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

[Peter Zijlstra]

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 ?

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.