linux-kernel - Re: [patch 1/4] spinlock: lockbreak cleanup

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Date:	Thu, 01 Nov 2007 11:39:59 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, Ingo Molnar <mingo@...e.hu>,
	riel <riel@...hat.com>
Subject: Re: [patch 1/4] spinlock: lockbreak cleanup

On Thu, 2007-11-01 at 15:29 +0100, Nick Piggin wrote:
> On Thu, Nov 01, 2007 at 03:06:05PM +0100, Peter Zijlstra wrote:
> > On Thu, 2007-11-01 at 15:02 +0100, Nick Piggin wrote:
> > 
> > > Rename need_lockbreak to spin_needbreak, make it use spin_is_contended to
> > > decouple it from the spinlock implementation, and make it typesafe (rwlocks
> > > do not have any need_lockbreak sites -- why do they even get bloated up
> > > with that break_lock then?).
> > 
> > IIRC Lee has a few patches floating about that do introduce lockbreak
> > stuff for rwlocks.
> 
> Well that would be a good reason to introduce a break_lock for them,
> but previously not so much... we have rwlocks in some slightly space
> critical structures (vmas, inodes, etc).
> 
> I guess it was done to make the "template" hacks eaiser. I don't really
> find that in good taste, especially for important core infrastructure.
> Anyway.

Actually, what I had/have is a cond_resched_rwlock() that I needed to
convert the i_mmap_lock() to rw for testing reclaim scalability.  [I've
seen a large system running an Oracle OLTP load hang spitting "cpu soft
lockup" messages with all cpus spinning on a i_mmap_lock spin lock.]
One of the i_mmap_lock paths uses cond_resched_lock() for spin locks.
To do a straight forward conversion [and maybe that isn't the right
approach], I created the cond_resched_rwlock() function by generalizing
the cond_sched_lock() code and creating both spin and rw lock wrappers.
I took advantage of the fact that, currently, need_lockbreak() is a
macro and that both spin and rw locks have/had the break_lock member.
Typesafe functions would probably be preferrable, if we want to keep
break_lock for rw spin locks.

Here's the most recent posting:

	http://marc.info/?l=linux-mm&m=118980356306014&w=4

See the changes to sched.[ch].  Should apply to 23-mm1 with offsets and
minor fixup in fs/inode.c.

Lee

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