cpufreq_update_util(9)

💅 cpufreq_update_util(9) — like, the Kernel API Manual, bestie 🎀

Okay so picture this: we're deep in the kernel, and there's this one little function whose entire job is to take a note about CPU utilization changes. That's it. That's the whole vibe. It's giving "quiet main character who actually runs everything." ✨

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🌸 NAME 🌸

cpufreq_update_util — take a note about CPU utilization changes. 📝

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🦋 SYNOPSIS 🦋

void cpufreq_update_util (struct rq *rq , unsigned int flags );

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💖 ARGUMENTS 💖

• rq — the runqueue you're carrying out the update for. Like, this is the runqueue, bestie, no cap.
• flags — the update reason flags. the "why are we even here" of it all.

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✨ DESCRIPTION ✨

Okay so here's the tea ☕ — this function gets called by the scheduler, on the CPU whose utilization is being updated. That's the whole setup.

🚨 Important vibe check: it can only be called from RCU-sched read-side critical sections. Like, that's not a suggestion, that's the rule. Don't be out here calling it from anywhere else, fr fr. 😭

Now, the way cpufreq is currently arranged, it needs to evaluate the CPU performance state (that's frequency/voltage, iykyk ⚡) on a regular basis, otherwise it can get stuck in a completely inadequate performance level for too long. And that is not guaranteed to happen if the updates are only triggered from CFS and DL — because those updates might just... not be coming in, if only RT tasks are active all the time (or if there are RT tasks only). 🙈

So as a workaround for that whole issue, this function gets called periodically by the RT sched class, to trigger extra cpufreq updates and keep it from stalling. But ngl? That really is a band-aid. 🩹 Going forward it should be replaced with solutions targeted more specifically at RT tasks. Periodt. 💅

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👑 SEE ALSO 👑

• Kernel file ./kernel/sched/sched.h available_idle_cpu(9)
• highest_flag_domain(9)
• arch_scale_freq_capacity(9)

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and that's a wrap, bestie 🫶 this little icon lives in ./kernel/sched, dated October 2026, and it's cpufreq_update_util(9) all day every day. go forth and update those utilizations 💜🌸