linux-kernel - Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

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Date:	Thu, 11 Aug 2016 19:27:52 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Christoph Hellwig <hch@....de>,
	"Huang, Ying" <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bob Peterson <rpeterso@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, LKP <lkp@...org>
Subject: Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

On Thu, Aug 11, 2016 at 5:54 PM, Dave Chinner <david@...morbit.com> wrote:
>
> So, removing mark_page_accessed() made the spinlock contention
> *worse*.
>
>   36.51%  [kernel]  [k] _raw_spin_unlock_irqrestore
>    6.27%  [kernel]  [k] copy_user_generic_string
>    3.73%  [kernel]  [k] _raw_spin_unlock_irq
>    3.55%  [kernel]  [k] get_page_from_freelist
>    1.97%  [kernel]  [k] do_raw_spin_lock
>    1.72%  [kernel]  [k] __block_commit_write.isra.30

I don't recall having ever seen the mapping tree_lock as a contention
point before, but it's not like I've tried that load either. So it
might be a regression (going back long, I suspect), or just an unusual
load that nobody has traditionally tested much.

Single-threaded big file write one page at a time, was it?

The mapping tree lock has been around forever (it used to be a rw-lock
long long ago), but I wonder if we might have moved more stuff into it
(memory accounting comes to mind) causing much worse contention or
something.

Hmm. Just for fun, I googled "tree_lock contention". It's shown up
before - back in 2006, and it was you hitting it back then too.

There was an even older one (related to AIM7, interesting) which was
what caused the tree_lock to become a rw-lock back in 2005 (but then
Nick Piggin made it a spinlock again in 2008).

So it's not unheard of, but it certainly hasn't been a big issue.

That's the only obvious ones I found (apart from some btrfs issues,
but btrfs has a completely different notion of tree locking, so those
are not about the same thing).

                 Linus