linux-kernel - Re: [PATCH v9 0/8] Parallel CPU bringup for x86

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Date:   Wed, 22 Feb 2023 07:08:04 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Usama Arif <usama.arif@...edance.com>, tglx@...utronix.de,
        kim.phillips@....com, arjan@...ux.intel.com, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        x86@...nel.org, pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com
Subject: Re: [PATCH v9 0/8] Parallel CPU bringup for x86_64

On Wed, Feb 22, 2023 at 5:44 AM David Woodhouse <dwmw2@...radead.org> wrote:
>
> On Wed, 2023-02-15 at 14:54 +0000, Usama Arif wrote:
> > The main change over v8 is dropping the patch to avoid repeated saves of MTRR
> > at boot time. It didn't make a difference to smpboot time and is independent
> > of parallel CPU bringup, so if needed can be explored in a separate patchset.
> >
> > The patches have also been rebased to v6.2-rc8 and retested and the
> > improvement in boot time is the same as v8.
>
> Thanks for picking this up, Usama.
>
> So the next thing that might be worth looking at is allowing the APs
> all to be running their hotplug thread simultaneously, bringing
> themselves from CPUHP_BRINGUP_CPU to CPUHP_AP_ONLINE. This series eats
> the initial INIT/SIPI/SIPI latency, but if there's any significant time
> in the AP hotplug thread, that could be worth parallelising.
>
> There may be further wins in the INIT/SIPI/SIPI too. Currently we
> process each CPU at a time, sending INIT, SIPI, waiting 10µs and
> sending another SIPI.
>
> What if we sent the first INIT+SIPI to all CPUs, then did another pass
> sending another SIPI only to those which hadn't already started running
> and set their bit in cpu_initialized_mask ?
>
> Might not be worth it, and there's an added complexity that they all
> have to wait for each other (on the real mode trampoline lock) before
> they can take their turn and get as far as setting their bit in
> cpu_initialized_mask. So we'd probably end up sending the second SIPI
> to most of them *anyway*.

Speaking of next steps, I have a followup patchset ready to go that
removes the global variables initial_stack, initial_gs, and
early_gdt_descr.  Should I send that now or wait until this patchset
lands in -tip?

--
Brian Gerst