linux-kernel - Re: Switching TestSetPageLocked to trylock

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Message-Id: <200808051341.53282.nickpiggin@yahoo.com.au>
Date:	Tue, 5 Aug 2008 13:41:53 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Switching TestSetPageLocked to trylock_page

On Tuesday 05 August 2008 13:28, Linus Torvalds wrote:
> On Tue, 5 Aug 2008, Nick Piggin wrote:
> > On Thursday 31 July 2008 17:26, Nick Piggin wrote:
> > > Hi,
> > >
> > > I'm wondering if I could get a patch merged which changes all
> > > TestSetPageLocked and replaces them with trylock_page?
> >
> > Yes? No?
> >
> > The alternative is try to merge it via -mm or -next, but that just
> > wastes everybodies time with conflicts of having these differences
> > between -mm and mainline.
>
> Heh. I had just been _assuming_ this would go through -mm, since it's
> exactly the kind of thing that usually does go through there.
>
> So I hadn't even really considered it.

OK... it just causes Andrew headaches I suspect. But if he prefers
to hold onto it for an entire release cycle... Andrew?

> I don't mind the patch per se, but can you give some background on what
> the pending optimization is that makes such a big difference?

Using the lock semantics bitops is the first one. While it is true
that we could just hack them into TestSetPageLocked, I really prefer
callers to require at least a cursory glance to convert them, and
understand that this is a lock lock, and not a test_and_set bitop
with full barrier semantics.

lock semantics bitops obviously doesn't help x86 a bit. It really
helps powerpc though.

The part that helps x86 is another patch further down my stack, which
can avoid looking at the hashed page waitqueue at unlock_page-time.
Basically this eliminates the entire page waitqueue from the cache
footprint of fastpath workloads where the lock is uncontended (often:
page faulting, write(2), writeout etc).

I'll send all that stuff off to -mm for 2.6.28 if this gets upstream
(or into -mm).
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