Re: [tip:x86/asm] x86: Speed up ___preempt_schedule*() by using THUNK helpers

[Oleg Nesterov]

On 10/03, Andy Lutomirski wrote:
>
> Is this thing missing a flags or cc clobber:
>
> # define __preempt_schedule() asm ("call ___preempt_schedule")

Well, calling.h says "rflags is clobbered", so probably yes?


> Also, I'm at a loss as to wtf all this code is doing.
>
> There's preempt_schedule, which appears to be a normal C function.
>
> There's ___preempt_schedule, which is written in assembly and calls
> preempt_schedule.
>
> Then there's __preempt_schedule, which is an inline assembler function
> that calls ___preempt_schedule.
>
> Is this all just to make the call sequence for preempt_schedule shorter?

Yes, please look at 1a338ac32ca630f67df25b4a16436cccc314e997

    sched, x86: Optimize the preempt_schedule() call

    Remove the bloat of the C calling convention out of the
    preempt_enable() sites by creating an ASM wrapper which allows us to
    do an asm("call ___preempt_schedule") instead.

Oleg.

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