From: David Woodhouse <dwmw2@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>,
Usama Arif <usama.arif@bytedance.com>,
kim.phillips@amd.com, brgerst@gmail.com
Cc: piotrgorski@cachyos.org, oleksandr@natalenko.name,
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, gpiccoli@igalia.com
Subject: Re: [PATCH v16 3/8] cpu/hotplug: Add dynamic parallel bringup states before CPUHP_BRINGUP_CPU
Date: Thu, 23 Mar 2023 22:49:40 +0000 [thread overview]
Message-ID: <47150207bfb76bb98aff678cf7c91f245e9f5dd9.camel@infradead.org> (raw)
In-Reply-To: <874jqb8588.ffs@tglx>
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On Thu, 2023-03-23 at 23:36 +0100, Thomas Gleixner wrote:
> On Tue, Mar 21 2023 at 19:40, Usama Arif wrote:
> > void bringup_nonboot_cpus(unsigned int setup_max_cpus)
> > {
> > + unsigned int n = setup_max_cpus - num_online_cpus();
> > unsigned int cpu;
> >
> > + /*
> > + * An architecture may have registered parallel pre-bringup states to
> > + * which each CPU may be brought in parallel. For each such state,
> > + * bring N CPUs to it in turn before the final round of bringing them
> > + * online.
> > + */
> > + if (n > 0) {
> > + enum cpuhp_state st = CPUHP_BP_PARALLEL_DYN;
> > +
> > + while (st <= CPUHP_BP_PARALLEL_DYN_END && cpuhp_hp_states[st].name) {
>
>
> There is no point in special casing this. All architectures can invoke
> the CPUHP_BP_* states before CPUHP_BRINGUP_CPU for each to be brought up
> CPU first. So this can be made unconditional and common exercised code.
>
There were three paragraphs in the commit message explaining why I
didn't want to do that. It didn't work for x86 before I started, and I
haven't reviewed *every* other architecture to ensure that it will work
there. It was opt-in for a reason. :)
> Aside of that this dynamic state range is pretty pointless. There is
> really nothing dynamic here and there is no real good reason to have
> four dynamic parallel states just because.
The original patch set did use more than one state; the plan to do
microcode updates in parallel does involve doing at least one more, I
believe.
https://lore.kernel.org/all/eb6717dfc4ceb99803c0396f950db7c3231c75ef.camel@infradead.org/
> The only interesting thing after CPUHP_BP_PREPARE_DYN_END and before
> CPUHP_BP_BRINGUP is a state which kicks the AP into life, i.e. we can
> just hardcode that as CPUHP_BP_PARALLEL_STARTUP.
>
> Thanks,
>
> tglx
> ---
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -133,6 +133,20 @@ enum cpuhp_state {
> CPUHP_MIPS_SOC_PREPARE,
> CPUHP_BP_PREPARE_DYN,
> CPUHP_BP_PREPARE_DYN_END = CPUHP_BP_PREPARE_DYN + 20,
> + /*
> + * This is an optional state if the architecture supports parallel
> + * startup. It's used to send the startup IPI so that the APs can
> + * run in parallel through the low level startup code instead of
> + * sending the IPIs one by one in CPUHP_BRINGUP_CPU. This avoids
> + * waiting for the AP to react and shortens the serialized bringup.
> + */
> + CPUHP_BP_PARALLEL_STARTUP,
> +
> + /*
> + * Fully per AP serialized bringup from here on. If the
> + * architecture does no register the CPUHP_BP_PARALLEL_STARTUP
> + * state, this step sends the startup IPI first.
> + */
Not sure I'd conceded that yet; the APs do their *own* bringup from
here, and that really ought to be able to run in parallel.
> CPUHP_BRINGUP_CPU,
>
> /*
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next prev parent reply other threads:[~2023-03-23 22:50 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-21 19:40 [PATCH v16 0/8] Parallel CPU bringup for x86_64 Usama Arif
2023-03-21 19:40 ` [PATCH v16 1/8] cpu/hotplug: Move idle_thread_get() to <linux/smpboot.h> Usama Arif
2023-03-21 19:40 ` [PATCH v16 2/8] cpu/hotplug: Reset task stack state in _cpu_up() Usama Arif
2023-03-22 11:06 ` Mark Rutland
2023-03-21 19:40 ` [PATCH v16 3/8] cpu/hotplug: Add dynamic parallel bringup states before CPUHP_BRINGUP_CPU Usama Arif
2023-03-23 22:36 ` Thomas Gleixner
2023-03-23 22:49 ` David Woodhouse [this message]
2023-03-23 23:48 ` Thomas Gleixner
2023-03-24 0:06 ` David Woodhouse
2023-03-23 23:05 ` Thomas Gleixner
2023-03-23 23:12 ` David Woodhouse
2023-03-24 1:16 ` Thomas Gleixner
2023-03-24 9:31 ` David Woodhouse
2023-03-24 13:57 ` Thomas Gleixner
2023-03-24 15:33 ` David Woodhouse
2023-03-24 14:07 ` Thomas Gleixner
2023-03-24 9:46 ` Thomas Gleixner
2023-03-24 10:00 ` David Woodhouse
2023-03-27 18:48 ` David Woodhouse
2023-03-21 19:40 ` [PATCH v16 4/8] x86/smpboot: Split up native_cpu_up into separate phases and document them Usama Arif
2023-03-21 19:40 ` [PATCH v16 5/8] x86/smpboot: Support parallel startup of secondary CPUs Usama Arif
2023-03-21 19:40 ` [PATCH v16 6/8] x86/smpboot: Send INIT/SIPI/SIPI to secondary CPUs in parallel Usama Arif
2023-03-21 19:40 ` [PATCH v16 7/8] x86/smpboot: Serialize topology updates for secondary bringup Usama Arif
2023-03-21 19:40 ` [PATCH v16 8/8] x86/smpboot: Allow parallel bringup for SEV-ES Usama Arif
2023-03-22 22:47 ` Borislav Petkov
2023-03-23 8:32 ` David Woodhouse
2023-03-23 8:51 ` Borislav Petkov
2023-03-23 9:04 ` David Woodhouse
2023-03-23 14:23 ` Brian Gerst
2023-03-27 17:47 ` Borislav Petkov
2023-03-27 18:14 ` David Woodhouse
2023-03-27 19:14 ` Tom Lendacky
2023-03-27 19:32 ` Borislav Petkov
2023-03-23 13:16 ` Tom Lendacky
[not found] ` <751f572f940220775054dc09324b20b929d7d66d.camel@amazon.co.uk>
2023-03-23 18:28 ` Tom Lendacky
2023-03-23 22:13 ` Borislav Petkov
2023-03-23 22:30 ` [EXTERNAL][PATCH " David Woodhouse
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