linux-kernel - Re: [PATCH] mm: Compute mTHP order efficiently

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Message-ID: <ZugxqJ-CjEi5lEW_@casper.infradead.org>
Date: Mon, 16 Sep 2024 14:24:56 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ryan.roberts@....com,
	anshuman.khandual@....com, baohua@...nel.org, hughd@...gle.com,
	ioworker0@...il.com, wangkefeng.wang@...wei.com,
	baolin.wang@...ux.alibaba.com, gshan@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: Compute mTHP order efficiently

On Fri, Sep 13, 2024 at 02:49:02PM +0530, Dev Jain wrote:
> We use pte_range_none() to determine whether contiguous PTEs are empty
> for an mTHP allocation. Instead of iterating the while loop for every
> order, use some information, which is the first set PTE found, from the
> previous iteration, to eliminate some cases. The key to understanding
> the correctness of the patch is that the ranges we want to examine
> form a strictly decreasing sequence of nested intervals.

This is a lot more complicated.  Do you have any numbers that indicate
that it's faster?  Yes, it's fewer memory references, but you've gone
from a simple linear scan that's easy to prefetch to an exponential scan
that might confuse the prefetchers.