From: David Woodhouse <dwmw2@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>,
Usama Arif <usama.arif@bytedance.com>,
arjan@linux.intel.com
Cc: 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 v6 01/11] x86/apic/x2apic: Fix parallel handling of cluster_mask
Date: Tue, 07 Feb 2023 11:27:20 +0000 [thread overview]
Message-ID: <921cfe295fcd398168e5454e01193045de312688.camel@infradead.org> (raw)
In-Reply-To: <d37f3af69df09ff542024ed93a37865b28dfa86e.camel@infradead.org>
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On Tue, 2023-02-07 at 10:57 +0000, David Woodhouse wrote:
>
> > + /*
> > + * On post boot hotplug iterate over the present CPUs to handle the
> > + * case of partial clusters as they might be presented by
> > + * virtualization.
> > + */
> > + for_each_present_cpu(cpu_i) {
>
>
> So... if this CPU was *present* at boot time (and if any other CPU in
> this cluster was present), it will already have a cluster_mask.
>
> Which means we get here in two cases:
>
> • This CPU wasn't actually present (was just 'possible') at boot time.
> (Is that actually a thing that happens?)
>
> • This CPU was present but no other CPU in this cluster was actually
> brought up at boot time so the cluster_mask wasn't allocated.
>
> The code looks right, I don't grok the comment about partial clusters
> and virtualization, and would have worded it something along the above
> lines?
As I get my head around that, I think the code needs to change too.
What if we *unplug* the only CPU in a cluster (present→possible), then
add a new one in the same cluster? The new one would get a new
cluster_mask. Which is kind of OK for now but then if we re-add the
original CPU it'd continue to use its old cluster_mask.
Now, that's kind of weird if it's physical CPUs because that cluster is
within a given chip, isn't it? But with virtualization maybe that's
something that could happen, and it doesn't hurt to be completely safe
by using for_each_possible_cpu() instead?
Now looks like this:
/*
* On post boot hotplug for a CPU which was not present at boot time,
* iterate over all possible CPUs (even those which are not present
* any more) to find any existing cluster mask.
*/
for_each_possible_cpu(cpu_i) {
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next prev parent reply other threads:[~2023-02-07 11:27 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-02 21:56 [PATCH v6 00/11] Parallel CPU bringup for x86_64 Usama Arif
2023-02-02 21:56 ` [PATCH v6 01/11] x86/apic/x2apic: Fix parallel handling of cluster_mask Usama Arif
2023-02-06 23:20 ` Thomas Gleixner
2023-02-07 10:57 ` David Woodhouse
2023-02-07 11:27 ` David Woodhouse [this message]
2023-02-07 14:24 ` Thomas Gleixner
2023-02-07 19:53 ` David Woodhouse
2023-02-07 20:58 ` Thomas Gleixner
2023-02-07 14:22 ` Thomas Gleixner
2023-02-02 21:56 ` [PATCH v6 02/11] cpu/hotplug: Move idle_thread_get() to <linux/smpboot.h> Usama Arif
2023-02-06 23:33 ` Thomas Gleixner
2023-02-07 1:24 ` Paul E. McKenney
2023-02-02 21:56 ` [PATCH v6 03/11] cpu/hotplug: Add dynamic parallel bringup states before CPUHP_BRINGUP_CPU Usama Arif
2023-02-06 23:43 ` Thomas Gleixner
2023-02-02 21:56 ` [PATCH v6 04/11] x86/smpboot: Reference count on smpboot_setup_warm_reset_vector() Usama Arif
2023-02-06 23:48 ` Thomas Gleixner
[not found] ` <57195f701f6d1d70ec440c9a28cbee4cfb81dc41.camel@amazon.co.uk>
2023-02-07 14:39 ` Thomas Gleixner
2023-02-07 16:50 ` Sean Christopherson
2023-02-07 19:48 ` [EXTERNAL][PATCH " David Woodhouse
2023-02-02 21:56 ` [PATCH v6 05/11] x86/smpboot: Split up native_cpu_up into separate phases and document them Usama Arif
2023-02-06 23:59 ` Thomas Gleixner
2023-02-02 21:56 ` [PATCH v6 06/11] x86/smpboot: Support parallel startup of secondary CPUs Usama Arif
2023-02-02 22:30 ` David Woodhouse
2023-02-02 22:50 ` [External] " Usama Arif
2023-02-03 8:14 ` David Woodhouse
2023-02-03 14:41 ` Arjan van de Ven
2023-02-03 18:17 ` Sean Christopherson
2023-02-07 0:07 ` Thomas Gleixner
2023-02-02 21:56 ` [PATCH v6 07/11] x86/smpboot: Disable parallel boot for AMD CPUs Usama Arif
2023-02-03 19:48 ` Kim Phillips
[not found] ` <d5ec64236ba75f0d3f3718fb69b2cb9169d8af0a.camel@amazon.co.uk>
2023-02-03 21:45 ` Kim Phillips
2023-02-03 22:25 ` [EXTERNAL][PATCH " David Woodhouse
2023-02-04 9:07 ` [PATCH " David Woodhouse
2023-02-04 10:09 ` David Woodhouse
2023-02-04 15:40 ` David Woodhouse
2023-02-04 18:18 ` Arjan van de Ven
2023-02-04 22:31 ` David Woodhouse
2023-02-05 22:13 ` [External] " Usama Arif
2023-02-06 8:05 ` David Woodhouse
2023-02-06 12:11 ` Usama Arif
2023-02-06 18:07 ` Sean Christopherson
2023-02-06 17:58 ` Kim Phillips
2023-02-07 16:27 ` Kim Phillips
2023-02-07 0:23 ` Thomas Gleixner
2023-02-07 10:04 ` David Woodhouse
2023-02-07 14:44 ` Thomas Gleixner
2023-02-07 0:09 ` Thomas Gleixner
[not found] ` <cbd9e88e738dc0c479e87121ca82431731905c73.camel@amazon.co.uk>
2023-02-07 14:46 ` Thomas Gleixner
2023-02-02 21:56 ` [PATCH v6 08/11] x86/smpboot: Send INIT/SIPI/SIPI to secondary CPUs in parallel Usama Arif
2023-02-07 0:28 ` Thomas Gleixner
2023-02-02 21:56 ` [PATCH v6 09/11] x86/mtrr: Avoid repeated save of MTRRs on boot-time CPU bringup Usama Arif
2023-02-02 21:56 ` [PATCH v6 10/11] x86/smpboot: Serialize topology updates for secondary bringup Usama Arif
2023-02-02 21:56 ` [PATCH v6 11/11] x86/smpboot: reuse timer calibration Usama Arif
2023-02-07 0:31 ` Thomas Gleixner
2023-02-07 23:16 ` Arjan van de Ven
2023-02-07 23:55 ` Thomas Gleixner
2023-02-05 19:17 ` [PATCH v6 00/11] Parallel CPU bringup for x86_64 Russ Anderson
2023-02-06 8:28 ` David Woodhouse
2023-02-06 12:18 ` [External] " Usama Arif
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