CLOCK_AUX stepping

[Miroslav Lichvar <mlichvar@redhat.com>]

There is an issue with the new system auxiliary clocks. When I make
a larger step of a CLOCK_AUX clock (by clock_settime() or
adjtimex(ADJ_SETOFFSET)), the system slows down significantly to
almost being unusable. This didn't happen with the original
tglx/timers/ptp/driver-auxclock branch, but happens with 6.17-rc1
and later.

Reproducer:
- echo 1 > /sys/kernel/time/aux_clocks/0/aux_clock_enable
- git clone -b staging https://github.com/mlichvar/linuxptp.git
- cd linuxptp && make
- ./phc_ctl CLOCK_AUX0 set

"echo 0 > .../aux_clock_enable" revives the system.

I'm not sure if this isn't just a symptom, but the top functions
reported by perf are:
  87.10%  swapper          [kernel.kallsyms]       [k] queued_spin_lock_slowpath                                    
   6.84%  rcu_exp_gp_kthr  [kernel.kallsyms]       [k] queued_spin_lock_slowpath                                    
   1.90%  rcu_exp_gp_kthr  [kernel.kallsyms]       [k] smp_call_function_single                                     

-- 
Miroslav Lichvar