From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754333Ab2GJIVR (ORCPT ); Tue, 10 Jul 2012 04:21:17 -0400 Received: from mail-wi0-f172.google.com ([209.85.212.172]:61366 "EHLO mail-wi0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753299Ab2GJIVK (ORCPT ); Tue, 10 Jul 2012 04:21:10 -0400 Date: Tue, 10 Jul 2012 10:21:04 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Linus Torvalds , hpa@zytor.com, eranian@google.com, linux-kernel@vger.kernel.org, fweisbec@gmail.com, akpm@linux-foundation.org, tglx@linutronix.de, linux-tip-commits@vger.kernel.org, Robert Richter Subject: Re: [tip:perf/core] perf/x86: Fix USER/KERNEL tagging of samples Message-ID: <20120710082104.GA11187@gmail.com> References: <1341598329.7709.57.camel@twins> <1341832997.3462.41.camel@twins> <20120709184145.GA7666@gmail.com> <1341906848.3462.92.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1341906848.3462.92.camel@twins> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Mon, 2012-07-09 at 20:41 +0200, Ingo Molnar wrote: > > > +static unsigned long get_segment_base(unsigned int segment) > > > +{ > > > + struct desc_struct *desc; > > > + int idx = segment >> 3; > > > + > > > + if ((segment & SEGMENT_TI_MASK) == SEGMENT_LDT) { > > > + if (idx > LDT_ENTRIES) > > > + return 0; > > > + > > > + desc = current->active_mm->context.ldt; > > > + } else { > > > + if (idx > GDT_ENTRIES) > > > + return 0; > > > + > > > + desc = __this_cpu_ptr(&gdt_page.gdt[0]); > > > + } > > > + > > > + return get_desc_base(desc + idx); > > > > Shouldn't idx be checked against active_mm->context.ldt.size, > > not LDT_ENTRIES (which is really just an upper limit)? > > Ah indeed, fixed that. Another boundary condition would be when we intentionally twiddle the GDT: such as during suspend or during BIOS upcalls. Can we then get a PMU interrupt? If yes then this will probably result in garbage: > > > + desc = __this_cpu_ptr(&gdt_page.gdt[0]); it won't outright crash, we don't ever deallocate our GDT - but it will return a garbage RIP. Then there's also all the Xen craziness with segments ... Both ought to be rare an uninteresting - but then again, segmented execution is already rare and uninteresting to begin with. So, instead of trying to discover all these weird x86 cases - with little to no testing done after that - I thought that it might be more future proof to just handle the cases we are explicitly interested in: flat code, and pounce in some well defined way in all the other situations by returning the RIP to an empty __X86_LEGACY_SEGMENTED_CODE() symbol. That way we will at least give *some* useful information to the poor segmented code user, if the profile says: 21.32% [kernel] [k] __X86_LEGACY_SEGMENTED_CODE 11.01% [kernel] [k] kallsyms_expand_symbol 8.29% [kernel] [k] vsnprintf 7.37% libc-2.15.so [.] __strcmp_sse42 6.93% perf [.] symbol_filter 4.20% perf [.] kallsyms__parse 3.92% [kernel] [k] format_decode 3.62% [kernel] [k] string.isra.4 3.59% [kernel] [k] memcpy 3.11% [kernel] [k] strnlen then the user at least knows that there's 21% of overhead in some sort of segmented x86 code. Or if they *really* want to resolve that, they can take your patch and add symbol decoding to user-space and test it all. KISS and such. Linus? Thanks, Ingo