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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <091f517d-e7dc-4c10-b1ac-39658f31f0ed@arm.com>
Date: Tue, 17 Sep 2024 09:25:27 +0530
From: Dev Jain <dev.jain@....com>
To: Matthew Wilcox <willy@...radead.org>
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 9/16/24 18:54, Matthew Wilcox wrote:
> 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.

I do have some numbers, I tested with a simple program, and also used
ktime API, with the latter, enclosing from "order = highest_order(orders)"
till "pte_unmap(pte)" (enclosing the entire while loop), a rough average
estimate is that without the patch, it takes 1700 ns to execute, with the
patch, on an average it takes 80 - 100ns less. I cannot think of a good
testing program...

For the prefetching thingy, I am still doing a linear scan, and in each
iteration, with the patch, the range I am scanning is going to strictly
lie inside the range I would have scanned without the patch. Won't the
compiler and the CPU still do prefetching, but on a smaller range; where
does the prefetcher get confused? I confess, I do not understand this
very well.