From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [patch 09/10] SLUB: Do our own locking via slab_lock and slab_unlock. Date: Wed, 31 Oct 2007 12:17:33 +1100 References: <20071028033156.022983073@sgi.com> <200710301550.55199.nickpiggin@yahoo.com.au> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200710311217.34162.nickpiggin@yahoo.com.au> Sender: owner-linux-mm@kvack.org Return-Path: To: Christoph Lameter Cc: Matthew Wilcox , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Pekka Enberg List-ID: On Wednesday 31 October 2007 05:32, Christoph Lameter wrote: > On Tue, 30 Oct 2007, Nick Piggin wrote: > > Is this actually a speedup on any architecture to roll your own locking > > rather than using bit spinlock? > > It avoids one load from memory when allocating and the release is simply > writing the page->flags back. Less instructions. OK, but it probably isn't a measurable speedup, even on microbenchmarks, right? And many architectures have to have more barriers around cmpxchg than they do around a test_and_set_bit_lock, so it may even be slower on some. > > I am not exactly convinced that smp_wmb() is a good idea to have in your > > unlock, rather than the normally required smp_mb() that every other open > > coded lock in the kernel is using today. If you comment every code path > > where a load leaking out of the critical section would not be a problem, > > then OK it may be correct, but I still don't think it is worth the > > maintenance overhead. > > I thought you agreed that release semantics only require a write barrier? Not in general. > The issue here is that other processors see the updates before the > updates to page-flags. > > A load leaking out of a critical section would require that the result of > the load is not used to update other information before the slab_unlock > and that the source of the load is not overwritten in the critical > section. That does not happen in sluib. That may be the case, but I don't think there is enough performance justification to add a hack like this. ia64 for example is going to do an mf for smp_wmb so I doubt it is a clear win. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org