From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from NAM11-BN8-obe.outbound.protection.outlook.com (mail-bn8nam11on2080.outbound.protection.outlook.com [40.107.236.80]) (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 1760C255E47 for ; Mon, 12 May 2025 19:08:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=fail smtp.client-ip=40.107.236.80 ARC-Seal:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747076882; cv=fail; b=HP3Ipb78l6XgO8NScS7CnqOoU28xHjT/tK/rphxE+MP0bS2r7LnAYURllLEvvpRn//nt6IUCKoLjYqAzgYogGMz69En0q66qhTFwXLhcKioDx7jC7GeZXoDxv8djrE5RoTHF6BM3L5Zxh5lBIZ4UY9uPEo/oDtWdYPlLChD4Tq4= ARC-Message-Signature:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1747076882; c=relaxed/simple; bh=f+fV1Nipa2LIu6o2hGksPPWAU0NmB9WPU5LH3bjCaIA=; h=Date:From:To:Cc:Subject:Message-ID:References:Content-Type: Content-Disposition:In-Reply-To:MIME-Version; b=Y6qMjtXdZhiFg8BuaTCwMm8vdxXZdmJ+La+FVysN7euAP3/BCs8lb5OZXJgc2MkCr1NKE2lMA96v5grTAMWrZ55gnAAI2n/AuDDsN2iO3eWj/qCwGcx89KISUKJblrQwlvhPSbH5+jyOLTzuTxyGUOCn6mY7CQhQ+fB5psn3D4M= ARC-Authentication-Results:i=2; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com; spf=fail smtp.mailfrom=nvidia.com; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b=gIFjZKk7; arc=fail smtp.client-ip=40.107.236.80 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=nvidia.com Authentication-Results: smtp.subspace.kernel.org; spf=fail smtp.mailfrom=nvidia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=Nvidia.com header.i=@Nvidia.com header.b="gIFjZKk7" ARC-Seal: i=1; a=rsa-sha256; s=arcselector10001; d=microsoft.com; cv=none; b=Q6+bH2yXTQ60j+s2tJ/2rYZHnadG/KNP1ajqSXMngV/izvqe82ApjwsoEnPJHY/ceuV1V31Wr7dg9/Uji9BtushpKm2WIggNNn/Oe8QrHh+bCQsLe5JQe9He0+y53F7r5nJVBVQVinXVepTspFXe+a3Sy8VOEYpWXLi3BXerOhtPMN9TDyvd0ibwSnPquOtRL5erVoJh9A/znIvcoxn5RRUqcf9Ddx4N/THdW8v3BPLWstLcE5+nXDWPQRWvcavAB6tuvq7tEi0eF9oT5olFEhD7Q4QvUEfnbsC5LetPIuviQXl4x3wsTYrD0pc+reG5780Sgbi7LCPtA1ugQKSreg== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com; s=arcselector10001; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-AntiSpam-MessageData-ChunkCount:X-MS-Exchange-AntiSpam-MessageData-0:X-MS-Exchange-AntiSpam-MessageData-1; bh=lqK6tFZOQudU/tWVocNHf98dUfYGG7phoMGAX/84DVY=; b=ueCW+v+BYqa0NLW8Zvazbcp5GvnQ2zPwrhsbOQFKOyV8BF7EB3Lj4+NYc8/5BLhwfL2OQlyAIjo7k+PAbb2/VJfMWwuAHxRanjkO/BeRjSDkDGXpJ9jvCEErQwkJlOk11bD462Qtj+Hz+ithsRvWPnh5CxHAW/p064Kj7C48HFJMO6Bokd9n3O5wk7W6mab6eZrWSIL26pi/7G7uOCrsEc7EwUb1I43w185pbZ1M8aq/9cJmWQap6niF0BVep2LuBEHe8uSQsZm1cB8+gjD7bEIEpd39N3RgJGyIshaXOYxfN9LdVcHnSr2F8lln8VAE0356M/KPwFgQ1YFc80o30Q== ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=nvidia.com; dmarc=pass action=none header.from=nvidia.com; dkim=pass header.d=nvidia.