CPU_NEED_RESCHED(9)

💅 NAME 💅

okay so bestie, welcome to CPU_NEED_RESCHED(9) from the Kernel Developer’s Manual — and the name is literally cpu_need_resched — context switch notification. that’s the vibe. ✨

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🎀 SYNOPSIS 🎀

#include <sys/cpu.h>

void
cpu_need_resched(struct cpu_info *ci, struct lwp *l, int flags);

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💖 DESCRIPTION 💖

okay so cpu_need_resched() is the machine-independent interface for the scheduler to notify machine-dependent code that a context switch from the current LWP l, on the cpu ci, is required. it’s giving “hey bestie, we gotta switch things up” energy. 💅

this event may occur if a higher priority LWP appears on the run queue, or if the current LWP has exceeded its time slice. l is the last LWP observed running on the CPU. and no cap, it may no longer be running, because cpu_need_resched() can be called without holding scheduler locks. 😭

here’s the flag tea:

• RESCHED_KPREEMPT — if this flag is specified in flags and the __HAVE_PREEMPTION C pre-processor macro is defined in <machine/intr.h>, machine-dependent code should make a context switch happen as soon as possible even if the CPU is running in kernel mode. if RESCHED_KPREEMPT is not specified, then RESCHED_UPREEMPT is specified instead. slay. 🔥
• RESCHED_IDLE — if this flag is specified in flags, the last thread observed running on the CPU was the idle LWP.
• RESCHED_REMOTE — if this flag is specified in flags, the request is not for the current CPU. the opposite also holds true. if ci is not the current processor, cpu_need_resched() typically issues an inter processor call to the processor to make it notice the need of a context switch as soon as possible.

bestie, important: cpu_need_resched() is always called with kernel preemption disabled. always. 👑

typically, the cpu_need_resched() function will perform the following operations:

• Set a per-processor flag which is checked by userret(9) when returning to user-mode execution.
• Post an asynchronous software trap (AST).
• Send an inter processor interrupt to wake up cpu_idle(9) and/or force an user process across the user/kernel boundary, thus making a trip through userret().

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🦋 SEE ALSO 🦋

• sched_4bsd(9)
• userret(9)

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mwah 💖 this CPU_NEED_RESCHED(9) page is from Debian, dated November 17, 2019 — periodt, that’s the footer tea ☕✨