linux-kernel - Re: [RFC] Cancellable MCS spinlock rework

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Date:	Fri, 4 Jul 2014 09:49:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Waiman Long <waiman.long@...com>,
	Davidlohr Bueso <davidlohr@...com>,
	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	mingo@...nel.org, linux-kernel@...r.kernel.org, riel@...hat.com,
	akpm@...ux-foundation.org, hpa@...or.com, andi@...stfloor.org,
	James.Bottomley@...senpartnership.com, rostedt@...dmis.org,
	tim.c.chen@...ux.intel.com, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [RFC] Cancellable MCS spinlock rework

On Thu, Jul 03, 2014 at 01:51:48PM -0700, Jason Low wrote:
> On Thu, 2014-07-03 at 16:35 -0400, Waiman Long wrote:
> > On 07/03/2014 02:34 PM, Jason Low wrote:
> > > On Thu, 2014-07-03 at 10:09 -0700, Davidlohr Bueso wrote:
> > >> On Thu, 2014-07-03 at 09:31 +0200, Peter Zijlstra wrote:
> > >>> On Wed, Jul 02, 2014 at 10:30:03AM -0700, Jason Low wrote:
> > >>>> Would potentially reducing the size of the rw semaphore structure by 32
> > >>>> bits (for all architectures using optimistic spinning) be a nice
> > >>>> benefit?
> > >>> Possibly, although I had a look at the mutex structure and we didn't
> > >>> have a hole to place it in, unlike what you found with the rwsem.
> > >> Yeah, and currently struct rw_semaphore is the largest lock we have in
> > >> the kernel. Shaving off space is definitely welcome.
> > > Right, especially if it could help things like xfs inode.
> > >
> > 
> > I do see a point in reducing the size of the rwsem structure. However, I 
> > don't quite understand the point of converting pointers in the 
> > optimistic_spin_queue structure to atomic_t.
> 
> Converting the pointers in the optimistic_spin_queue to atomic_t would
> mean we're fully operating on atomic operations instead of using the
> potentially racy cmpxchg + ACCESS_ONCE stores on the pointers.
> 
> If we're in the process of using the CPU numbers in atomic_t, I thought
> we might as well fix that as well since it has actually been shown to
> result in lockups on some architectures. We can then avoid needing to
> implement the tricky architecture workarounds for optimistic spinning.
> Wouldn't that be a "nice-have"?

Nah, I think those archs are fundamentally broken at the moment, we
should not make code harder to read and or more complex just for them.

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