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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BACEBC433EF for ; Thu, 14 Jul 2022 15:01:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239834AbiGNPB1 (ORCPT ); Thu, 14 Jul 2022 11:01:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39450 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239817AbiGNPB0 (ORCPT ); Thu, 14 Jul 2022 11:01:26 -0400 Received: from fanzine2.igalia.com (fanzine.igalia.com [178.60.130.6]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A2EF45E33B; Thu, 14 Jul 2022 08:01:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=igalia.com; s=20170329; h=Content-Transfer-Encoding:Content-Type:In-Reply-To:From: References:Cc:To:Subject:MIME-Version:Date:Message-ID:Sender:Reply-To: Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender: Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help:List-Unsubscribe: List-Subscribe:List-Post:List-Owner:List-Archive; bh=SD+E46jleghc1zhKhR9PveNf5aZVNxzYDkkNEuLw6xs=; b=jPDf1sXvRlPt3kdJgvEDxX2uey IoOgOI2HYUqGtfmOWtAn/eLsTaubeRmEpDtlThTCaZPwh0qo4P1axzKtPseySrD5Z6TKVVf6Gx+J8 gwHyY7HD7pLZ1C4EqAV8nJIwcWGj3NENks9j2m0vCDOYMwuT0t/E8/6AY7TNYUuCW6X3CRo4Li2Mg xHCkAj6UAHswTSl4wssGSfxEJ09ev0/AyK87B61JTyjy5th087ygs0og9jduneGmc4yOxzyUrwNim vtVDigEUVgARQqcOg7FQ/RTTbMkFszxNB9lNYZHadrXPaLU5tdUk2FYt8K8vRzOMUWBB0z3KTOu63 yl6BYIoA==; Received: from [177.139.47.106] (helo=[192.168.15.109]) by fanzine2.igalia.com with esmtpsa (Cipher TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_128_GCM:128) (Exim) id 1oC0L8-00GV8X-3S; Thu, 14 Jul 2022 17:01:18 +0200 Message-ID: <8bfd13a7-ed02-00dd-63a1-7144f2e55ef0@igalia.com> Date: Thu, 14 Jul 2022 12:00:59 -0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [RFC] futex2: add NUMA awareness Content-Language: en-US To: Andrey Semashev Cc: linux-api@vger.kernel.org, fweimer@redhat.com, linux-kernel@vger.kernel.org, Darren Hart , Peter Zijlstra , Ingo Molnar , Thomas Gleixner , libc-alpha@sourceware.org, Davidlohr Bueso , Steven Rostedt , Sebastian Andrzej Siewior References: <36a8f60a-69b2-4586-434e-29820a64cd88@igalia.com> <74ba5239-27b0-299e-717c-595680cd52f9@gmail.com> From: =?UTF-8?Q?Andr=c3=a9_Almeida?= In-Reply-To: <74ba5239-27b0-299e-717c-595680cd52f9@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Andrey, Thanks for the feedback. Às 08:01 de 14/07/22, Andrey Semashev escreveu: > On 7/14/22 06:18, André Almeida wrote: [...] >> >> Feedback? Who else should I CC? > > Just a few questions: > > Do I understand correctly that notifiers won't be able to wake up > waiters unless they know on which node they are waiting? > If userspace is using NUMA_FLAG, yes. Otherwise all futexes would be located in the default node, and userspace doesn't need to know which one is the default. > Is it possible to wait on a futex on different nodes? Yes, given that you specify `.hint = id` with the proper node id. > > Is it possible to wake waiters on a futex on all nodes? When a single > (or N, where N is not "all") waiter is woken, which node is selected? Is > there a rotation of nodes, so that nodes are not skewed in terms of > notified waiters? Regardless of which node the waiter process is running, what matter is in which node the futex hash table is. So for instance if we have: struct futex32_numa f = {.value = 0, hint = 2}; And now we add some waiters for this futex: Thread 1, running on node 3: futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL); Thread 2, running on node 0: futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL); Thread 3, running on node 2: futex_wait(&f, 0, FUTEX_NUMA | FUTEX_32, NULL); And then, Thread 4, running on node 3: futex_wake(&f, 2, FUTEX_NUMA | FUTEX_32); Now, two waiter would wake up (e.g. T1 and T3, node 3 and 2) and they are from different nodes. futex_wake() doesn't provide guarantees of which waiter will be selected, so I can't say which node would be selected. There's no policy for fairness/starvation for futex_wake(). Do you think this would be important for the NUMA case? Let me know if this clarifies your questions.