Re: [PATCH] clocksource: Scale the max retry number of watchdog read according to CPU numbers

[Paul E. McKenney]

On Fri, Jan 26, 2024 at 11:19:50AM -0500, Waiman Long wrote:
> On 1/26/24 08:45, Feng Tang wrote:
> > Hi Paul,
> > 
> > On Fri, Jan 26, 2024 at 04:27:19AM -0800, Paul E. McKenney wrote:
> > > On Fri, Jan 26, 2024 at 05:12:50PM +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 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.
> > > > 
> > > > Tested-by: Jin Wang <jin1.wang@xxxxxxxxx>
> > > > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> > > > ---
> > > >   kernel/time/clocksource.c | 9 +++++++++
> > > >   1 file changed, 9 insertions(+)
> > > > 
> > > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> > > > index c108ed8a9804..f15283101906 100644
> > > > --- a/kernel/time/clocksource.c
> > > > +++ b/kernel/time/clocksource.c
> > > > @@ -226,6 +226,15 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
> > > >   	u64 wd_end, wd_end2, wd_delta;
> > > >   	int64_t wd_delay, wd_seq_delay;
> > > > +	/*
> > > > +	 * If no user changes the default value, scale the retry threshold
> > > > +	 * according to CPU numbers. As per test, the more CPU a platform has,
> > > > +	 * the bigger read latency is found during boot time or under stress
> > > > +	 * work load. Increase the try nubmer to reduce false alarms.
> > > > + 	 */
> > > > +	if (max_cswd_read_retries == 2)
> > > I like the self-adjusting behavior!
> > Thanks!
> > > But why not make max_cswd_read_retries be a long (instead of a ulong)
> > > defaulting to -1.  Then when someone sets it to a specific positive
> > > value, they get exactly that value, while leaving it -1 gets the default
> > > CPU-scaling behavior.  Zero and other negative values should get a
> > > warning in order to reserve the for possible future use.
> > Good suggestions!
> > 
> > > I also suggest doing the adjustment at boot time, for example, using
> > > an early_initcall().  That way the test code also sees the scaled value.
> > I also thought about doing the adjustment once in early boot phase
> > using num_possible_cpus(), but gave up as that parameters could be
> > changed runtime using sysfs's module parameter interface, and cpu
> > runtime hotplugging.
> > 
> > Since the watchdog timer only happens (if not skipped) every 500 ms,
> > how about doing the ilog2 math everytime, like below:
> > 
> > diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
> > index 1d42d4b17327..9104bdecf34e 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;
> > +extern long max_cswd_read_retries;
> >   void clocksource_verify_percpu(struct clocksource *cs);
> >   #endif /* _LINUX_CLOCKSOURCE_H */
> > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> > index c108ed8a9804..867bb36e6dad 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 +225,17 @@ 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 = max_cswd_read_retries;
> > +	
> > +	if (max_cswd_read_retries <= 0) {
> > +		if (max_cswd_read_retries != -1)
> > +			pr_warn_once("max_cswd_read_retries has been set a invalid number: %d\n",
> > +				max_cswd_read_retries);
> > -	for (nretries = 0; nretries <= max_cswd_read_retries; nretries++) {
> > +		max_retries = ilog2(num_online_cpus()) + 1;
> > +	} 		
> > +
> > +	for (nretries = 0; nretries <= max_retries; nretries++) {
> >   		local_irq_disable();
> >   		*wdnow = watchdog->read(watchdog);
> >   		*csnow = cs->read(cs);
> > @@ -238,7 +247,7 @@ 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);
> >   			}
> 
> The max_cswd_read_retries value is also used in
> kernel/time/clocksource-wdtest.c. You will have to apply similar logic to
> clocksource-wdtest.c if it is not done once in early_init.

Good point!  If it is not done in an early_init(), could we please
have a function for the common code?

							Thanx, Paul