From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f42.google.com (mail-wg0-f42.google.com [74.125.82.42]) by kanga.kvack.org (Postfix) with ESMTP id 579D16B0035 for ; Sun, 24 Aug 2014 06:05:09 -0400 (EDT) Received: by mail-wg0-f42.google.com with SMTP id l18so11831612wgh.1 for ; Sun, 24 Aug 2014 03:05:08 -0700 (PDT) Received: from mail-we0-x22c.google.com (mail-we0-x22c.google.com [2a00:1450:400c:c03::22c]) by mx.google.com with ESMTPS id a15si5312821wiw.9.2014.08.24.03.05.07 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 24 Aug 2014 03:05:07 -0700 (PDT) Received: by mail-we0-f172.google.com with SMTP id x48so12294105wes.31 for ; Sun, 24 Aug 2014 03:05:07 -0700 (PDT) Date: Sun, 24 Aug 2014 12:05:03 +0200 From: Ingo Molnar Subject: Re: [PATCH] [v4] warn on performance-impacting configs aka. TAINT_PERFORMANCE Message-ID: <20140824100503.GA929@gmail.com> References: <20140822205625.657E9890@viggo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140822205625.657E9890@viggo.jf.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: linux-kernel@vger.kernel.org, dave.hansen@linux.intel.com, peterz@infradead.org, mingo@redhat.com, ak@linux.intel.com, tim.c.chen@linux.intel.com, akpm@linux-foundation.org, cl@linux.com, penberg@kernel.org, linux-mm@kvack.org, kirill@shutemov.name, lauraa@codeaurora.org, davej@redhat.com * Dave Hansen wrote: > > From: Dave Hansen > > Changes from v3: > * remove vestiges of TAINT_PERFORMANCE > * change filename to check-configs > * fix typos in description > * print out CONFIG_FOO=y > * tone down warning message > * add KMEMCHECK and GCOV > * add PROVE_LOCKING, but keep LOCKDEP since _anything_ selecting > it will cause scalaing issues at least. But, move LOCKDEP > below LOCK_STAT and PROVE_LOCKING. > * no more perfo-mance (missing an 'r') > * temporary variable in lieu of multiple ARRAY_SIZE() > * break early out of snprintf() loop > > Changes from v2: > * remove tainting and stack track > * add debugfs file > * added a little text to guide folks who want to add more > options > > Changes from v1: > * remove schedstats > * add DEBUG_PAGEALLOC and SLUB_DEBUG_ON > > -- > > I have more than once myself been the victim of an accidentally- > enabled kernel config option being mistaken for a true > performance problem. > > I'm sure I've also taken profiles or performance measurements > and assumed they were real-world when really I was measuring the > performance with an option that nobody turns on in production. > > A warning like this late in boot will help remind folks when > these kinds of things are enabled. We can also teach tooling to > look for and capture /sys/kernel/debug/config_debug . > > As for the patch... > > I originally wanted this for CONFIG_DEBUG_VM, but I think it also > applies to things like lockdep and slab debugging. See the patch > for the list of offending config options. I'm open to adding > more, but this seemed like a good list to start. > > The compiler is smart enough to really trim down the code when > the array is empty. An objdump -d looks like this: > > lib/check-configs.o: file format elf64-x86-64 > > Disassembly of section .init.text: > > 000000000000000 : > 0: 55 push %rbp > 1: 31 c0 xor %eax,%eax > 3: 48 89 e5 mov %rsp,%rbp > 6: 5d pop %rbp > 7: c3 retq > > This could be done with Kconfig and an #ifdef to save us 8 bytes > of text and the entry in the late_initcall() section. Doing it > this way lets us keep the list of these things in one spot, and > also gives us a convenient way to dump out the name of the > offending option. > > For anybody that *really* cares, I put the whole thing under > CONFIG_DEBUG_KERNEL in the Makefile. > > The messages look like this: > > [ 3.865297] INFO: Be careful when using this kernel for performance measurement. > [ 3.868776] INFO: Potentially performance-altering options enabled: > [ 3.871558] CONFIG_LOCKDEP=y > [ 3.873326] CONFIG_SLUB_DEBUG_ON=y > > Signed-off-by: Dave Hansen > Cc: Peter Zijlstra > Cc: Ingo Molnar > Cc: ak@linux.intel.com > Cc: tim.c.chen@linux.intel.com > Cc: Andrew Morton > Cc: Christoph Lameter > Cc: Pekka Enberg > Cc: linux-kernel@vger.kernel.org > Cc: linux-mm@kvack.org > Cc: kirill@shutemov.name > Cc: lauraa@codeaurora.org > Cc: davej@redhat.com Looks good to me in principle. (haven't tested it) Reviewed-by: Ingo Molnar Thanks, Ingo -- 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