Messages in this thread | | | Date | Sun, 28 Jan 2024 13:15:31 -0500 | Subject | Re: [PATCH v2] clocksource: Scale the max retry number of watchdog read according to CPU numbers | From | Waiman Long <> |
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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@kernel.org> > Cc: Waiman Long <longman@redhat.com> > Signed-off-by: Feng Tang <feng.tang@intel.com> > Tested-by: Jin Wang <jin1.wang@intel.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? Alternatively, you can make clocksource_max_watchdog_read_retries() an inline function in a header file and keep the max_cswd_read_retries export.
Cheers, Longman
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