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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,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 46A33C38BF9 for ; Mon, 24 Feb 2020 21:22:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 16C35222C2 for ; Mon, 24 Feb 2020 21:22:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727851AbgBXVWi (ORCPT ); Mon, 24 Feb 2020 16:22:38 -0500 Received: from out01.mta.xmission.com ([166.70.13.231]:46662 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727709AbgBXVWh (ORCPT ); Mon, 24 Feb 2020 16:22:37 -0500 Received: from in02.mta.xmission.com ([166.70.13.52]) by out01.mta.xmission.com with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1j6LBT-0007lJ-1K; Mon, 24 Feb 2020 14:22:35 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1j6LBR-0002HL-BN; Mon, 24 Feb 2020 14:22:34 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Linus Torvalds Cc: Feng Tang , Oleg Nesterov , Jiri Olsa , Peter Zijlstra , kernel test robot , Ingo Molnar , Vince Weaver , Jiri Olsa , Alexander Shishkin , Arnaldo Carvalho de Melo , Arnaldo Carvalho de Melo , "Naveen N. Rao" , Ravi Bangoria , Stephane Eranian , Thomas Gleixner , LKML , lkp@lists.01.org, andi.kleen@intel.com, "Huang\, Ying" References: <20200205123216.GO12867@shao2-debian> <20200205125804.GM14879@hirez.programming.kicks-ass.net> <20200221080325.GA67807@shbuild999.sh.intel.com> <20200221132048.GE652992@krava> <20200223141147.GA53531@shbuild999.sh.intel.com> <20200224003301.GA5061@shbuild999.sh.intel.com> <20200224021915.GC5061@shbuild999.sh.intel.com> Date: Mon, 24 Feb 2020 15:20:26 -0600 In-Reply-To: (Linus Torvalds's message of "Mon, 24 Feb 2020 12:47:14 -0800") Message-ID: <87a757znqd.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1j6LBR-0002HL-BN;;;mid=<87a757znqd.fsf@x220.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/SSyf6yZIP/k5uVMtCOs9svimc0568GaY= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Linus Torvalds writes: > [ Adding a few more people that tend to be involved in signal > handling. Just in case - even if they probably don't care ] > > On Mon, Feb 24, 2020 at 12:09 PM Linus Torvalds > wrote: >> >> TOTALLY UNTESTED patch attached. It may be completely buggy garbage, >> but it _looks_ trivial enough. > > I've tested it, and the profiles on the silly microbenchmark look much > nicer. Now it's just the sigpending update shows up, the refcount case > clearly still occasionally happens, but it's now in the noise. > > I made slight changes to the __sigqueue_alloc() case to generate > better code: since we now use that atomic_inc_return() anyway, we > might as well then use the value that is returned for the > RLIMIT_SIGPENDING check too, instead of reading it again. > > That might avoid another potential cacheline bounce, plus the > generated code just looks better. > > Updated (and now slightly tested!) patch attached. > > It would be interesting if this is noticeable on your benchmark > numbers. I didn't actually _time_ anything, I just looked at profiles. > > But my setup clearly isn't going to see the horrible contention case > anyway, so my timing numbers wouldn't be all that interesting. > > Linus I keep looking at your patch and wondering if there isn't a way to remove the uid refcount entirely on this path. Linus I might be wrong but I have this sense that your change will only help when signal delivery is backed up. I expect in the common case there won't be any pending signals outstanding for the user. Not that I see anything bad jumping out at me from your patch. Eric > kernel/signal.c | 23 ++++++++++++++--------- > 1 file changed, 14 insertions(+), 9 deletions(-) > > diff --git a/kernel/signal.c b/kernel/signal.c > index 9ad8dea93dbb..5b2396350dd1 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -413,27 +413,32 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi > { > struct sigqueue *q = NULL; > struct user_struct *user; > + int sigpending; > > /* > * Protect access to @t credentials. This can go away when all > * callers hold rcu read lock. > + * > + * NOTE! A pending signal will hold on to the user refcount, > + * and we get/put the refcount only when the sigpending count > + * changes from/to zero. > */ > rcu_read_lock(); > - user = get_uid(__task_cred(t)->user); > - atomic_inc(&user->sigpending); > + user = __task_cred(t)->user; > + sigpending = atomic_inc_return(&user->sigpending); > + if (sigpending == 1) > + get_uid(user); > rcu_read_unlock(); > > - if (override_rlimit || > - atomic_read(&user->sigpending) <= > - task_rlimit(t, RLIMIT_SIGPENDING)) { > + if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) { > q = kmem_cache_alloc(sigqueue_cachep, flags); > } else { > print_dropped_signal(sig); > } > > if (unlikely(q == NULL)) { > - atomic_dec(&user->sigpending); > - free_uid(user); > + if (atomic_dec_and_test(&user->sigpending)) > + free_uid(user); > } else { > INIT_LIST_HEAD(&q->list); > q->flags = 0; > @@ -447,8 +452,8 @@ static void __sigqueue_free(struct sigqueue *q) > { > if (q->flags & SIGQUEUE_PREALLOC) > return; > - atomic_dec(&q->user->sigpending); > - free_uid(q->user); > + if (atomic_dec_and_test(&q->user->sigpending)) > + free_uid(q->user); > kmem_cache_free(sigqueue_cachep, q); > } >