Re: [PATCH] x86/tsc: Extend the watchdog check exemption to 4S/8S machine

[Feng Tang]

On Sun, Oct 09, 2022 at 03:01:32PM +0200, Peter Zijlstra wrote:
> On Sun, Oct 09, 2022 at 01:12:09PM +0800, Feng Tang wrote:
> > There is report again that the tsc clocksource on a 4 sockets x86
> > Skylake server was wrongly judged as 'unstable' by 'jiffies' watchdog,
> > and disabled [1]. Also we got silimar reports on 8 sockets platform
> > from internal test.
> > 
> > Commit b50db7095fe0 ("x86/tsc: Disable clocksource watchdog for TSC
> > on qualified platorms") was introduce to deal with these false
> > alarms of tsc unstable issues, covering qualified platforms for 2
> > sockets or smaller ones.
> > 
> > Extend the exemption also to 4/8 sockets to fix the issue.
> > 
> > Rui also proposed another way to disable 'jiffies' as clocksource
> > watchdog [2], which can also solve this specific problem in an
> > architecture independent way, with one limitation that some tsc false
> > alarms are reported by other watchdogs like HPET in post-boot time,
> > while 'jiffies' is mostly used in boot phase before hardware
> > clocksources are initialized.
> > 
> > [1]. https://lore.kernel.org/all/9d3bf570-3108-0336-9c52-9bee15767d29@xxxxxxxxxx/
> > [2]. https://lore.kernel.org/all/bd5b97f89ab2887543fc262348d1c7cafcaae536.camel@xxxxxxxxx/
> > 
> > Reported-by: Yu Liao <liaoyu15@xxxxxxxxxx>
> > Tested-by: Yu Liao <liaoyu15@xxxxxxxxxx>
> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> > ---
> >  arch/x86/kernel/tsc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index cafacb2e58cc..b4ea79cb1d1a 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -1217,7 +1217,7 @@ static void __init check_system_tsc_reliable(void)
> >  	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> >  	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
> >  	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
> > -	    nr_online_nodes <= 2)
> > +	    nr_online_nodes <= 8)
> 
> So you're saying all 8 socket systems since Broadwell (?) are TSC
> sync'ed ?

No, I didn't mean that. I haven't got chance to any 8 sockets
machine, and I got a report last month that on one 8S machine,
the TSC was judged 'unstable' by HPET as watchdog.

> AFAIK there is no architectural guarantee for >4 sockets to have a sane
> TSC. If there is one, the above should be limited to architectures that
> conform. 

Thanks for the note! Yes, we should be very cautious for 8 sockets
machine. Will limit the max sockets to 4, which was also originally
suggested by Thomas.

Thanks,
Feng