Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH] use x86 cpu park to speedup smp_init in kexec situation | Date | Wed, 16 Dec 2020 16:31:53 +0100 |
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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
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