noise for large systems, which has been
> verified fine in 4 days reboot test on the 8-socket machine.
>
> Also as suggested by Thomas, remove parameter 'max_cswd_read_retries'
> which was originally introduced to cover this.
>
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> Tested-by: Jin Wang <jin1.wang@...el.com>
> Tested-by: Paul E. McKenney <paulmck@...nel.org>
> Reviewed-by: Waiman Long <longman@...hat.com>
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> ---
> Changelog:
>
> since v4:
> * Add a helper function to be shared by different files (Paul)
>
> since v3:
> * Remove clocksource's module parameter 'max_cswd_read_retries' (Thomas)
> * Use "ilog4" instead of ilog2 for max retry calculation, and
> may be adjusted later (Paul)
>
> since v2:
> * Fix the unexported symbol of helper function being used by
> kernel module issue (Waiman)
>
> since v1:
> * Add santity check for user input value of 'max_cswd_read_retries'
> and a helper function for getting max retry nubmer (Paul)
> * Apply the same logic to watchdog test code (Waiman)
>
>
>
> Documentation/admin-guide/kernel-parameters.txt | 6 ------
> include/linux/clocksource.h | 13 ++++++++++++-
> kernel/time/clocksource-wdtest.c | 13 +++++++------
> kernel/time/clocksource.c | 9 ++++-----
> tools/testing/selftests/rcutorture/bin/torture.sh | 2 +-
> 5 files changed, 24 insertions(+), 19 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 31b3a25680d0..763e96dcf8b1 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -679,12 +679,6 @@
> loops can be debugged more effectively on production
> systems.
>
> - clocksource.max_cswd_read_retries= [KNL]
> - Number of clocksource_watchdog() retries due to
> - external delays before the clock will be marked
> - unstable. Defaults to two retries, that is,
> - three attempts to read the clock under test.
> -
> clocksource.verify_n_cpus= [KNL]
> Limit the number of CPUs checked for clocksources
> marked with CLOCK_SOURCE_VERIFY_PERCPU that
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index 1d42d4b17327..13cf36fe27ac 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -291,7 +291,18 @@ 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;
> +static inline int clocksource_get_max_watchdog_retry(void)
> +{
> + /*
> + * When system is in boot phase or under heavy workload, there could
> + * be random big latency during clocksource/watchdog read, so add
> + * some retry to filter the noise latency. As the latency's frequency
> + * and maximum value goes up with the CPU numbers relatively, chose
> + * the max retry number according to CPU numbers.
> + */
> + return (ilog2(num_online_cpus()) / 2 + 1);
> +}
> +
> void clocksource_verify_percpu(struct clocksource *cs);
>
> #endif /* _LINUX_CLOCKSOURCE_H */
> diff --git a/kernel/time/clocksource-wdtest.c b/kernel/time/clocksource-wdtest.c
> index df922f49d171..2a5abda307e1 100644
> --- a/kernel/time/clocksource-wdtest.c
> +++ b/kernel/time/clocksource-wdtest.c
> @@ -105,7 +105,7 @@ static int wdtest_func(void *arg)
> {
> unsigned long j1, j2;
> char *s;
> - int i;
> + int i, max_retries;
>
> schedule_timeout_uninterruptible(holdoff * HZ);
>
> @@ -139,18 +139,19 @@ static int wdtest_func(void *arg)
> 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)
> + max_retries = clocksource_get_max_watchdog_retry();
> + for (i = 0; i <= max_retries + 1; i++) {
> + if (i <= 1 && i < max_retries)
> s = "";
> - else if (i <= max_cswd_read_retries)
> + else if (i <= max_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);
> + pr_info("--- Watchdog with %dx error injection, %d retries%s.\n", i, max_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) !=
> + WARN_ON_ONCE((i <= max_retries) !=
> !(clocksource_wdtest_ktime.flags & CLOCK_SOURCE_UNSTABLE));
> wdtest_ktime_clocksource_reset();
> }
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 3052b1f1168e..7fcb8d06b7cd 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -210,9 +210,6 @@ void clocksource_mark_unstable(struct clocksource *cs)
> spin_unlock_irqrestore(&watchdog_lock, flags);
> }
>
> -ulong max_cswd_read_retries = 2;
> -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);
>
> @@ -227,8 +224,10 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
> unsigned int nretries;
> u64 wd_end, wd_end2, wd_delta;
> int64_t wd_delay, wd_seq_delay;
> + int max_retries;
>
> - for (nretries = 0; nretries <= max_cswd_read_retries; nretries++) {
> + max_retries = clocksource_get_max_watchdog_retry();
> + for (nretries = 0; nretries <= max_retries; nretries++) {
> local_irq_disable();
> *wdnow = watchdog->read(watchdog);
> *csnow = cs->read(cs);
> @@ -240,7 +239,7 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
> wd_delay = clocksource_cyc2ns(wd_delta, watchdog->mult,
> watchdog->shift);
> if (wd_delay <= WATCHDOG_MAX_SKEW) {
> - if (nretries > 1 || nretries >= max_cswd_read_retries) {
> + if (nretries > 1 || nretries >= max_retries) {
> pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n",
> smp_processor_id(), watchdog->name, nretries);
> }
> diff --git a/tools/testing/selftests/rcutorture/bin/torture.sh b/tools/testing/selftests/rcutorture/bin/torture.sh
> index d5a0d8a33c27..bbac5f4b03d0 100755
> --- a/tools/testing/selftests/rcutorture/bin/torture.sh
> +++ b/tools/testing/selftests/rcutorture/bin/torture.sh
> @@ -567,7 +567,7 @@ then
> torture_bootargs="rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 tsc=watchdog"
> torture_set "clocksourcewd-1" tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 45s --configs TREE03 --kconfig "CONFIG_TEST_CLOCKSOURCE_WATCHDOG=y" --trust-make
>
> - torture_bootargs="rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 clocksource.max_cswd_read_retries=1 tsc=watchdog"
> + torture_bootargs="rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 tsc=watchdog"
> torture_set "clocksourcewd-2" tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 45s --configs TREE03 --kconfig "CONFIG_TEST_CLOCKSOURCE_WATCHDOG=y" --trust-make
>
> # In case our work is already done...
> --
> 2.34.1
>
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