Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755586AbWKQJ3z (ORCPT ); Fri, 17 Nov 2006 04:29:55 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755587AbWKQJ3z (ORCPT ); Fri, 17 Nov 2006 04:29:55 -0500 Received: from brick.kernel.dk ([62.242.22.158]:57094 "EHLO kernel.dk") by vger.kernel.org with ESMTP id S1755586AbWKQJ3y (ORCPT ); Fri, 17 Nov 2006 04:29:54 -0500 Date: Fri, 17 Nov 2006 10:29:25 +0100 From: Jens Axboe To: "Paul E. McKenney" Cc: Alan Stern , Linus Torvalds , Thomas Gleixner , Ingo Molnar , LKML , john stultz , David Miller , Arjan van de Ven , Andrew Morton , Andi Kleen , manfred@colorfullife.com, oleg@tv-sign.ru Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync Message-ID: <20061117092925.GT7164@kernel.dk> References: <20061117065128.GA5452@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20061117065128.GA5452@us.ibm.com> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4881 Lines: 109 On Thu, Nov 16 2006, Paul E. McKenney wrote: > On Thu, Nov 16, 2006 at 10:06:25PM -0500, Alan Stern wrote: > > On Thu, 16 Nov 2006, Linus Torvalds wrote: > > > > > > > > > > > On Thu, 16 Nov 2006, Alan Stern wrote: > > > > On Thu, 16 Nov 2006, Linus Torvalds wrote: > > > > > > > > > > Paul, it would be _really_ nice to have some way to just initialize > > > > > that SRCU thing statically. This kind of crud is just crazy. > > > > > > > > I looked into this back when SRCU was first added. It's essentially > > > > impossible to do it, because the per-cpu memory allocation & usage APIs > > > > are completely different for the static and the dynamic cases. > > > > > > I don't think that's how you'd want to do it. > > > > > > There's no way to do an initialization of a percpu allocation statically. > > > That's pretty obvious. > > > > Hmmm... What about DEFINE_PER_CPU in include/asm-generic/percpu.h > > combined with setup_per_cpu_areas() in init/main.c? So long as you want > > all the CPUs to start with the same initial values, it should work. > > > > > What I'd suggest instead, is to make the allocation dynamic, and make it > > > inside the srcu functions (kind of like I did now, but I did it at a > > > higher level). > > > > > > Doing it at the high level was trivial right now, but we may well end up > > > hitting this problem again if people start using SRCU more. Right now I > > > suspect the cpufreq notifier is the only thing that uses SRCU, and it > > > already showed this problem with SRCU initializers. > > > > > > So I was more thinking about moving my "one special case high level hack" > > > down lower, down to the SRCU level, so that we'll never see _more_ of > > > those horrible hacks. We'll still have the hacky thing, but at least it > > > will be limited to a single place - the SRCU code itself. > > > > Another possible approach (but equally disgusting) is to use this static > > allocation approach, and have the SRCU structure include both a static and > > a dynamic percpu pointer together with a flag indicating which should be > > used. > > I am actually taking some suggestions you made some months ago. At the > time, I rejected them because they injected extra branches into the > fastpath. However, recent experience indicates that you (Alan Stern) > were right and I was wrong -- turns out that the update-side overhead > cannot be so lightly disregarded, which forces memory barriers (but > neither atomics nor cache misses) into the fastpath. If some application > ends up being provably inconvenienced by the read-side overhead, they old > implementation can be re-introduced under a different name or some such. > > So, here is my current plan: > > o Add NULL checks on srcu_struct_array to srcu_read_lock(), > srcu_read_unlock(), and synchronize_srcu. These will > acquire the mutex and attempt to initialize. If out > of memory, they will use the new hardluckref field. > > o Add memory barriers to srcu_read_lock() and srcu_read_unlock(). > > o Also add a memory barrier or two to synchronize_srcu(), which, > in combination with those in srcu_read_lock() and srcu_read_unlock(), > permit removing two of the three synchronize_sched() calls > in synchronize_srcu(), decreasing its latency by roughly > a factor of three. > > This change should have the added benefit of making > synchronize_srcu() much easier to understand. > > o I left out the super-fastpath synchronize_srcu() because > after sleeping on it, it scared me silly. Might be OK, > but needs careful thought. The fastpath is of the form: > > if (srcu_readers_active(sp) == 0) { > smp_mb(); > return; > } > > prior to the mutex_lock() in synchronize_srcu(). It works for me, but the overhead is still large. Before it would take 8-12 jiffies for a synchronize_srcu() to complete without there actually being any reader locks active, now it takes 2-3 jiffies. So it's definitely faster, and as suspected the loss of two of three synchronize_sched() cut down the overhead to a third. It's still too heavy for me, by far the most calls I do to synchronize_srcu() doesn't have any reader locks pending. I'm still a big advocate of the fastpath srcu_readers_active() check. I can understand the reluctance to make it the default, but for my case it's "safe enough", so if we could either export srcu_readers_active() or export a synchronize_srcu_fast() (or something like that), then SRCU would be a good fit for barrier vs plug rework. > Attached is a patch that compiles, but probably goes down in flames > otherwise. Works here :-) -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/