Question

I knwow that “The need_resched flag is a message to the kernel that the scheduler should be invoked as soon as possible because another process deserves to run”, but I’m missing some points:

who is in charge of checking this flag? There is a kernel process ad hoc? In the end, probably, the only question I’d like to make is: what does the kernel do to call schedule() as soon as possible after the flag is set?

I hope I’ve been clear,

Stefano

Answer 1

Typically this flag will be checked in the interrupt and syscall return paths – if it’s set when returning from an interrupt or syscall, and (in the interrupt case) the kernel is preemptible, then the kernel will invoke one of the rescheduling functions prior to returning.

For example, in arch/x86/kernel/entry_32.S:

 358 ENTRY(resume_userspace)
 359        LOCKDEP_SYS_EXIT
 360        DISABLE_INTERRUPTS(CLBR_ANY)    # make sure we don't miss an interrupt
 361                                        # setting need_resched or sigpending
 362                                        # between sampling and the iret
 363        TRACE_IRQS_OFF
 364        movl TI_flags(%ebp), %ecx
 365        andl $_TIF_WORK_MASK, %ecx      # is there any work to be done on
 366                                        # int/exception return?
 367        jne work_pending
 368        jmp restore_all
 369 END(ret_from_exception)

Note that need_resched is a bit flag in TI_flags, and _TIF_WORK_MASK includes most TI_flags bits, including the one for need_resched. There are a number of other similar checks, and the return-to-kernel path also has an explicit check as well.