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RE: [PATCH] xen/arm: Fix sibling mask setup to only include online CPUs
- To: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- From: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>
- Date: Wed, 28 Oct 2026 07:31:47 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, "michal.orzel@xxxxxxx" <michal.orzel@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>
- Delivery-date: Wed, 28 Oct 2026 07:32:14 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH] xen/arm: Fix sibling mask setup to only include online CPUs
Hello Volodymyr,
> -----Original Message-----
> From: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
> Sent: Wednesday, October 28, 2026 11:13 AM
> To: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; michal.orzel@xxxxxxx; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Bertrand Marquis
> <bertrand.marquis@xxxxxxx>
> Subject: Re: [PATCH] xen/arm: Fix sibling mask setup to only include online
> CPUs
>
> Hello Hirokazu-san,
>
> Hirokazu Takahashi <taka@xxxxxxxxxxxxx> writes:
>
> > Populate the CPU sibling map only with CPUs that are brought
> > online. Currently, all CPUs in cpu_possible_map are added,
> > which leaves CPUs that failed to boot in the sibling masks and
> > may lead the scheduler to make incorrect decisions.
>
> I think that this is a scheduler issue then. A CPU has siblings
> regardless of if they are running or not. Also, your change will affect
> CPU Hotplug functionality, as with your patch sibling mask will change
> in runtime. But partially - only in cpu_up, but not on cpu_down. I am
> not sure that schedulers can handle updates to siblings mask in runtime.
>
> Could you please elaborate how it affects scheduler behaviour right now?
In x86 Xen, __cpu_disable() is called in the cpu_down() path, where the target
CPU is removed from the CPU sibling map (via remove_siblinginfo()).
I think ARM64 Xen should follow suit.
> > Fixes: 701190abb3d5 ("xen/arm: Set correct per-cpu cpu_core_mask")
> > Signed-off-by: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>
> > ---
> > xen/arch/arm/smpboot.c | 18 +++++++++++++++---
> > 1 file changed, 15 insertions(+), 3 deletions(-)
> >
> > diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
> > index 8c9072f3c1..2d01896cf0 100644
> > --- a/xen/arch/arm/smpboot.c
> > +++ b/xen/arch/arm/smpboot.c
Thank you,
Hirokazu Takahashi.
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