timers: Tag (hr)timer softirq as hotplug safe

Specific stress involving frequent CPU-hotplug operations, such as
running rcutorture for example, may trigger the following message:

  NOHZ tick-stop error: local softirq work is pending, handler #02!!!"

This happens in the CPU-down hotplug process, after
CPUHP_AP_SMPBOOT_THREADS whose teardown callback parks ksoftirqd, and
before the target CPU shuts down through CPUHP_AP_IDLE_DEAD. In this
fragile intermediate state, softirqs waiting for threaded handling may be
forever ignored and eventually reported by the idle task as in the above
example.

However some vectors are known to be safe as long as the corresponding
subsystems have teardown callbacks handling the migration of their
events. The above error message reports pending timers softirq although
this vector can be considered as hotplug safe because the
CPUHP_TIMERS_PREPARE teardown callback performs the necessary migration
of timers after the death of the CPU. Hrtimers also have a similar
hotplug handling.

Therefore this error message, as far as (hr-)timers are concerned, can
be considered spurious and the relevant softirq vectors can be marked as
hotplug safe.

Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230912104406.312185-6-frederic@kernel.org

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