Re: [PATCH] use x86 cpu park to speedup smp_init in kexec situation

[Thomas Gleixner]

Kai,

On Wed, Dec 16 2020 at 22:18, shenkai wrote:
> After some tests, the conclusion that time cost is from deep C-state 
> turns out to be wrong
>
> Sorry for that.

No problem.

> In kexec case, first let APs spinwait like what I did  in that patch,
> but wake APs up by sending apic INIT and SIPI  interrupts as normal
> procedure instead of writing to some address and there is no
> acceleration (time cost is still 210ms).

Ok.

> So can we say that the main time cost is from apic INIT and SIPI
> interrupts and the handling of them instead of deep C-state?

That's a fair conclusion.

> I didn't test with play_dead() because in kexec case, one new kernel
> will be started and APs can't be waken up by normal interrupts like in
> hibernate case for the irq vectors are gone with the old kernel.
>
> Or maybe I didn't get the point correctly?

Not exactly, but your experiment answered the question already.

My point was that the regular kexec unplugs the APs which then end up in
play_dead() and trying to use the deepest C-state via mwait(). So if the
overhead would be related to getting them out of a deep C-state then
forcing that play_dead() to use the HLT instruction or the most shallow
C-state with mwait() would have brought an improvement, right?

But obviously the C-state in which the APs are waiting is not really
relevant, as you demonstrated that the cost is due to INIT/SIPI even
with spinwait, which is what I suspected.

OTOH, the advantage of INIT/SIPI is that the AP comes up in a well known
state.

Thanks,

        tglx