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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 87DE1C55ABD for ; Mon, 16 Nov 2020 09:52:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3766F20738 for ; Mon, 16 Nov 2020 09:52:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728432AbgKPJLA (ORCPT ); Mon, 16 Nov 2020 04:11:00 -0500 Received: from outbound-smtp08.blacknight.com ([46.22.139.13]:50697 "EHLO outbound-smtp08.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727789AbgKPJK7 (ORCPT ); Mon, 16 Nov 2020 04:10:59 -0500 Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp08.blacknight.com (Postfix) with ESMTPS id 923C71C43BF for ; Mon, 16 Nov 2020 09:10:57 +0000 (GMT) Received: (qmail 12950 invoked from network); 16 Nov 2020 09:10:57 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 16 Nov 2020 09:10:56 -0000 Date: Mon, 16 Nov 2020 09:10:54 +0000 From: Mel Gorman To: Peter Zijlstra , Will Deacon Cc: Davidlohr Bueso , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Loadavg accounting error on arm64 Message-ID: <20201116091054.GL3371@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, I got cc'd internal bug report filed against a 5.8 and 5.9 kernel that loadavg was "exploding" on arch64 on a machines acting as a build servers. It happened on at least two different arm64 variants. That setup is complex to replicate but fortunately can be reproduced by running hackbench-process-pipes while heavily overcomitting a machine with 96 logical CPUs and then checking if loadavg drops afterwards. With an MMTests clone, I reproduced it as follows ./run-mmtests.sh --config configs/config-workload-hackbench-process-pipes --no-monitor testrun; \ for i in `seq 1 60`; do cat /proc/loadavg; sleep 60; done Load should drop to 10 after about 10 minutes and it does on x86-64 but remained at around 200+ on arm64. The reproduction case simply hammers the case where a task can be descheduling while also being woken by another task at the same time. It takes a long time to run but it makes the problem very obvious. The expectation is that after hackbench has been running and saturating the machine for a long time. Commit dbfb089d360b ("sched: Fix loadavg accounting race") fixed a loadavg accounting race in the generic case. Later it was documented why the ordering of when p->sched_contributes_to_load is read/updated relative to p->on_cpu. This is critical when a task is descheduling at the same time it is being activated on another CPU. While the load/stores happen under the RQ lock, the RQ lock on its own does not give any guarantees on the task state. Over the weekend I convinced myself that it must be because the implementation of smp_load_acquire and smp_store_release do not appear to implement acquire/release semantics because I didn't find something arm64 that was playing with p->state behind the schedulers back (I could have missed it if it was in an assembly portion as I can't reliablyh read arm assembler). Similarly, it's not clear why the arm64 implementation does not call smp_acquire__after_ctrl_dep in the smp_load_acquire implementation. Even when it was introduced, the arm64 implementation differed significantly from the arm implementation in terms of what barriers it used for non-obvious reasons. Unfortunately, making that work similar to the arch-independent version did not help but it's not helped that I know nothing about the arm64 memory model. I'll be looking again today to see can I find a mistake in the ordering for how sched_contributes_to_load is handled but again, the lack of knowledge on the arm64 memory model means I'm a bit stuck and a second set of eyes would be nice :( -- Mel Gorman SUSE Labs 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=-5.