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=Nvidia.com; s=selector2; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=lqK6tFZOQudU/tWVocNHf98dUfYGG7phoMGAX/84DVY=; b=gIFjZKk71+bE8CpD3MPXYcY6nFOlj31ZhKj8j0ce8REL9mB3z4ykoRYQcSNqPUvF0TaD3bBeXqH1IS96YiVmEtnZ8qcMKRZ/U4lkcNlVJm2bjvy+q3/pYWBV/LdsAHC4wm08+f2ic7U/Rm2PK92Fa8KRHK8FizKRi5dshCdIRtf/v5hiAfX+NcR0w2M/opW38cRWkAg94+lfYAIrMr7dbKgyxmUn6drGUM8kq+ld+ux3bkCV54Dt5UBRV/BSGumdksu3IJIOj33y1nwdHHF7/K7XkEy/7UFgIM9YqtVvMSoi6mv7o2Ur/39K+ok98plOHNCnDTC74ka7zM/QlLne7g== Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from LV8PR12MB9620.namprd12.prod.outlook.com (2603:10b6:408:2a1::19) by MW5PR12MB5624.namprd12.prod.outlook.com (2603:10b6:303:19d::19) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8699.20; Mon, 12 May 2025 19:07:57 +0000 Received: from LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::1b59:c8a2:4c00:8a2c]) by LV8PR12MB9620.namprd12.prod.outlook.com ([fe80::1b59:c8a2:4c00:8a2c%7]) with mapi id 15.20.8722.027; Mon, 12 May 2025 19:07:57 +0000 Date: Mon, 12 May 2025 21:07:49 +0200 From: Andrea Righi To: David Vernet Cc: Tejun Heo , Changwoo Min , linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] sched_ext/idle: Make scx_bpf_select_cpu_and() usable from any context Message-ID: References: <20250512151743.42988-1-arighi@nvidia.com> <20250512151743.42988-3-arighi@nvidia.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MI2P293CA0002.ITAP293.PROD.OUTLOOK.COM (2603:10a6:290:45::17) To LV8PR12MB9620.namprd12.prod.outlook.com (2603:10b6:408:2a1::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LV8PR12MB9620:EE_|MW5PR12MB5624:EE_ X-MS-Office365-Filtering-Correlation-Id: fd5e021a-d3f4-4ac1-d153-08dd91884e43 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024; X-Microsoft-Antispam-Message-Info: =?us-ascii?Q?bn2sGJ77zmqcR7jxG9W6m9ziY5HWRfOHXZXi0JwA8KN1OUywfHlOjqxcT7Oo?= =?us-ascii?Q?sNJBdFsNBMRhSBZvmHR2nMJY3oQZgYfdrZVUKuOeF1NGvRuTLPWiaUc9jNJP?= =?us-ascii?Q?Ni/rltXW5VGai1YnJtpip5eZRYWtdnbb3kxmsvXflvqTc6s5hQJim4at3clu?= =?us-ascii?Q?A8GBC6E133gxCtie6N1hh9AZrqMg+KovUDYW5oQxzTYZMhzNaDE0ddC4QCQi?= =?us-ascii?Q?ONUM+MJv8QVbAv3VJ6Qu0zhS1w26WbKAvsT/sTJ9HTAmGqPGqjZOl+5gD9Pr?= =?us-ascii?Q?n1p2UutZ2uRtCjSz/hJ4rUhCOtaVm5CNCa6mhxXQJ+Poh6J8wR4TuP3o77DO?= =?us-ascii?Q?We7LtRhuX3YjNHV10B/I2AE8IkT8YWSnclO+gnKODW2KigiM273EX9pIZzhx?= =?us-ascii?Q?PtUF0k/2Go/Aive17SL3Tcf4nQF3suEqDEY4QDM37OGnnsfaxdaNo9AUF0wx?= =?us-ascii?Q?uH/NRCBTt41VyxhUp5345/kIOqJ8O+ZVRWzC84d5SslFIVwZwvaxEY3bvmQa?= =?us-ascii?Q?U4siOorEGeeXDn1nAgV0fS7spBIGqUdUt+lCyD6TqVVK64RKgTHK3Of1pAzL?