[PATCH] sched: Don't define sched_clock_irqtime as static key

[Yafang Shao <laoar.shao@gmail.com>]

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);

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/kvm/37a79ba3-9ce0-479c-a5b0-2bd75d573ed3@stanley.mountain/
Debugged-by: Dan Carpenter <dan.carpenter@linaro.org>
Debugged-by: Sean Christopherson <seanjc@google.com>
Debugged-by: Michal Koutný <mkoutny@suse.com>
Fixes: 8722903cbb8f ("sched: Define sched_clock_irqtime as static key")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Peter Zijlstra" <peterz@infradead.org>
Cc: Vincent Guittot" <vincent.guittot@linaro.org>
---
 kernel/sched/cputime.c | 8 ++++----
 kernel/sched/sched.h   | 4 ++--
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 5d9143dd0879..c7904ce2345a 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -9,8 +9,6 @@
 
 #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
@@ -24,14 +22,16 @@ DEFINE_STATIC_KEY_FALSE(sched_clock_irqtime);
  */
 DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
 
+static 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,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 38e0e323dda2..ab16d3d0e51c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3259,11 +3259,11 @@ 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;
 }
 
 /*
-- 
2.43.5