[PATCH v2] x86/tsc: Extend watchdog check exemption to 4-Sockets platform

[Feng Tang]

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].

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 to 4 sockets to fix the issue.

We also got similar reports on 8 sockets platform from internal test,
but as Peter pointed out, there was tsc sync issues for 8-sockets
platform, and it'd better be handled architecture by architecture,
instead of directly changing the threshold to 8 here.

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>
---

Hi reviewers:

In the v1 review cycle, Dave raised the issue that 'nr_online_nodes'
is not accurate, and could have problem with fakenuma (numa=fake=4 in
cmdline) case and system with CPU-less HBM/PMEM nodes, which we have
discussed in https://lore.kernel.org/lkml/Y0UgeUIJSFNR4mQB@feng-clx/
and will post the solution as a separate RFC patch.

Thanks,
Feng

Changelog:
  
  Since v1:
  * Change the max socket number from 8 to 4, as Peter Zijlstra
    pointed the 8S machine could have tsc sync issue, and should
    not be exempted generally
  
 arch/x86/kernel/tsc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index cafacb2e58cc..1fa3fdf43159 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1209,7 +1209,7 @@ static void __init check_system_tsc_reliable(void)
 	 *  - TSC which does not stop in C-States
 	 *  - the TSC_ADJUST register which allows to detect even minimal
 	 *    modifications
-	 *  - not more than two sockets. As the number of sockets cannot be
+	 *  - not more than four sockets. As the number of sockets cannot be
 	 *    evaluated at the early boot stage where this has to be
 	 *    invoked, check the number of online memory nodes as a
 	 *    fallback solution which is an reasonable estimate.
@@ -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 <= 4)
 		tsc_disable_clocksource_watchdog();
 }
 
-- 
2.34.1