linux-kernel - Re: linux-next regression: SNP Guest boot hangs with certain cpu/mem config combination

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Message-ID: <CAMj1kXGLWYrfEzdDXy1wriBycx0DPQ_kL==Jkw_sDW5f0KxEHw@mail.gmail.com>
Date: Fri, 28 Mar 2025 10:17:16 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Steven Rostedt <rostedt@...dmis.org>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Tom Lendacky <thomas.lendacky@....com>, 
	"Aithal, Srikanth" <sraithal@....com>, Jason Baron <jbaron@...mai.com>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Linux-Next Mailing List <linux-next@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, 
	"Roth, Michael" <Michael.Roth@....com>
Subject: Re: linux-next regression: SNP Guest boot hangs with certain cpu/mem
 config combination

On Fri, 28 Mar 2025 at 10:09, Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> On Fri, Mar 28, 2025 at 10:28:19AM +0200, Kirill A. Shutemov wrote:
> > On Thu, Mar 27, 2025 at 07:39:22PM +0200, Kirill A. Shutemov wrote:
> > > On Thu, Mar 27, 2025 at 11:02:24AM -0400, Steven Rostedt wrote:
> > > > On Thu, 27 Mar 2025 16:43:43 +0200
> > > > "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> > > >
> > > > > > > The only option I see so far is to drop static branch from this path.
> > > > > > >
> > > > > > > But I am not sure if it the only case were we use static branch from CPU
> > > > > > > hotplug callbacks.
> > > > > > >
> > > > > > > Any other ideas?
> > > > > >
> > > > > >
> > > > > > Hmmm, didn't take too close a look here, but there is the
> > > > > > static_key_slow_dec_cpuslocked() variant, would that work here? Is the issue
> > > > > > the caller may or may not have the cpu_hotplug lock?
> > > > >
> > > > > Yes. This is generic page alloc path and it can be called with and without
> > > > > the lock.
> > > >
> > > > Note, it's not the static_branch that is an issue, it's enabling/disabling
> > > > the static branch that is. Changing a static branch takes a bit of work as
> > > > it does modify the kernel text.
> > > >
> > > > Is it possible to delay the update via a workqueue?
> > >
> > > Ah. Good point. Should work. I'll give it try.
> >
> > The patch below fixes problem for me.
>
> Ah. No, it won't work. We can get there before workqueues are initialized:
> mm_core_init() is called before workqueue_init_early().
>
> We cannot queue a work. :/
>
> Steven, any other ideas?
>

Why is the use of a static key required here?