__rcu_irq_enter_check_tick(9)

✨ NAME ✨

okay bestie, this is the Kernel API Manual page for __rcu_irq_enter_check_tick(9) 💅. the whole vibe is honestly right there in the name: it enables the scheduler tick on a CPU if RCU needs it. that's it, that's the function. ✨

🎀 SYNOPSIS 🎀

void __rcu_irq_enter_check_tick (void );

🌸 ARGUMENTS 🌸

• void — no arguments. like, zero. this function takes literally nothing and asks for nothing. 🫶

💅 DESCRIPTION 💅

so here's the tea ☕: the scheduler tick is not normally enabled when CPUs enter the kernel from nohz_full userspace execution. and honestly, that makes sense — nohz_full userspace execution is an RCU quiescent state, and the time executing in the kernel is quite short. 💖

except, of course, when it isn't 😭. and it is not hard to cause a large system to spend tens of seconds or even minutes looping in the kernel, which can cause a number of problems, including RCU CPU stall warnings. not iconic. not slay. very much giving "why is this CPU stuck." 💀

so therefore — yes, therefore, bestie — if a nohz_full CPU fails to report a quiescent state in a timely manner, the RCU grace-period kthread sets that CPU's ->rcu_urgent_qs flag. the expectation is that the next interrupt or exception will invoke this function, which will turn on the scheduler tick. and that scheduler tick will enable RCU to detect that CPU's quiescent states — for example, due to cond_resched calls in CONFIG_PREEMPT=n kernels. the tick will be disabled once a quiescent state is reported for this CPU. periodt. ✨

and like, in carefully tuned systems, there might never be an interrupt or exception. in that case, the RCU grace-period kthread will eventually cause one to happen. however, in less carefully controlled environments, this function allows RCU to get what it needs without creating otherwise useless interruptions. iconic behavior, honestly 🫶💖

🦋 SEE ALSO 🦋

Kernel file ./kernel/rcu/tree.c is the source-file bestie, and here are all the related manual pages — no skips, no crumbs, fr fr:

• rcu_softirq_qs(9)
• rcu_watching_snap_stopped_since(9)
• rcu_is_cpu_rrupt_from_idle(9)
• rcu_irq_exit_check_preempt(9)
• rcu_is_watching(9)
• rcu_set_gpwrap_lag(9)
• call_rcu_hurry(9)
• call_rcu(9)
• synchronize_rcu(9)
• get_completed_synchronize_rcu_full(9)
• get_state_synchronize_rcu(9)
• get_state_synchronize_rcu_full(9)
• start_poll_synchronize_rcu(9)
• start_poll_synchronize_rcu_full(9)
• poll_state_synchronize_rcu(9)
• poll_state_synchronize_rcu_full(9)
• cond_synchronize_rcu(9)
• cond_synchronize_rcu_full(9)
• rcu_barrier(9)
• rcu_barrier_throttled(9)

mwah 💋 — source/version ./kernel/rcu, date October 2026, and this whole page lives under the Kernel API Manual. bye bestie, go report a quiescent state 🌈👑