linux-kernel - Re: regression/bisected commit 773688a6cb24b0b3c2ba40354d883348a2befa38 make my system completely unusable under high load

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Date: Tue, 20 Feb 2024 19:51:07 +0100
From: Marco Elver <elver@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>, Andrey Konovalov <andreyknvl@...il.com>, glider@...gle.com, 
	dvyukov@...gle.com, eugenis@...gle.com, Oscar Salvador <osalvador@...e.de>, 
	Linux List Kernel Mailing <linux-kernel@...r.kernel.org>, 
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: regression/bisected commit 773688a6cb24b0b3c2ba40354d883348a2befa38
 make my system completely unusable under high load

On Tue, 20 Feb 2024 at 19:16, Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 2/20/24 18:30, Andrew Morton wrote:
> > On Tue, 20 Feb 2024 10:37:03 +0500 Mikhail Gavrilov <mikhail.v.gavrilov@...il.com> wrote:
> >
> >> On Tue, Feb 20, 2024 at 4:50 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >> > >
> >> > > I'm all confused.
> >> > >
> >> > > 4434a56ec209 ("stackdepot: make fast paths lock-less again") was
> >> > > mainlined for v6.8-rc3.
> >> >
> >> > Uh sorry, I just trusted the info that it's not merged and didn't verify
> >> > it myself. Yeah, I can see it is there.
> >> >
> >>
> >> Wait, I am talk about these two patches which is not merged yet:
> >> [PATCH v2 1/2] stackdepot: use variable size records for non-evictable entries
> >> [PATCH v2 2/2] kasan: revert eviction of stack traces in generic mode
> >> https://lore.kernel.org/linux-mm/20240129100708.39460-1-elver@google.com/
> >
> > A can move those into the 6.8-rc hotfixes queue, and it appears a
> > cc:stable will not be required.
> >
> > However I'm not seeing anything in the changelogs to indicate that
> > we're fixing a dramatic performance regression, nor why that
> > regressions is occurring.

It's primarily fixing a regression of memory usage overhead for
stackdepot users in general. Performance is mostly fixed, but patch
2/2 ("kasan: revert eviction of stack traces in generic mode") also
helps with KASAN performance because entries that were being
repeatedly evicted-then-reallocated are just allocated once and with
increasing system uptime the slow path will be taken much less.

> We also seem have an unhappy bot with the 2/2 patch :/ although it's not yet
> clear if it's a genuine issue.
>
> https://lore.kernel.org/all/202402201506.b7e4b9b6-oliver.sang@intel.com/

While it would be nice if 6.8 would not regress over 6.7 (performance
is mostly fixed, memory usage is not), waiting for confirmation what
the rcutorture issue from the bot is about might be good.

Mikhail: since you are testing mainline, in about 4 weeks the fixes
should then reach 6.9-rc in the next merge window. Until then, if it's
not too difficult for you, you can apply those 2 patches in your own
tree.

Thanks,
-- Marco