Regression: x86/tsc: Fix mark_tsc_unstable()

[Jeremy Cline]

Hi folks,

A few Fedora users have reported[0] a regression starting in v4.16.8
where the boot will hang ~1/3 of the time with the following RCU stall
warning:

INFO: rcu_sched detected stalls on CPUs/tasks:
o1-...!: (0 ticks this GP) idle=688/0/0 softirq=171/171 fqs=0
o(detected by 0, t=60002 jiffies, g=-142, c=-143, q=9)
Sending NMI from CPU 0 to CPU 1:
NMI backtrace for cpu 1 skipped: idling at
acpi_processor_ffh_cstate_enter+0x65/0xb0
rcu_sched kthread starved for 60002 jiffies! g18446744073709551474
c1844674407370955143 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x402 -> cpu=1
RCU grace-period kthread stack dump:
rcu_sched       I    0     9      2 0x80000000
Call Trace:
 ? __schedule+0x234/0x850
 schedule+0x28/0x80
 schedule_timeout+0x166/0x380
 ? __next_timer_interrupt+0xc0/0xc0
 rcu_gp_kthread+0x368/0x830
 ? rcu_process_callbacks+0x4f0/0x4f0
 kthread+0x112/0x130
 ? kthread_create_worker_on_cpu+0x70/0x70
 ret_from_fork+0x35/0x40

A user has bisected the problem to the v4.16 commit 1ab4ca7c59d4
("x86/tsc: Fix mark_tsc_unstable()"). According to the reporter,
explicitly setting "tsc=" on the kernel command line causes the boot to
always succeed. All the users have Thinkpad T500s or T400s (Core 2 Duos)

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1579925


Thanks,
Jeremy