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

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Message-ID: <20250327110224.68c69c78@gandalf.local.home>
Date: Thu, 27 Mar 2025 11:02:24 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Jason Baron <jbaron@...mai.com>, Peter Zijlstra <peterz@...radead.org>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Ard Biesheuvel <ardb@...nel.org>, Tom
 Lendacky <thomas.lendacky@....com>, "Aithal, Srikanth" <sraithal@....com>,
 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 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?

-- Steve