sched/clock: Avoid false sharing for sched_clock_irqtime

sched/clock: Avoid false sharing for sched_clock_irqtime

Read-mostly sched_clock_irqtime may share the same cacheline with
frequently updated nohz struct. Make it as static_key to avoid
false sharing issue.

The only user of disable_sched_clock_irqtime()
is tsc_.*mark_unstable() which may be invoked under atomic context
and require a workqueue to disable static_key. But both of them
calls clear_sched_clock_stable() just before doing
disable_sched_clock_irqtime(). We can reuse
"sched_clock_work" to also disable sched_clock_irqtime().

One additional case need to handle is if the tsc is marked unstable
before late_initcall() phase, sched_clock_work will not be invoked
and sched_clock_irqtime will stay enabled although clock is unstable:
  tsc_init()
    enable_sched_clock_irqtime() # irqtime accounting is enabled here
    ...
    if (unsynchronized_tsc()) # true
      mark_tsc_unstable()
        clear_sched_clock_stable()
          __sched_clock_stable_early = 0;
          ...
          if (static_key_count(&sched_clock_running.key) == 2)
            # Only happens at sched_clock_init_late()
            __clear_sched_clock_stable(); # Never executed
  ...

  # late_initcall() phase
  sched_clock_init_late()
    if (__sched_clock_stable_early) # Already false
      __set_sched_clock_stable(); # sched_clock is never marked stable
  # TSC unstable, but sched_clock_work won't run to disable irqtime

So we need to disable_sched_clock_irqtime() in sched_clock_init_late()
if clock is unstable.

Reported-by: Benjamin Lei <benjamin.lei@intel.com>
Suggested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Tianyou Li <tianyou.li@intel.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://patch.msgid.link/20260127072509.2627346-1-wangyang.guo@intel.com