From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx183.postini.com [74.125.245.183]) by kanga.kvack.org (Postfix) with SMTP id 338D86B0068 for ; Sat, 4 Aug 2012 19:02:49 -0400 (EDT) Date: Sat, 4 Aug 2012 23:59:10 +0100 From: "Dr. David Alan Gilbert" Subject: Re: [RFC] page-table walkers vs memory order Message-ID: <20120804225910.GB1255@gallifrey> References: <1343064870.26034.23.camel@twins> <20120804143719.GB10459@redhat.com> <20120804220245.GB3307@linux.vnet.ibm.com> <20120804224705.GD10459@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120804224705.GD10459@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: "Paul E. McKenney" , Hugh Dickins , Peter Zijlstra , Linus Torvalds , Rik van Riel , Andrew Morton , Nick Piggin , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org * Andrea Arcangeli (aarcange@redhat.com) wrote: > On Sat, Aug 04, 2012 at 03:02:45PM -0700, Paul E. McKenney wrote: > > OK, I'll bite. ;-) > > :)) > > > The most sane way for this to happen is with feedback-driven techniques > > involving profiling, similar to what is done for basic-block reordering > > or branch prediction. The idea is that you compile the kernel in an > > as-yet (and thankfully) mythical pointer-profiling mode, which records > > the values of pointer loads and also measures the pointer-load latency. > > If a situation is found where a given pointer almost always has the > > same value but has high load latency (for example, is almost always a > > high-latency cache miss), this fact is recorded and fed back into a > > subsequent kernel build. This subsequent kernel build might choose to > > speculate the value of the pointer concurrently with the pointer load. > > > > And of course, when interpreting the phrase "most sane way" at the > > beginning of the prior paragraph, it would probably be wise to keep > > in mind who wrote it. And that "most sane way" might have little or > > no resemblance to anything that typical kernel hackers would consider > > anywhere near sanity. ;-) > > I see. The above scenario is sure fair enough assumption. We're > clearly stretching the constraints to see what is theoretically > possible and this is a very clear explanation of how gcc could have an > hardcoded "guessed" address in the .text. > > Next step to clearify now, is how gcc can safely dereference such a > "guessed" address without the kernel knowing about it. > > If gcc would really dereference a guessed address coming from a > profiling run without kernel being aware of it, it would eventually > crash the kernel with an oops. gcc cannot know what another CPU will > do with the kernel pagetables. It'd be perfectly legitimate to > temporarily move the data at the "guessed address" to another page and > to update the pointer through stop_cpu during some weird "cpu > offlining scenario" or anything you can imagine. I mean gcc must > behave in all cases so it's not allowed to deference the guessed > address at any given time. A compiler could decide to dereference it using a non-faulting load, do the calculations or whatever on the returned value of the non-faulting load, and then check whether the load actually faulted, and whether the address matched the prediction before it did a store based on it's guess. Dave -- -----Open up your eyes, open up your mind, open up your code ------- / Dr. David Alan Gilbert | Running GNU/Linux | Happy \ \ gro.gilbert @ treblig.org | | In Hex / \ _________________________|_____ http://www.treblig.org |_______/ -- 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