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 E34AFC4742C for ; Mon, 16 Nov 2020 09:11:33 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 74E7B206B7 for ; Mon, 16 Nov 2020 09:11:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="w8IslP9x" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 74E7B206B7 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=techsingularity.net Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:MIME-Version:Message-ID:Subject:To:From:Date: Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender :Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:References:List-Owner; bh=M5z+ImV9dB38VUB9Y9fxrejaMuXb0OckZhbpXKbz7k8=; b=w8IslP9x+R6lvz+axijI4f0ybg G7iaYkSJYolPZKkd3y9sBLoqn0HXthQLohajVo/I5VXK9YIFG2sBnk9wwJFDPSgI52qKd69W5O/sz gS8i5KFS8HzW9M8fr4a9POKCb1gTX+r+lrmywiuv8ID3YjLxBYTq44dr5Q6l3L9ZGc6eMLz5GLjdO WsZwgQyCq2juR1Eb9IrNxetInLMLqtlJ8O7d7QmzvUOZ9n+3LOn8joq8fcX66hWHFsla6FbUPAPd0 9Nu3DsB4ZgA40OMGB2S8HfzNyVK8eSMVrVtKWSnx6f+qdXpi5eL+OiUaNhBZBFr6jqMpvnD1PUrOL DhaR0HWA==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1keaXV-00018z-6I; Mon, 16 Nov 2020 09:11:09 +0000 Received: from outbound-smtp29.blacknight.com ([81.17.249.32]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1keaXP-00017i-9j for linux-arm-kernel@lists.infradead.org; Mon, 16 Nov 2020 09:11:07 +0000 Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp29.blacknight.com (Postfix) with ESMTPS id 7B91ABEDA5 for ; Mon, 16 Nov 2020 09:10:57 +0000 (GMT) Received: (qmail 12950 invoked from network); 16 Nov 2020 09:10:57 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.22.4]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 16 Nov 2020 09:10:56 -0000 Date: Mon, 16 Nov 2020 09:10:54 +0000 From: Mel Gorman To: Peter Zijlstra , Will Deacon Subject: Loadavg accounting error on arm64 Message-ID: <20201116091054.GL3371@techsingularity.net> MIME-Version: 1.0 Content-Disposition: inline User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201116_041103_587184_1519822E X-CRM114-Status: GOOD ( 15.98 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Davidlohr Bueso , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi, I got cc'd internal bug report filed against a 5.8 and 5.9 kernel that loadavg was "exploding" on arch64 on a machines acting as a build servers. It happened on at least two different arm64 variants. That setup is complex to replicate but fortunately can be reproduced by running hackbench-process-pipes while heavily overcomitting a machine with 96 logical CPUs and then checking if loadavg drops afterwards. With an MMTests clone, I reproduced it as follows ./run-mmtests.sh --config configs/config-workload-hackbench-process-pipes --no-monitor testrun; \ for i in `seq 1 60`; do cat /proc/loadavg; sleep 60; done Load should drop to 10 after about 10 minutes and it does on x86-64 but remained at around 200+ on arm64. The reproduction case simply hammers the case where a task can be descheduling while also being woken by another task at the same time. It takes a long time to run but it makes the problem very obvious. The expectation is that after hackbench has been running and saturating the machine for a long time. Commit dbfb089d360b ("sched: Fix loadavg accounting race") fixed a loadavg accounting race in the generic case. Later it was documented why the ordering of when p->sched_contributes_to_load is read/updated relative to p->on_cpu. This is critical when a task is descheduling at the same time it is being activated on another CPU. While the load/stores happen under the RQ lock, the RQ lock on its own does not give any guarantees on the task state. Over the weekend I convinced myself that it must be because the implementation of smp_load_acquire and smp_store_release do not appear to implement acquire/release semantics because I didn't find something arm64 that was playing with p->state behind the schedulers back (I could have missed it if it was in an assembly portion as I can't reliablyh read arm assembler). Similarly, it's not clear why the arm64 implementation does not call smp_acquire__after_ctrl_dep in the smp_load_acquire implementation. Even when it was introduced, the arm64 implementation differed significantly from the arm implementation in terms of what barriers it used for non-obvious reasons. Unfortunately, making that work similar to the arch-independent version did not help but it's not helped that I know nothing about the arm64 memory model. I'll be looking again today to see can I find a mistake in the ordering for how sched_contributes_to_load is handled but again, the lack of knowledge on the arm64 memory model means I'm a bit stuck and a second set of eyes would be nice :( -- Mel Gorman SUSE Labs _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel