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Date: Mon, 29 Jan 2024 10:17:56 +0800
From: Feng Tang <feng.tang@...el.com>
To: Waiman Long <longman@...hat.com>
CC: John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
Stephen Boyd <sboyd@...nel.org>, <linux-kernel@...r.kernel.org>,
<paulmck@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Jin Wang
<jin1.wang@...el.com>
Subject: Re: [PATCH v2] clocksource: Scale the max retry number of watchdog
read according to CPU numbers
On Sun, Jan 28, 2024 at 01:15:31PM -0500, Waiman Long wrote:
>
> On 1/28/24 09:31, 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 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 on large system, 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', 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>
> > ---
> > Changelog:
> >
> > 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 | 2 +-
> > kernel/time/clocksource-wdtest.c | 12 +++++++-----
> > kernel/time/clocksource.c | 26 +++++++++++++++++++++-----
> > 3 files changed, 29 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> > index 1d42d4b17327..8d84d2f79622 100644
> > --- a/include/linux/clocksource.h
> > +++ b/include/linux/clocksource.h
> > @@ -291,7 +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;
> > +long clocksource_max_watchdog_read_retries(void);
> > 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..00e359c80822 100644
> > --- a/kernel/time/clocksource.c
> > +++ b/kernel/time/clocksource.c
> > @@ -208,9 +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);
> > -EXPORT_SYMBOL_GPL(max_cswd_read_retries);
> > +static long max_cswd_read_retries = -1;
> > +module_param(max_cswd_read_retries, long, 0644);
> > static int verify_n_cpus = 8;
> > module_param(verify_n_cpus, int, 0644);
> > @@ -220,13 +219,30 @@ enum wd_read_status {
> > WD_READ_SKIP
> > };
> > +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 has been set a invalid number: %ld\n",
> > + max_cswd_read_retries);
> > +
> > + max_retries = ilog2(num_online_cpus()) + 1;
> > + }
> > + return max_retries;
> > +}
>
> Should you add "EXPORT_SYMBOL_GPL(clocksource_max_watchdog_read_retries)"
> since clocksource-wdtest.c is likely to be in a kernel module?
Yes, you are right. I tested the wdtest as built-in and missed this.
> Alternatively, you can make clocksource_max_watchdog_read_retries() an
> inline function in a header file and keep the max_cswd_read_retries export.
Yep, will go this way and restore exporting 'max_cswd_read_retries'. Thanks!
- Feng
>
> Cheers,
> Longman
>
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