linux-kernel - Re: [PATCH 0/3] x86: Make 5-level paging support unconditional for x86-64

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Message-ID: <20240731111147.GA33588@noisy.programming.kicks-ass.net>
Date: Wed, 31 Jul 2024 13:11:47 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: 20240621164406.256314-1-kirill.shutemov@...ux.intel.com,
	kirill.shutemov@...ux.intel.com, ardb@...nel.org, bp@...en8.de,
	brijesh.singh@....com, corbet@....net, dave.hansen@...ux.intel.com,
	hpa@...or.com, jan.kiszka@...mens.com, jgross@...e.com,
	kbingham@...nel.org, linux-doc@...r.kernel.org,
	linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, luto@...nel.org, michael.roth@....com,
	mingo@...hat.com, rick.p.edgecombe@...el.com, sandipan.das@....com,
	thomas.lendacky@....com, x86@...nel.org
Subject: Re: [PATCH 0/3] x86: Make 5-level paging support unconditional for
 x86-64

On Wed, Jul 31, 2024 at 11:15:05AM +0200, Thomas Gleixner wrote:
> On Wed, Jul 31 2024 at 14:27, Shivank Garg wrote:
> > lmbench:lat_pagefault: Metric- page-fault time (us) - Lower is better
> >                 4-Level PT              5-Level PT		% Change
> > THP-never       Mean:0.4068             Mean:0.4294		5.56
> >                 95% CI:0.4057-0.4078    95% CI:0.4287-0.4302
> >
> > THP-Always      Mean: 0.4061            Mean: 0.4288		% Change
> >                 95% CI: 0.4051-0.4071   95% CI: 0.4281-0.4295	5.59
> >
> > Inference:
> > 5-level page table shows increase in page-fault latency but it does
> > not significantly impact other benchmarks.
> 
> 5% regression on lmbench is a NONO.
> 
> 5-level page tables add a cost in every hardware page table walk. That's
> a matter of fact and there is absolutely no reason to inflict this cost
> on everyone.
> 
> The solution to this to make the 5-level mechanics smarter by evaluating
> whether the machine has enough memory to require 5-level tables and
> select the depth at boot time.

I gotta mention (again) that its a pain we can't mix and match like
s390. They default run their userspace on 4 level, even if the kernel
runs 5. Only silly daft userspace that needs more than insane amounts of
memory get 5 level.