linux-kernel - Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges

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Date:	Fri, 29 Feb 2008 13:34:34 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
cc:	Nick Piggin <nickpiggin@...oo.com.au>, akpm@...ux-foundation.org,
	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges

On Fri, 29 Feb 2008, Andrea Arcangeli wrote:

> On Fri, Feb 29, 2008 at 01:03:16PM -0800, Christoph Lameter wrote:
> > That means we need both the anon_vma locks and the i_mmap_lock to become 
> > semaphores. I think semaphores are better than mutexes. Rik and Lee saw 
> > some performance improvements because list can be traversed in parallel 
> > when the anon_vma lock is switched to be a rw lock.
> 
> The improvement was with a rw spinlock IIRC, so I don't see how it's
> related to this.

AFAICT The rw semaphore fastpath is similar in performance to a rw 
spinlock. 
 
> Perhaps the rwlock spinlock can be changed to a rw semaphore without
> measurable overscheduling in the fast path. However theoretically

Overscheduling? You mean overhead?

> speaking the rw_lock spinlock is more efficient than a rw semaphore in
> case of a little contention during the page fault fast path because
> the critical section is just a list_add so it'd be overkill to
> schedule while waiting. That's why currently it's a spinlock (or rw
> spinlock).

On the other hand a semaphore puts the process to sleep and may actually 
improve performance because there is less time spend in a busy loop. 
Other processes may do something useful and we stay off the contended 
cacheline reducing traffic on the interconnect.
 
> preempt-rt runs quite a bit slower, or we could rip spinlocks out of
> the kernel in the first place ;)

The question is why that is the case and it seesm that there are issues 
with interrupt on/off that are important here and particularly significant 
with the SLAB allocator (significant hacks there to deal with that issue). 
The fastpath that we have in the works for SLUB may address a large 
part of that issue because it no longer relies on disabling interrupts.
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