From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from fra-out-012.esa.eu-central-1.outbound.mail-perimeter.amazon.com (fra-out-012.esa.eu-central-1.outbound.mail-perimeter.amazon.com [52.57.120.243]) (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 D8DAED27E for ; Tue, 4 Nov 2025 21:05:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=52.57.120.243 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762290337; cv=none; b=l+xeFgoDojhDx7rh3m8tPRwrYiI8bUGkz6XU2LlIKVygmsP5LQN7tz5U6j3RgbCMdj5zDIw5ZoblmULDtr6mfBtd6OZknXWBf1CF8aixK8hRqVLoYYgcu8JYViuRo7ZVKZponbOf9sMf2pEccj2EePmXVq/fx7dG0amJ0HYhiO0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762290337; c=relaxed/simple; bh=GH18lwwYtCKq866HjM9UdFupw9yNx9rOTZY1n71DgQ4=; h=From:To:CC:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=TaiN7WV/O2/W7GHqyuakAh8sCoEbo6VFdjeWubNbRosQYIBHQX+WxUrGZAy9eMCHfv5qpUGWSkWfkb8dqUE0zGSMEt8JL0lDy1BnEy8pI+gXJzd/hRL/zBhIpn+QuKAcEO5xBgyIDkzWfGkLLWvOsmxV7DlsnK7ZHfqZs+Y/dHc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.com; spf=pass smtp.mailfrom=amazon.com; dkim=pass (2048-bit key) header.d=amazon.com header.i=@amazon.com header.b=k5G6h3Yq; arc=none smtp.client-ip=52.57.120.243 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=amazon.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=amazon.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=amazon.com header.i=@amazon.com header.b="k5G6h3Yq" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazoncorp2; t=1762290336; x=1793826336; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=GBVtyKVdeWeCdJHd/Z3Byw+JnZ+6jxftwA94Z83YXP8=; b=k5G6h3YqxqH7oMm7h42Fcx9bu90JUdwChEk/RXjkLPoGmvMVw4fBjnGX 6UPVOlAFXld9LVwCc/90vXPqPxFDu0kuPVBQkHAaJv0nbX707e6rJwKKe 9MkxseUmhQyjeAt3hFEfDUYorf236QaU7c/fObNJiOHDAVsmCr9ZAxdiH e9lrA02a47y3bFF8sSJ+f27GvJHQ/ovOX7fq21SxnhBumRqLjn9i7saRT +fbZJfBduuWO0+aPAoRJHRvFx0NoqR2aEpd6TSIkn3lnK0GEclOxEz3xI UBE7sZTU0aAGgtv5JEFymc8jMMK7SKRpHWpdSjl1abcue03bDgSRkg5jX Q==; X-CSE-ConnectionGUID: 33JROTSMR7u3ZKHb+J2ADA== X-CSE-MsgGUID: uldWNatgQoCVaTWxi/4Hiw== X-IronPort-AV: E=Sophos;i="6.19,280,1754956800"; d="scan'208";a="4578549" Received: from ip-10-6-11-83.eu-central-1.compute.internal (HELO smtpout.naws.eu-central-1.prod.farcaster.email.amazon.dev) ([10.6.11.83]) by internal-fra-out-012.esa.eu-central-1.outbound.mail-perimeter.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Nov 2025 21:05:18 +0000 Received: from EX19MTAEUA001.ant.amazon.com [54.240.197.233:21123] by smtpin.naws.eu-central-1.prod.farcaster.email.amazon.dev [10.0.26.203:2525] with esmtp (Farcaster) id a34df2cf-195e-4b65-8eaa-ce12a1e249ce; Tue, 4 Nov 2025 21:05:18 +0000 (UTC) X-Farcaster-Flow-ID: a34df2cf-195e-4b65-8eaa-ce12a1e249ce Received: from EX19D003EUB001.ant.amazon.com (10.252.51.97) by EX19MTAEUA001.ant.amazon.com (10.252.50.50) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.29; Tue, 4 Nov 2025 21:05:17 +0000 Received: from u5934974a1cdd59.ant.amazon.com (10.146.13.221) by EX19D003EUB001.ant.amazon.com (10.252.51.97) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.29; Tue, 4 Nov 2025 21:05:13 +0000 From: Fernand Sieber To: kernel test robot CC: , , , , Peter Zijlstra , , Subject: Re: [tip:sched/core] [sched/fair] 79104becf4: BUG:kernel_NULL_pointer_dereference,address Date: Tue, 4 Nov 2025 23:04:55 +0200 Message-ID: <20251104210456.652800-1-sieberf@amazon.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <202510211205.1e0f5223-lkp@intel.com> References: <202510211205.1e0f5223-lkp@intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ClientProxiedBy: EX19D045UWA004.ant.amazon.com (10.13.139.91) To EX19D003EUB001.ant.amazon.com (10.252.51.97) Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Hi Peter, I spent some time today investigating this report. The crash happens when a proxy task yields. Since it probably doesn't make sense that a task blocking the best pick yields, a simple workaround is to ignore the yield in this case: --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8993,6 +8993,11 @@ static void yield_task_fair(struct rq *rq) if (unlikely(rq->nr_running == 1)) return; + /* Don't yield if we're running a proxy task */ + if (rq->donor && rq->donor != curr) { + return; + } + However, more generally, I am not sure that the logic in update_min_vruntime() is sound when we are running a proxy task, which I suspect is the ultimate root cause of the problem. It seems to assume that cfs_rq->curr is the running task, which is not the case. In my troubleshooting I have seen inconsistent calculations with underflows of cfs_rq->avg_vruntime and avg_vruntime(cfs_rq) being lower than min_vruntime. I'll see if I can invest more time diving into this, in the meantime do you have any thoughts? Thanks, --Fernand Amazon Development Centre (South Africa) (Proprietary) Limited 29 Gogosoa Street, Observatory, Cape Town, Western Cape, 7925, South Africa Registration Number: 2004 / 034463 / 07