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Date: Mon, 07 Jul 2014 10:22:28 -0700
From: Jason Low <jason.low2@...com>
To: Peter Zijlstra <peterz@...radead.org>
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 Fri, 2014-07-04 at 09:51 +0200, Peter Zijlstra wrote:
> On Thu, Jul 03, 2014 at 06:07:23PM -0700, Jason Low wrote:
> > On Thu, 2014-07-03 at 16:35 -0400, Waiman Long wrote:
> >
> > > 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. The structure is cacheline
> > > aligned and there is no saving in size. Converting them to atomic_t does
> > > have a bit of additional overhead of converting the encoded cpu number
> > > back to the actual pointer.
> > >
> > > So my suggestion is to just change what is stored in the mutex and rwsem
> > > structure to atomic_t, but keep the pointers in the
> > > optimistic_spin_queue structure.
> >
> > Peter, would you prefer going with the above?
> >
> > If we were to keep the pointers to the next and prev nodes in the struct
> > optimistic_spin_queue instead of converting them to atomic_t to store
> > their cpu #, we'd still need to keep track of the cpu #. In the unqueue
> > phase of osq_lock, we might have to reload prev = node->prev which we
> > then may cmpxchg() it with the lock tail.
> >
> > The method we can think of so far would be to add a regular int variable
> > to optimistic_spin_queue and initialize it to the CPU #, during the time
> > we also initialize node->locked and node->next at the beginning of
> > osq_lock. The cost wouldn't be much of an issue since
> > optimistic_spin_queue is cache aligned.
>
> Let me try and have an actual look at the patch;
Okay, I will be sending out the patchset I had so that there's something
more concrete.
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