= =?us-ascii?Q?LggvuE5Kt5EUvzsIV4JIZR5iTbaTGXyTU+33S3s1Peuc6x0eBksbwhGKtKf8?= =?us-ascii?Q?XJA7ycr9jS6oKENTWT7hYdMCE8PPXdqujkVoml45zzc5IZFFeiLD75K+M5Me?= =?us-ascii?Q?wpq7qTuCqa/5AikPOee0ySnDveozS0sR/eH0wPNXHrtQRbtYDdcg031Wpha4?= =?us-ascii?Q?1GeH1K53tAV7aEMRCmcOCq9sNP0f8H7wiQ+1shjwJhZ7bTmOo4zIKLTtc4JC?= =?us-ascii?Q?Mu4FzsxX0bnAs4g7iGn3EnJqIbLRQzFNmwmng4Qz+fAUhzxR89ouysri5nvc?= =?us-ascii?Q?MnFw7Sd6RybVj6ClFdCDNaEBWJ1xPQJB4Bc48LqZX2pb/dGwWNdpJ/EBwi9z?= =?us-ascii?Q?ViqXmd57iHMMgNYnPbTgWNvyPugE0DmTTJMeu4+tBzriEEWtlmyW57HNtAr9?= =?us-ascii?Q?WgG9pJv9nVRLwX1jyDwRgYDfSm7dJoSOVFUHTZ1QQg3TlG7EXqzjlP9oHQD2?= =?us-ascii?Q?6t+r965jKasoL4pqugjQgEmU9fnJSE+XGsXxF4cFV+DQglMMf6KNU9N+3ELV?= =?us-ascii?Q?nv36kad6gdXZFdxFZHcHTAv+XaBEXpeYz0c8UQsXtt0Ef0MlU+ZjsYHVw2q1?= =?us-ascii?Q?kB2a1e6lkqiQ8cWhMVKnaBptx5svgf/ZI5d5i1xER2TT743sk+oANObB+0/u?= =?us-ascii?Q?JDYPOV6lRwvn7MbtmbEFlND4LVCQQ60uQxp0J1DOwc1GF35CPbLHnxRZTt8y?= =?us-ascii?Q?MXLWpFw27rA+S+TfVMHPdnY61mkqxCcjCwkB2Vz0uc8+WrggkA8HhCrtKQ6h?= =?us-ascii?Q?nlhk85Mom9+1fo2qPXru9k/B+r+BRAPJ8W3GM6n2Qgp8bhJOAqDdwF8PL21u?= =?us-ascii?Q?TwVXCZ86V1U=3D?= X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:LV8PR12MB9620.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230040)(376014)(366016)(1800799024);DIR:OUT;SFP:1101; X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?evSLfQBRqc+fkT3B3j1RQdwhYMMmwcWl0n1mo7TUydzuYr2MCzAOyKita8hJ?= =?us-ascii?Q?vkqXZNiBPOUlgrjMurRmVD9MLKq62XlIZYfMI++jTQm10EkRIogGjDLii/vt?= =?us-ascii?Q?H9p16C9PwR19a8O9TstTQcmoH5giWU9iY0RjBRXbaVXmcpyL1HfdnNEDu/7C?= =?us-ascii?Q?g1LhAzjtRIiRx3C7iuQvTEVoxdHM64b/e0YrdRdrlQa47lRzXyMEFg0lo8jX?= =?us-ascii?Q?HlxbMCixJgz0QMqRw8U9ZJkuUd21JrbOwijSrHKZ/hAFBV27WHo6Uf7nWOYK?= =?us-ascii?Q?HCrjXO7GCrXsu7NgbJ+IHmegSX/MZodKMiuzZrDu+PCRymCT5wOdLzQTHIHY?= =?us-ascii?Q?Wd974aruTeD7NXC6kP+/xBG1QYOYg5z8fZ8mYmweNlWd00E922AHyr796HOl?= =?us-ascii?Q?dhYYOP7Ni8EYUwqL+UCPbbVmBYz6PNC3zlzL//9yJKGWFe7EmTn3+MBnnJsC?= =?us-ascii?Q?bIVFwC4a9TN76XHlu+RXmvjIuEdTJp5TrmJwYwlimi7mVbS98IfKKbWb+SPP?= =?us-ascii?Q?+n8h6ly4lqDZxXIm0sXT7VpB3xX0yjwnJ/0hDNiqEeg9UnSwWqzBd1oDEkzb?= =?us-ascii?Q?+BuBi3E2H6Pb5bbncO1YNPk44piq+4EN3q4dCANHz0KKsngAXGCB7q56l3Vy?= =?us-ascii?Q?X0ZTD+4zEcFU+r7NhqoSjRw6CfcanTPr/vdxQHBw0CYKCTW/mR1eUgikBbMV?= =?us-ascii?Q?N89fpKqszcP1FeWcOCKGtgL+mGkPrll3tQeWj9I4tHwX9ZeQaes67zQuGylH?= =?us-ascii?Q?krLRJpisuE2uaxQ0chtvUsBHoDx21ON41ksKDi9aWy28rse9LkSE4SK51dwG?= =?us-ascii?Q?avKSC+4fgomU2SG7ip2F/b6rRN6JVuiC6/9JB4E6g43QQKUKVJjLXk8WtSbZ?= =?us-ascii?Q?euA21R9uaTUouxPmOiry3hbLFaljrQfrhGQPH9bg/ChUTBM8CCMruCB6Nv0r?= =?us-ascii?Q?eNZafUSGy9+6wu7lva6ZwqfGFavGdH4lGuSm4GTf62N5F+WzJlBX+fsPBtzN?