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 83577C10DCE for ; Sun, 8 Mar 2020 18:07:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 697E620637 for ; Sun, 8 Mar 2020 18:07:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726373AbgCHSHj (ORCPT ); Sun, 8 Mar 2020 14:07:39 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:56891 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726296AbgCHSHi (ORCPT ); Sun, 8 Mar 2020 14:07:38 -0400 Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jB0Kc-00082h-PX; Sun, 08 Mar 2020 19:07:19 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id DBBB11040A5; Sun, 8 Mar 2020 19:07:17 +0100 (CET) From: Thomas Gleixner To: Linus Torvalds , kernel test robot Cc: Peter Zijlstra , Jann Horn , LKML , x86-ml , Ingo Molnar , Borislav Petkov , lkp@lists.01.org Subject: Re: [futex] 8019ad13ef: will-it-scale.per_process_ops -97.8% regression In-Reply-To: References: <20200308140200.GO5972@shao2-debian> Date: Sun, 08 Mar 2020 19:07:17 +0100 Message-ID: <87h7yy90ve.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Linus Torvalds writes: > [ Just a re-send without html crud that makes all the lists unhappy. > I'm still on the road, the flight I was supposed to be on yesterday > got cancelled.. ] > > I do note that the futex hashing seems to be broken by that commit. Or > at least it's questionable. It keeps hashing on "both.word", and > doesn't use the u64 field at all for hashing. > > Maybe I'm mis-reading it - I didn't apply the patch, I just looked at > the patch and my source base separately. > > But the 98% regression sure says something went wrong ;) Right you are. The pointer needs to be the starting point as it moved ahead of word, which means it starts at word and hashes word and offset and an extra u32 beyond the end of the key. Thanks, tglx ---- diff --git a/kernel/futex.c b/kernel/futex.c index e14f7cd45dbd..9f3251349f65 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -385,8 +385,8 @@ static inline int hb_waiters_pending(struct futex_hash_bucket *hb) */ static struct futex_hash_bucket *hash_futex(union futex_key *key) { - u32 hash = jhash2((u32*)&key->both.word, - (sizeof(key->both.word)+sizeof(key->both.ptr))/4, + u32 hash = jhash2((u32*)&key->both.ptr, + (sizeof(key->both.ptr) + sizeof(key->both.word)) / 4, key->both.offset); return &futex_queues[hash & (futex_hashsize - 1)]; }