linux-kernel - Re: [PATCH v3 0/2] Reduce CPU consumption after panic

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Message-ID: <20250501085528.GR4439@noisy.programming.kicks-ass.net>
Date: Thu, 1 May 2025 10:55:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Carlos Bilbao <bilbao@...edu>
Cc: Andrew Morton <akpm@...ux-foundation.org>, seanjc@...gle.com,
	carlos.bilbao@...nel.org, tglx@...utronix.de, jan.glauber@...il.com,
	pmladek@...e.com, jani.nikula@...el.com,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	takakura@...inux.co.jp, john.ogness@...utronix.de, x86@...nel.org
Subject: Re: [PATCH v3 0/2] Reduce CPU consumption after panic

On Wed, Apr 30, 2025 at 01:54:11PM -0500, Carlos Bilbao wrote:

> > All that said... the default more or less does for(;;) { mdelay(100) },
> > if you have a modern chip that should not end up using much power at
> > all. That should end up in delay_halt_tpause() or delay_halt_mwaitx()
> > (depending on you being on Intel or AMD). And spend most its time in
> > deep idle states.
> > 
> > Is something not working?
> 
> Well, in my experiments, that’s not what happened -- halting the CPU in VMs
> reduced CPU usage by around 70%.

Because you're doing VMs, and VMs create problems where there weren't
any before. IOW you get to keep the pieces.

Specifically, VMs do VMEXIT on HLT and this is what's working for you.

On real hardware though, HLT gets you C1, while both TPAUSE and MWAITX
can probably get you deeper C states. As such, HLT is probably a
regression on power.

> How would folks feel about adding something like
> /proc/sys/kernel/halt_after_panic, disabled by default? It would help in
> the Linux use cases I care about (e.g., virtualized environments), without
> affecting others.

What's wrong with any of the existing options? Fact remains you need to
configure your VMs properly.