sched: Don't define sched_clock_irqtime as static key

JIRA: https://issues.redhat.com/browse/RHEL-78821

commit b9f2b29
Author: Yafang Shao <[email protected]>
Date:   Wed Feb 5 11:24:38 2025 +0800

    sched: Don't define sched_clock_irqtime as static key

    The sched_clock_irqtime was defined as a static key in commit 8722903
    ('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);

    [[email protected]: reported a build error in the prev version]

    Closes: https://lore.kernel.org/kvm/[email protected]/
    Fixes: 8722903 ("sched: Define sched_clock_irqtime as static key")
    Reported-by: Dan Carpenter <[email protected]>
    Debugged-by: Dan Carpenter <[email protected]>
    Debugged-by: Sean Christopherson <[email protected]>
    Debugged-by: Michal Koutný <[email protected]>
    Signed-off-by: Yafang Shao <[email protected]>
    Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
    Reviewed-by: Vincent Guittot <[email protected]>
    Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Phil Auld <[email protected]>

Author: auldp

kernel/sched/cputime.c

@@ -5,8 +5,6 @@
 
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
 
-DEFINE_STATIC_KEY_FALSE(sched_clock_irqtime);
-
 /*
  * There are no locks covering percpu hardirq/softirq time.
  * They are only modified in vtime_account, on corresponding CPU
@@ -20,14 +18,16 @@ DEFINE_STATIC_KEY_FALSE(sched_clock_irqtime);
  */
 DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
 
+int sched_clock_irqtime;
+
 void enable_sched_clock_irqtime(void)
 {
-	static_branch_enable(&sched_clock_irqtime);
+	sched_clock_irqtime = 1;
 }
 
 void disable_sched_clock_irqtime(void)
 {
-	static_branch_disable(&sched_clock_irqtime);
+	sched_clock_irqtime = 0;
 }
 
 static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,

kernel/sched/sched.h

@@ -3029,11 +3029,11 @@ struct irqtime {
 };
 
 DECLARE_PER_CPU(struct irqtime, cpu_irqtime);
-DECLARE_STATIC_KEY_FALSE(sched_clock_irqtime);
+extern int sched_clock_irqtime;
 
 static inline int irqtime_enabled(void)
 {
-	return static_branch_likely(&sched_clock_irqtime);
+	return sched_clock_irqtime;
 }
 
 /*