sched: Don't define sched_clock_irqtime as static key

sched: Don't define sched_clock_irqtime as static key

The sched_clock_irqtime was defined as a static key in commit 8722903cbb8f
('sched: Define sched_clock_irqtime as static key'). However, this change
introduces a 'sleeping in atomic context' warning, as shown below:

	arch/x86/kernel/tsc.c:1214 mark_tsc_unstable()
	warn: sleeping in atomic context

As analyzed by Dan, the affected code path is as follows:

vcpu_load() <- disables preempt
-> kvm_arch_vcpu_load()
   -> mark_tsc_unstable() <- sleeps

virt/kvm/kvm_main.c
   166  void vcpu_load(struct kvm_vcpu *vcpu)
   167  {
   168          int cpu = get_cpu();
                          ^^^^^^^^^^
This get_cpu() disables preemption.

   169
   170          __this_cpu_write(kvm_running_vcpu, vcpu);
   171          preempt_notifier_register(&vcpu->preempt_notifier);
   172          kvm_arch_vcpu_load(vcpu, cpu);
   173          put_cpu();
   174  }

arch/x86/kvm/x86.c
  4979          if (unlikely(vcpu->cpu != cpu) || kvm_check_tsc_unstable()) {
  4980                  s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
  4981                                  rdtsc() - vcpu->arch.last_host_tsc;
  4982                  if (tsc_delta < 0)
  4983                          mark_tsc_unstable("KVM discovered backwards TSC");

arch/x86/kernel/tsc.c
    1206 void mark_tsc_unstable(char *reason)
    1207 {
    1208         if (tsc_unstable)
    1209                 return;
    1210
    1211         tsc_unstable = 1;
    1212         if (using_native_sched_clock())
    1213                 clear_sched_clock_stable();
--> 1214         disable_sched_clock_irqtime();
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
kernel/jump_label.c
   245  void static_key_disable(struct static_key *key)
   246  {
   247          cpus_read_lock();
                ^^^^^^^^^^^^^^^^
This lock has a might_sleep() in it which triggers the static checker
warning.

   248          static_key_disable_cpuslocked(key);
   249          cpus_read_unlock();
   250  }

Let revert this change for now as {disable,enable}_sched_clock_irqtime
are used in many places, as pointed out by Sean, including the following:

The code path in clocksource_watchdog():

  clocksource_watchdog()
  |
  -> spin_lock(&watchdog_lock);
     |
     -> __clocksource_unstable()
        |
        -> clocksource.mark_unstable() == tsc_cs_mark_unstable()
           |
           -> disable_sched_clock_irqtime()

And the code path in sched_clock_register():

	/* Cannot register a sched_clock with interrupts on */
	local_irq_save(flags);

	...

	/* Enable IRQ time accounting if we have a fast enough sched_clock() */
	if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
		enable_sched_clock_irqtime();

	local_irq_restore(flags);

[lkp@intel.com: reported a build error in the prev version]

Closes: https://lore.kernel.org/kvm/37a79ba3-9ce0-479c-a5b0-2bd75d573ed3@stanley.mountain/
Fixes: 8722903cbb8f ("sched: Define sched_clock_irqtime as static key")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Debugged-by: Dan Carpenter <dan.carpenter@linaro.org>
Debugged-by: Sean Christopherson <seanjc@google.com>
Debugged-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20250205032438.14668-1-laoar.shao@gmail.com