linux-kernel - Re: darktable performance regression on AMD systems caused by "mm: align larger anonymous mappings on THP boundaries"

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Message-ID: <20241024124953.5d77c0b3@mordecai.tesarici.cz>
Date: Thu, 24 Oct 2024 12:49:53 +0200
From: Petr Tesarik <ptesarik@...e.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Thorsten Leemhuis <regressions@...mhuis.info>, Rik van Riel
 <riel@...riel.com>, Matthias <matthias@...enbinder.de>, Andrew Morton
 <akpm@...ux-foundation.org>, Linux kernel regressions list
 <regressions@...ts.linux.dev>, LKML <linux-kernel@...r.kernel.org>,
 Linux-MM <linux-mm@...ck.org>, Yang Shi <yang@...amperecomputing.com>
Subject: Re: darktable performance regression on AMD systems caused by "mm:
 align larger anonymous mappings on THP boundaries"

On Thu, 24 Oct 2024 12:23:48 +0200
Vlastimil Babka <vbabka@...e.cz> wrote:

> On 10/24/24 11:58, Vlastimil Babka wrote:
> > On 10/24/24 09:45, Thorsten Leemhuis wrote:  
> >> Hi, Thorsten here, the Linux kernel's regression tracker.
> >> 
> >> Rik, I noticed a report about a regression in bugzilla.kernel.org that
> >> appears to be caused by the following change of yours:
> >> 
> >> efa7df3e3bb5da ("mm: align larger anonymous mappings on THP boundaries")
> >> [v6.7]
> >> 
> >> It might be one of those "some things got faster, a few things became
> >> slower" situations. Not sure. Felt odd that the reporter was able to
> >> reproduce it on two AMD systems, but not on a Intel system. Maybe there
> >> is a bug somewhere else that was exposed by this.  
> > 
> > It seems very similar to what we've seen with spec benchmarks such as cactus
> > and bisected to the same commit:
> > 
> > https://bugzilla.suse.com/show_bug.cgi?id=1229012
> > 
> > The exact regression varies per system. Intel regresses too but relatively
> > less. The theory is that there are many large-ish allocations that don't
> > have individual sizes aligned to 2MB and would have been merged, commit
> > efa7df3e3bb5da causes them to become separate areas where each aligns its
> > start at 2MB boundary and there are gaps between. This (gaps and vma
> > fragmentation) itself is not great, but most of the problem seemed to be
> > from the start alignment, which togethter with the access pattern causes
> > more TLB or cache missess due to limited associtativity.
> > 
> > So maybe darktable has a similar problem. A simple candidate fix could
> > change commit efa7df3e3bb5da so that the mapping size has to be a multiple
> > of THP size (2MB) in order to become aligned, right now it's enough if it's
> > THP sized or larger.  
> 
> Maybe this could be enough to fix the issue? (on 6.12-rc4)


Yes, this should work. I was unsure if thp_get_unmapped_area_vmflags()
differs in other ways from mm_get_unmapped_area_vmflags(), which might
still be relevant. I mean, does mm_get_unmapped_area_vmflags() also
prefer to allocate THPs if the virtual memory block is large enough?

Petr T

> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 9c0fb43064b5..a5297cfb1dfc 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -900,7 +900,8 @@ __get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  
>  	if (get_area) {
>  		addr = get_area(file, addr, len, pgoff, flags);
> -	} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> +	} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)
> +		   && IS_ALIGNED(len, PMD_SIZE)) {
>  		/* Ensures that larger anonymous mappings are THP aligned. */
>  		addr = thp_get_unmapped_area_vmflags(file, addr, len,
>  						     pgoff, flags, vm_flags);
>