linux-kernel - Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT

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Message-ID: <20241211140402.yf7gMExr@linutronix.de>
Date: Wed, 11 Dec 2024 15:04:02 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Arnd Bergmann <arnd@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>, linux-mm@...ck.org,
	linux-rt-devel@...ts.linux.dev, Ard Biesheuvel <ardb@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>,
	Jason Baron <jbaron@...mai.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Matthew Wilcox <willy@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Russell King <linux@...linux.org.uk>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

On 2024-12-11 14:48:11 [+0100], To Arnd Bergmann wrote:
> I guess if you have boxes with 4GiB+ and can proof that the performance
> improves without HIGHPTE (since you don't have to map the page table).
> The question is then how much of low mem has to be used instead and when
> does it start to hurt.

Some numbers have been been documented in commit
   14315592009c1 ("x86, mm: Allow highmem user page tables to be disabled at boot time")

and I would like cite:
| We could probably handwave up an argument for a threshold at 16G of total
| RAM.

which means HIGHPTE would make sense with >= 16GiB of memory.

Sebastian