Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757194Ab0LJUd1 (ORCPT ); Fri, 10 Dec 2010 15:33:27 -0500 Received: from casper.infradead.org ([85.118.1.10]:36995 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757070Ab0LJUd0 convert rfc822-to-8bit (ORCPT ); Fri, 10 Dec 2010 15:33:26 -0500 Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM From: Peter Zijlstra To: Christoph Lameter Cc: Eric Dumazet , Venkatesh Pallipadi , Russell King - ARM Linux , Mikael Pettersson , Ingo Molnar , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, John Stultz In-Reply-To: References: <20101208142814.GE9777@n2100.arm.linux.org.uk> <1291851079-27061-1-git-send-email-venki@google.com> <1291899120.29292.7.camel@twins> <1291917330.6803.7.camel@twins> <1291920939.6803.38.camel@twins> <1291936593.13513.3.camel@laptop> <1291975704.6803.59.camel@twins> <1291987065.6803.151.camel@twins> <1291987635.6803.161.camel@twins> <1291988866.6803.171.camel@twins> <1292001500.3580.268.camel@edumazet-laptop> <1292003346.13513.30.camel@laptop> <1292004859.3580.387.camel@edumazet-laptop> <1292006788.13513.43.camel@laptop> <1292011644.13513.61.camel@laptop> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 10 Dec 2010 21:32:37 +0100 Message-ID: <1292013157.13513.69.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2893 Lines: 90 On Fri, 2010-12-10 at 14:23 -0600, Christoph Lameter wrote: > On Fri, 10 Dec 2010, Peter Zijlstra wrote: > > > Its not about passing per-cpu pointers, its about passing long pointers. > > > > When I write: > > > > void foo(u64 *bla) > > { > > *bla++; > > } > > > > DEFINE_PER_CPU(u64, plop); > > > > void bar(void) > > { > > foo(__this_cpu_ptr(plop)); > > } > > > > I want gcc to emit the equivalent to: > > > > __this_cpu_inc(plop); /* incq %fs:(%0) */ > > > > Now I guess the C type system will get in the way of this ever working, > > since a long pointer would have a distinct type from a regular > > pointer :/ > > > > The idea is to use 'regular' functions with the per-cpu data in a > > transparent manner so as not to have to replicate all logic. > > That would mean you would have to pass information in the pointer at > runtime indicating that this particular pointer is a per cpu pointer. > > Code for the Itanium arch can do that because it has per cpu virtual > mappings. So you define a virtual area for per cpu data and then map it > differently for each processor. If we would have a different page table > for each processor then we could avoid using segment register and do the > same on x86. I don't think its a runtime issue, its a compile time issue. At compile time the compiler can see the argument is a long pointer: %fs:(addr,idx,size), and could propagate this into the caller. The above example will compute the effective address by doing something like: lea %fs:(addr,idx,size),%ebx and will then do something like inc (%ebx) Where it could easily have optimized this into: inc %fs:(addr,idx,size) esp when foo would be inlined. If its an actual call-site you need function overloading because a long pointer has a different signature from a regular pointer, and that is something C doesn't do. > > > Seems that you do not have that use case in mind. So a seqlock restricted > > > to a single processor? If so then you wont need any of those smp write > > > barriers mentioned earlier. A simple compiler barrier() is sufficient. > > > > The seqcount is sometimes read by different CPUs, but I don't see why we > > couldn't do what Eric suggested. > > But you would have to define a per cpu seqlock. Each cpu would have > its own seqlock. Then you could have this_cpu_read_seqcount_begin and > friends: > > Then you can do > > this_cpu_read_seqcount_begin(&bla) > Which to me seems to be exactly what Eric proposed.. > But then this seemed to be a discussion related to ARM. ARM does not have > optimized per cpu accesses. Nah, there's multiple issues all nicely mangled into one thread ;-) -- 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/