From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.7 required=3.0 tests=BAYES_00,DKIM_ADSP_CUSTOM_MED, FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A4B44C433ED for ; Thu, 8 Apr 2021 16:44:48 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id E75A161131 for ; Thu, 8 Apr 2021 16:44:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E75A161131 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-21180-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 20383 invoked by uid 550); 8 Apr 2021 16:44:40 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 20363 invoked from network); 8 Apr 2021 16:44:40 -0000 Date: Thu, 8 Apr 2021 18:44:26 +0200 From: Alexey Gladkov To: Linus Torvalds Cc: kernel test robot , 0day robot , LKML , lkp@lists.01.org, "Huang, Ying" , Feng Tang , zhengjun.xing@intel.com, Kernel Hardening , Linux Containers , Linux-MM , Andrew Morton , Christian Brauner , "Eric W . Biederman" , Jann Horn , Jens Axboe , Kees Cook , Oleg Nesterov Subject: Re: 08ed4efad6: stress-ng.sigsegv.ops_per_sec -41.9% regression Message-ID: <20210408164426.o5cfvv3ixowsto62@example.org> References: <7abe5ab608c61fc2363ba458bea21cf9a4a64588.1617814298.git.gladkov.alexey@gmail.com> <20210408083026.GE1696@xsang-OptiPlex-9020> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Apr 08, 2021 at 09:22:40AM -0700, Linus Torvalds wrote: > On Thu, Apr 8, 2021 at 1:32 AM kernel test robot wrote: > > > > FYI, we noticed a -41.9% regression of stress-ng.sigsegv.ops_per_sec due to commit > > 08ed4efad684 ("[PATCH v10 6/9] Reimplement RLIMIT_SIGPENDING on top of ucounts") > > Ouch. > > I *think* this test may be testing "send so many signals that it > triggers the signal queue overflow case". > > And I *think* that the performance degradation may be due to lots of > unnecessary allocations, because ity looks like that commit changes > __sigqueue_alloc() to do > > struct sigqueue *q = kmem_cache_alloc(sigqueue_cachep, flags); > > *before* checking the signal limit, and then if the signal limit was > exceeded, it will just be free'd instead. > > The old code would check the signal count against RLIMIT_SIGPENDING > *first*, and if there were m ore pending signals then it wouldn't do > anything at all (including not incrementing that expensive atomic > count). > > Also, the old code was very careful to only do the "get_user()" for > the *first* signal it added to the queue, and do the "put_user()" for > when removing the last signal. Exactly because those atomics are very > expensive. > > The new code just does a lot of these atomics unconditionally. Yes and right now I'm trying to rewrite this patch. > I dunno. The profile data in there is a bit hard to read, but there's > a lot more cachee misses, and a *lot* of node crossers: > > > 5961544 +190.4% 17314361 perf-stat.i.cache-misses > > 22107466 +119.2% 48457656 perf-stat.i.cache-references > > 163292 ą 3% +4582.0% 7645410 perf-stat.i.node-load-misses > > 227388 ą 2% +3708.8% 8660824 perf-stat.i.node-loads > > and (probably as a result) average instruction costs have gone up enormously: > > > 3.47 +66.8% 5.79 perf-stat.overall.cpi > > 22849 -65.6% 7866 perf-stat.overall.cycles-between-cache-misses > > and it does seem to be at least partly about "put_ucounts()": > > > 0.00 +4.5 4.46 perf-profile.calltrace.cycles-pp.put_ucounts.__sigqueue_free.get_signal.arch_do_signal_or_restart.exit_to_user_mode_prepare > > and a lot of "get_ucounts()". > > But it may also be that the new "get sigpending" is just *so* much > more expensive than it used to be. Thanks for decrypting this! I spent some time to understand this report and still wasn't sure I understood it. -- Rgrds, legion