LKML Archive on
 help / color / Atom feed
From: Linus Torvalds <>
To: kernel test robot <>
Cc: Jann Horn <>, LKML <>,
Subject: Re: [mm] fd4d9c7d0c: stress-ng.switch.ops_per_sec -30.5% regression
Date: Thu, 26 Mar 2020 09:57:45 -0700
Message-ID: <> (raw)
In-Reply-To: <20200326055723.GL11705@shao2-debian>

On Wed, Mar 25, 2020 at 10:57 PM kernel test robot
<> wrote:
> FYI, we noticed a -30.5% regression of stress-ng.switch.ops_per_sec due to commit:
> commit: fd4d9c7d0c71866ec0c2825189ebd2ce35bd95b8 ("mm: slub: add missing TID bump in kmem_cache_alloc_bulk()")

This looks odd.

I would not expect the update of c->tid to have that noticeable an
impact, even on a big machine that might be close to some scaling

It doesn't add any expensive atomic ops, and while it _could_ make a
percpu cacheline dirty, I think that cacheline should already be dirty
anyway under any load where this is noticeable. Plus this should be a
relatively cold path anyway.

So mind humoring me, and double-check that regression?

Of course, it might be another "just magic cache placement" detail
where code moved enough to make a difference.

Or maybe it really ends up causing new tid mismatches and we end up
failing the fast path in slub as a result. But looking at the stats
that changed in your message doesn't make me go "yeah, that looks like
a slub difference".

So before we look more at this, I'd like to make sure that the
regression is actually real, and not noise.



  reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26  5:57 kernel test robot
2020-03-26 16:57 ` Linus Torvalds [this message]
2020-03-27  8:46   ` Rong Chen
2020-03-30  1:12   ` [LKP] " Feng Tang

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='' \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on

Archives are clonable:
	git clone --mirror lkml/git/0.git
	git clone --mirror lkml/git/1.git
	git clone --mirror lkml/git/2.git
	git clone --mirror lkml/git/3.git
	git clone --mirror lkml/git/4.git
	git clone --mirror lkml/git/5.git
	git clone --mirror lkml/git/6.git
	git clone --mirror lkml/git/7.git
	git clone --mirror lkml/git/8.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ \
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone