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Date: Tue, 30 Jan 2024 05:55:33 -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>, Waiman Long <longman@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
Jin Wang <jin1.wang@...el.com>
Subject: Re: [PATCH v3] clocksource: Scale the max retry number of watchdog
read according to CPU numbers
On Mon, Jan 29, 2024 at 09:45:05PM +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 and 670 rounds of reboot test on the 8-socket machine.
>
> Also add sanity check for user input value for 'max_cswd_read_retries', make
> it self-adaptive by default, and provide a general helper for getting this
> max retry number as suggested by Paul and Waiman.
>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> 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>
> ---
> Changelog:
>
> 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)
>
> include/linux/clocksource.h | 18 +++++++++++++++++-
> kernel/time/clocksource-wdtest.c | 12 +++++++-----
> kernel/time/clocksource.c | 10 ++++++----
> 3 files changed, 30 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> index 1d42d4b17327..0483f7dd66a3 100644
> --- a/include/linux/clocksource.h
> +++ b/include/linux/clocksource.h
> @@ -291,7 +291,23 @@ 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;
> +extern long max_cswd_read_retries;
> +
> +static inline long clocksource_max_watchdog_read_retries(void)
> +{
> + long max_retries = max_cswd_read_retries;
> +
> + if (max_cswd_read_retries <= 0) {
> + /* santity check for user input value */
> + if (max_cswd_read_retries != -1)
> + pr_warn_once("max_cswd_read_retries was set with an invalid number: %ld\n",
> + max_cswd_read_retries);
> +
> + max_retries = ilog2(num_online_cpus()) + 1;
> + }
> + return max_retries;
> +}
> +
> 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..c70cea3c44a1 100644
> --- a/kernel/time/clocksource-wdtest.c
> +++ b/kernel/time/clocksource-wdtest.c
> @@ -106,6 +106,7 @@ static int wdtest_func(void *arg)
> unsigned long j1, j2;
> char *s;
> int i;
> + long max_retries;
>
> schedule_timeout_uninterruptible(holdoff * HZ);
>
> @@ -139,18 +140,19 @@ 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_max_watchdog_read_retries();
> + 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, %ld 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 c108ed8a9804..2e5a1d6c6712 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -208,8 +208,8 @@ 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);
> +long max_cswd_read_retries = -1;
> +module_param(max_cswd_read_retries, long, 0644);
> EXPORT_SYMBOL_GPL(max_cswd_read_retries);
> static int verify_n_cpus = 8;
> module_param(verify_n_cpus, int, 0644);
> @@ -225,8 +225,10 @@ 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;
> + long max_retries;
>
> - for (nretries = 0; nretries <= max_cswd_read_retries; nretries++) {
> + max_retries = clocksource_max_watchdog_read_retries();
> + for (nretries = 0; nretries <= max_retries; nretries++) {
> local_irq_disable();
> *wdnow = watchdog->read(watchdog);
> *csnow = cs->read(cs);
> @@ -238,7 +240,7 @@ 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);
> }
> --
> 2.34.1
>
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