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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 DC004C43219 for ; Wed, 24 Feb 2021 14:26:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A768164F03 for ; Wed, 24 Feb 2021 14:26:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237156AbhBXOY7 (ORCPT ); Wed, 24 Feb 2021 09:24:59 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39966 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234649AbhBXNS1 (ORCPT ); Wed, 24 Feb 2021 08:18:27 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9DE92C0617A7 for ; Wed, 24 Feb 2021 05:16:34 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=ROZDWadhTAJM+fwXVa9G4naTgOaLGoDbTKwN4LtxPhM=; b=X2Qou+gRQpUSm6XG/E7p582kaZ ARA62R3qfN1iaOtkxVCHstSEc2gh20xO720CJ37bbbLlLCjMhbrz5KCSbpsmGxDiyt1HtVMIuKeTW kFsS0R1/8Z5Tpu/SBXjudQnWSxlvJ9QIIMQlPd+PpctzCz899Bdfq0aLK+g+2WEC5idUTuXTx0mDZ Jq9rWwEwEVhyP8+Fko7/nbF0U6OuQ5nTkjzfISlQpJ+/1R7fx5vv3or5/BCaBQG/LTxhbG+4v9kzw jvlALFjsJ0xv1nWqa1Se3V5IOOBV7TC2k8AD0LCpgz6FkYvqxYRHZxF8y5Nko0dQPab+yIvWXCC/W Fm7dmeaw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lEu12-009Qti-Be; Wed, 24 Feb 2021 13:15:48 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2093630477A; Wed, 24 Feb 2021 14:15:42 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 0) id CE9B6203BF925; Wed, 24 Feb 2021 14:15:42 +0100 (CET) Message-ID: <20210224131355.357743989@infradead.org> User-Agent: quilt/0.66 Date: Wed, 24 Feb 2021 13:24:40 +0100 From: Peter Zijlstra To: Ingo Molnar , Thomas Gleixner Cc: Valentin Schneider , Vincent Guittot , Mel Gorman , Dietmar Eggemann , linux-kernel@vger.kernel.org, peterz@infradead.org, Andi Kleen Subject: [PATCH 1/6] sched: Fix migration_cpu_stop() requeueing References: <20210224122439.176543586@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When affine_move_task(p) is called on a running task @p, which is not otherwise already changing affinity, we'll first set p->migration_pending and then do: stop_one_cpu(cpu_of_rq(rq), migration_cpu_stop, &arg); This then gets us to migration_cpu_stop() running on the CPU that was previously running our victim task @p. If we find that our task is no longer on that runqueue (this can happen because of a concurrent migration due to load-balance etc.), then we'll end up at the: } else if (dest_cpu < 1 || pending) { branch. Which we'll take because we set pending earlier. Here we first check if the task @p has already satisfied the affinity constraints, if so we bail early [A]. Otherwise we'll reissue migration_cpu_stop() onto the CPU that is now hosting our task @p: stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop, &pending->arg, &pending->stop_work); Except, we've never initialized pending->arg, which will be all 0s. This then results in running migration_cpu_stop() on the next CPU with arg->p == NULL, which gives the by now obvious result of fireworks. The cure is to change affine_move_task() to always use pending->arg, furthermore we can use the exact same pattern as the SCA_MIGRATE_ENABLE case, since we'll block on the pending->done completion anyway, no point in adding yet another completion in stop_one_cpu(). This then gives a clear distinction between the two migration_cpu_stop() use cases: - sched_exec() / migrate_task_to() : arg->pending == NULL - affine_move_task() : arg->pending != NULL; And we can have it ignore p->migration_pending when !arg->pending. Any stop work from sched_exec() / migrate_task_to() is in addition to stop works from affine_move_task(), which will be sufficient to issue the completion. Fixes: 6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()") Signed-off-by: Peter Zijlstra (Intel) --- kernel/sched/core.c | 39 ++++++++++++++++++++++++++++----------- 1 file changed, 28 insertions(+), 11 deletions(-) --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1922,6 +1922,24 @@ static int migration_cpu_stop(void *data rq_lock(rq, &rf); pending = p->migration_pending; + if (pending && !arg->pending) { + /* + * This happens from sched_exec() and migrate_task_to(), + * neither of them care about pending and just want a task to + * maybe move about. + * + * Even if there is a pending, we can ignore it, since + * affine_move_task() will have it's own stop_work's in flight + * which will manage the completion. + * + * Notably, pending doesn't need to match arg->pending. This can + * happen when tripple concurrent affine_move_task() first sets + * pending, then clears pending and eventually sets another + * pending. + */ + pending = NULL; + } + /* * If task_rq(p) != rq, it cannot be migrated here, because we're * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because @@ -2194,10 +2212,6 @@ static int affine_move_task(struct rq *r int dest_cpu, unsigned int flags) { struct set_affinity_pending my_pending = { }, *pending = NULL; - struct migration_arg arg = { - .task = p, - .dest_cpu = dest_cpu, - }; bool complete = false; /* Can the task run on the task's current CPU? If so, we're done */ @@ -2235,6 +2249,12 @@ static int affine_move_task(struct rq *r /* Install the request */ refcount_set(&my_pending.refs, 1); init_completion(&my_pending.done); + my_pending.arg = (struct migration_arg) { + .task = p, + .dest_cpu = -1, /* any */ + .pending = &my_pending, + }; + p->migration_pending = &my_pending; } else { pending = p->migration_pending; @@ -2265,12 +2285,6 @@ static int affine_move_task(struct rq *r p->migration_flags &= ~MDF_PUSH; task_rq_unlock(rq, p, rf); - pending->arg = (struct migration_arg) { - .task = p, - .dest_cpu = -1, - .pending = pending, - }; - stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop, &pending->arg, &pending->stop_work); @@ -2283,8 +2297,11 @@ static int affine_move_task(struct rq *r * is_migration_disabled(p) checks to the stopper, which will * run on the same CPU as said p. */ + refcount_inc(&pending->refs); /* pending->{arg,stop_work} */ task_rq_unlock(rq, p, rf); - stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg); + + stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop, + &pending->arg, &pending->stop_work); } else {