Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756096AbWKRAhv (ORCPT ); Fri, 17 Nov 2006 19:37:51 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756101AbWKRAhv (ORCPT ); Fri, 17 Nov 2006 19:37:51 -0500 Received: from e31.co.us.ibm.com ([32.97.110.149]:44494 "EHLO e31.co.us.ibm.com") by vger.kernel.org with ESMTP id S1756096AbWKRAhu (ORCPT ); Fri, 17 Nov 2006 19:37:50 -0500 Date: Fri, 17 Nov 2006 16:38:59 -0800 From: "Paul E. McKenney" To: Alan Stern Cc: "Paul E. McKenney" , Jens Axboe , 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: <20061118003859.GG2632@us.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20061117191538.GA2632@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2571 Lines: 60 On Fri, Nov 17, 2006 at 02:27:15PM -0500, Alan Stern wrote: > On Fri, 17 Nov 2006, Paul E. McKenney wrote: > > > > 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. > > > > Good to hear, thank you for trying it out! > > > > > 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. > > > > OK, will export the interface. Do your queues have associated locking? > > > > > > Attached is a patch that compiles, but probably goes down in flames > > > > otherwise. > > > > > > Works here :-) > > > > I have at least a couple bugs that would show up under low-memory > > situations, will fix and post an update. > > Perhaps a better approach to the initialization problem would be to assume > that either: > > 1. The srcu_struct will be initialized before it is used, or > > 2. When it is used before initialization, the system is running > only one thread. Are these assumptions valid? If so, they would indeed simplify things a bit. > In other words, statically allocated SRCU strucures that get used during > system startup must be initialized before the system starts multitasking. > That seems like a reasonable requirement. > > This eliminates worries about readers holding mutexes. It doesn't > solve the issues surrounding your hardluckref, but maybe it makes them > easier to think about. For the moment, I cheaped out and used a mutex_trylock. If this can block, I will need to add a separate spinlock to guard per_cpu_ref allocation. Hmmm... How to test this? Time for the wrapper around alloc_percpu() that randomly fails, I guess. ;-) Thanx, Paul - 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/