From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-119.freemail.mail.aliyun.com (out30-119.freemail.mail.aliyun.com [115.124.30.119]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 41CAA274659 for ; Wed, 22 Oct 2025 09:00:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.119 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761123651; cv=none; b=VgfoSl1uwdQC/ataPBpHBKb1ed0SK+d8PU+/Ya4J9NjabGrz3wg2Qa4CmqIgbOr1pA0elB6iMqJtMWBXNFEEVSxwnD9iten8RmIKH7jqeN3u3a25VMN4ADqHQm39ufxHkR8LOEKw1jbQNoh4wY88TlvDJgAVVD6vmdniIln+VQQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761123651; c=relaxed/simple; bh=xM0mRvnCnxrq8qrBayle8Vn8kADKts0PdydU+a9IdEE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kX0z0O7SomWTt50flSDIJONwuLrGiFIrgsXdTKDf5WVouXasZGwgVi6MBiuviHWq4oGORXHNnsDuY8RJyv4r/NRbgu6HG5eFk1apnRUeFPTKpz1PW8PdYD+Y75jgRSCQEIa0BeS6U5KhWVxG0FZ0rMKnfD/P9gALvfvVSCLJQQY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=ptYYMjmX; arc=none smtp.client-ip=115.124.30.119 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="ptYYMjmX" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1761123640; h=Date:From:To:Subject:Message-ID:MIME-Version:Content-Type; bh=dvsliT5+X3hg8ePEtHv+3tm4bcT3hp4UnhYiD7EpFVE=; b=ptYYMjmXPKgBxtiRR2FcLDpyqH9vrXHcgtbt1X7WQX6o7Il+lK8Ihxw19T9TZKyQjDrdAfLG6RbYanSruc2QGXOPNzIwQ3d8uV+2ZF/84aywpv1gTSW5ffjyeiI2WhA416fp1N/Cpf9yf2s3OKInlVa2YIEGljvf/Nzb+vW0EKA= Received: from localhost(mailfrom:peng_wang@linux.alibaba.com fp:SMTPD_---0WqmHuQH_1761123638 cluster:ay36) by smtp.aliyun-inc.com; Wed, 22 Oct 2025 17:00:39 +0800 Date: Wed, 22 Oct 2025 17:00:38 +0800 From: Peng Wang To: Vincent Guittot Cc: Peter Zijlstra , mingo@redhat.com, juri.lelli@redhat.com, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, vdavydov.dev@gmail.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched/fair: Clear ->h_load_next after hierarchical load Message-ID: <20251022090038.GA88368@j38d01266.eu95sqa> Reply-To: Peng Wang References: <20251015124422.GD3419281@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=gb2312 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) On Wed, Oct 15, 2025 at 03:14:37PM +0200, Vincent Guittot wrote: > On Wed, 15 Oct 2025 at 14:44, Peter Zijlstra wrote: > > > > On Wed, Oct 15, 2025 at 08:19:50PM +0800, Peng Wang wrote: > > > > > We found that the task_group corresponding to the problematic se > > > is not in the parent task_group¡¯s children list, indicating that > > > h_load_next points to an invalid address. Consider the following > > > cgroup and task hierarchy: > > > > > > A > > > / \ > > > / \ > > > B E > > > / \ | > > > / \ t2 > > > C D > > > | | > > > t0 t1 > > > > > > Here follows a timing sequence that may be responsible for triggering > > > the problem: > > > > > > CPU X CPU Y CPU Z > > > wakeup t0 > > > set list A->B->C > > > traverse A->B->C > > > t0 exits > > > destroy C > > > wakeup t2 > > > set list A->E wakeup t1 > > > set list A->B->D > > > traverse A->B->C > > > panic > > > > > > CPU Z sets ->h_load_next list to A->B->D, but due to arm64 weaker memory > > > ordering, Y may observe A->B before it sees B->D, then in this time window, > > > it can traverse A->B->C and reach an invalid se. > > > > Hmm, I rather think we should ensure update_cfs_rq_h_load() is > > serialized against unregister_fair_sched_group(). > > The bug has been reported for v5.10 which probably don't have fixed > done "recently" > commit b027789e5e50 ("sched/fair: Prevent dead task groups from > regaining cfs_rq's") Hi, Vincent and Peter, We have already integrated this commit, but the bug persists. Do you think we should explicitly clear the h_load_next list? Even though update_cfs_rq_h_load runs under an RCU lock, ARM's weak memory ordering could still allow readers to observe stale values in the list. > > > > > And I'm thinking that really shouldn't be hard; note how > > sched_unregister_group() already has an RCU grace period. So all we need > > to ensure is that task_h_load() is called in a context that stops RCU > > grace periods (rcu_read_lock(), preempt_disable(), local_irq_disable(), > > local_bh_disable()). > > > > A very quick scan makes me think at the very least the usage in > > > > task_numa_migrate() > > task_numa_find_cpu() > > task_h_load() > > > > fails here; probably more.