= =?us-ascii?Q?2k0od3kuGXZgrxpQ/gRSHWTKmruKwEFyZ0hMXQ1tukEWaV6fKqQUVeOXO0Bq?= =?us-ascii?Q?myJi/5e2s0noNKt478Xc3fnUs2rbSKdU9p8h8bhIiUJiQHpUQHveFjjmta8Z?= =?us-ascii?Q?U822uJZlwG9Rw8eKPcAfUQBTMuGCpUG9dJoKaloxLIU0QvLKkRYxYVsKXOSW?= =?us-ascii?Q?zPxdc+6vb4+x/374AkSevQWMSwvMX/lp3ju+Rz8i5ETp0xIFSfsu9k7DaC/K?= =?us-ascii?Q?f6tDPPH5CqnE2z129ZVvC9Cb+GrzG1B8Mbw+V+FZ8MmXNIfFnZ/A/igx8SfX?= =?us-ascii?Q?JsF4cj8h/E2hkDANduYOFBoM8q/rzqO/r5mJKr74RK8vtibHHfavhZ2Sjjky?= =?us-ascii?Q?9KyfM5/unAS2UAIpkVxl6Ywka8fkRrlCsUgyzzbJkYPXTLS/UcvdJXjqfoJa?= =?us-ascii?Q?Gu1SaJj7/Hg86LxQNZLN4Ak9csYZbiubH7iQkbZWQsxrHwH5h8Os5A+UiSC7?= =?us-ascii?Q?uR+4a1fVnmYL1b1G0q4D+9xeqTFFXv92Pcr06PGGMQ/+2dd/8AawrXySgLMv?= =?us-ascii?Q?7c9bgHH4Ps+9A3iiKsRNx/m6FkOJDj6/P1NM4N4F?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: fd5e021a-d3f4-4ac1-d153-08dd91884e43 X-MS-Exchange-CrossTenant-AuthSource: LV8PR12MB9620.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 12 May 2025 19:07:57.2066 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 0Day74nEwaXmqCepb9ldrZkj3vIqio1abZzREQa3mrKsQrzAqyKl4hh64eoy6IHpTXJn951BosHfL2uez1Y1SA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW5PR12MB5624 On Mon, May 12, 2025 at 08:29:26PM +0200, Andrea Righi wrote: ... > > > + /* > > > + * If called from an unlocked context, try to acquire > > > + * cpus_read_lock() to avoid races with CPU hotplug. > > > + */ > > > + if (scx_kf_allowed_if_unlocked()) > > > + if (!cpus_read_trylock()) > > > + return -EBUSY; > > > > Hmm, so this now assumes that this function can be called without the rq lock > > held. I'm not sure if that's safe, as scx_select_cpu_dfl() queries p->cpus_ptr, > > which is protected by the rq lock. Additionally, scx_bpf_select_cpu_and() > > checks p->nr_cpus_allowed which is protected sometimes by pi_lock, sometimes by > > the rq lock, etc depending on its state. > > Yeah, it makes locking quite tricky. Maybe we can acquire the rq lock if > called from an unlocked context, instead of cpu_read_lock(), but then we > still have to deal with pi_lock. Actually that might work, since pi_lock should never be held when scx_kf_allowed_if_unlocked() is true, so basically we can do: if (scx_kf_allowed_if_unlocked()) rq = task_rq_lock(p, &rf); ... if (scx_kf_allowed_if_unlocked()) task_rq_unlock(rq, p, &rf); Or at least it should cover all current use cases. The tricky one is scx_bpf_select_cpu_and() being called via BPF test_run from a user-space task (scx_rustland_core). If we had a way to clearly identify a test_run context, we could restrict this to BPF_PROG_TYPE_STRUCT_OPS and BPF_PROG_TYPE_TEST_RUN (but as far as I can tell, the latter doesn't exist). Thanks, -Andrea