Date:   Wed, 22 Feb 2023 12:53:02 +0000
From:   David Woodhouse <dwmw2@infradead.org>
To:     Brian Gerst <brgerst@gmail.com>
CC:     Usama Arif <usama.arif@bytedance.com>, tglx@linutronix.de,
        kim.phillips@amd.com, arjan@linux.intel.com, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
        x86@kernel.org, pbonzini@redhat.com, paulmck@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        rcu@vger.kernel.org, mimoja@mimoja.de, hewenliang4@huawei.com,
        thomas.lendacky@amd.com, seanjc@google.com, pmenzel@molgen.mpg.de,
        fam.zheng@bytedance.com, punit.agrawal@bytedance.com,
        simon.evans@bytedance.com, liangma@liangbit.com
Subject: Re: [PATCH v9 0/8] Parallel CPU bringup for x86_64
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On 22 February 2023 12:08:04 GMT, Brian Gerst <brgerst@gmail=2Ecom> wrote:
>On Wed, Feb 22, 2023 at 5:44 AM David Woodhouse <dwmw2@infradead=2Eorg> w=
rote:
>>
>> 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=2E It didn't make a difference to smpboot time and is in=
dependent
>> > of parallel CPU bringup, so if needed can be explored in a separate p=
atchset=2E
>> >
>> > The patches have also been rebased to v6=2E2-rc8 and retested and the
>> > improvement in boot time is the same as v8=2E
>>
>> Thanks for picking this up, Usama=2E
>>
>> 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=2E This series eat=
s
>> the initial INIT/SIPI/SIPI latency, but if there's any significant time
>> in the AP hotplug thread, that could be worth parallelising=2E
>>
>> There may be further wins in the INIT/SIPI/SIPI too=2E Currently we
>> process each CPU at a time, sending INIT, SIPI, waiting 10=C2=B5s and
>> sending another SIPI=2E
>>
>> 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=2E So we'd probably end up sending the second SIPI
>> to most of them *anyway*=2E
>
>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=2E  Should I send that now or wait until this patchset
>lands in -tip?

Happy either way=2E Want to send it and we can take a look at whether to w=
ork it in with this?