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

[Vlastimil Babka <vbabka@suse.cz>]

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)

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);