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Fri, 1 Nov 2024 21:01:20 +0000 Message-ID: Date: Fri, 1 Nov 2024 16:01:16 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v3 7/7] docs: networking: Describe irq suspension To: Joe Damato , , , , , , , , , , , , , , , Martin Karsten , Bagas Sanjaya , "David S. Miller" , Simon Horman , "Jonathan Corbet" , "open list:DOCUMENTATION" , open list , "open list:BPF [MISC] :Keyword:(?:b|_)bpf(?:b|_)" References: <20241101004846.32532-1-jdamato@fastly.com> <20241101004846.32532-8-jdamato@fastly.com> Content-Language: en-US From: "Samudrala, Sridhar" In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BY5PR17CA0037.namprd17.prod.outlook.com (2603:10b6:a03:167::14) To PH0PR11MB4886.namprd11.prod.outlook.com (2603:10b6:510:33::22) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH0PR11MB4886:EE_|BN9PR11MB5289:EE_ X-MS-Office365-Filtering-Correlation-Id: 8a88c3ef-aea3-4271-b7bb-08dcfab85611 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014|921020; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?K1RNVHE3OG8xaDliU0EwUkVUbnlnMjQ1dER6S1l5K0oyby9CYzVkNWlta0t3?= =?utf-8?B?YXhrLzJESjdxSEFPTSs4cFNuVjNYTHd3K1JZOUNQamNyM2xkOHhLN2grRVpP?= =?utf-8?B?cDJHQU5kb29pRFhnVkdHemxTTU5MM2xSVGh6OHIyaHNvbkFwN1JjREZMMVhw?= =?utf-8?B?K3dPeTVvREw1KzlydE93NUJGdS9LQ2djYW83TWEzWGZjUDRXTmROMkFoSk1R?= =?utf-8?B?eDE1WmErVURCUFpCa0ZzOGs5T1hLeml2TmhhSTJUY2NIK21NcHdQNzQ0Wll0?= =?utf-8?B?OUcySDdpQi9TbkJLeGZHYlZ3OElrYUFBVGMvVG5CUXNLeHdCcGZpUVZ2Ukkz?= =?utf-8?B?RjdjTUIzaVZ4UVVEbDhnNHBnUlJiTXZPbHVqRzBWM0lZR0lMTlFDWEJvSWlK?= =?utf-8?B?NkpEMUpIZEdxcHJIbWNyaVZSVGRZaW1GWmxKTytwYXpQOExSeXJlNjYyT3Fk?= =?utf-8?B?TFlmaXNUV3BFOGlhQmpaODdDODI3REdZYzErSk1zMytXdFlicHNmVFQ4OG5j?= =?utf-8?B?MEdRKzNDdG9rcDNkTUc1NWsrSDFCTkQybkQyYWNFYkJDY2RNZmE0Z3NDemFR?= =?utf-8?B?NWZUQzJKOEt2TTVqYjVoMnRVRzFtQ0szWjZWdWt4eWpVU09SZ3RObndsTzdZ?= =?utf-8?B?Qkx0NFMyK0dEbjV2U05OR3YyVklmYkhseDlzZUlZSWxLeVlWSzJWT1gwNVc5?= =?utf-8?B?SzAzT1JEZWNWQXFzRjNJYmUxeGUwSFlhOEVyUjFCSmxMZXhpalhoVU5lb3Y1?= =?utf-8?B?eC9aTlNPdytSQVBGMlJzVXlDd05GRFBVYnAwR2h6RHhmOU9oZGtvNEJRR2Iv?= =?utf-8?B?NnkrMDNaSEtiMW43MUVEdWQzWk1aN1pxekpueG9CeXNudHhOb1BsL2dwYmha?= =?utf-8?B?V2JYRTQzODBBRWtlSHA5dkF1d25RMEFuZ21UR0h2WXE2SnJvRXIwQkcraC9T?= =?utf-8?B?T2UyN1ZtOHBMMUpCaXB4K1VobGRHMno4eGhWWmhHd01maE81elFFTWw2UVdY?= =?utf-8?B?V1V4cEd4emhMNDA4ZEhENHZBQ1dKaDVUUmFJaG1BTDRoREVvc1FkdzdoK3J5?= =?utf-8?B?ZVVzci9EV2VSRWludDhtTzBacVNVekF2b3M3Z2ludGZaaTdIeVJwS3ZHU0ZN?= =?utf-8?B?Q092TEdUSlRpN052Z09DZE81TmdvMDBESVUzWmpoOWtpNDJVbUdHVWdTUWpv?= =?utf-8?B?NFlZN1RuT3J4QTZzOW13c0VaRWRPYzNxbmlQcDdSOWN4ejNrWDhGeFltTGxY?= =?utf-8?B?YmhYYW5oQ3YxQ29pNFF3OFFrOHBvRmYwVVBocGxWbWJxZk1ONkZvam9IY3Qy?= =?utf-8?B?U1RnN2tiY25OcXI0clArRTl2enY1SFY2d3dTUTF6bWtNcWwwbUxrOW55dzV4?= =?utf-8?B?YllXaFRJN1Iwbi9WV0o5TXFnajRuNE84eWgrMmxPNXZGc3I2TDdwZDZ6Y1Vw?= =?utf-8?B?VzBRVmxYYmlKVFozMzlNd2RyVm90cGpJRXViOWVNenVJd1RSMm9JUmcwOFJB?= =?utf-8?B?UUdYanlhYWhaSFR6ZXI1M3BWRGZTK0RENk1pNE12NUdzNjRheEZGdzhxcUNX?= =?utf-8?B?U3VIYnFybVRNQnJkUXJhT2Q4TkZLZ3lPN3pJTEFZSzZsNTNTZnN6UGJNcmpI?= =?utf-8?B?UjZUWk00dGFYaGt5UTNRaVQ4N2EyNmVEVG82NmgzdXZTeG1EMVU3aXRCeUdO?= =?utf-8?B?YlhjTEkxQzJMVkIxMDZkdWJudHdRS1R5ZUJobGdvSTBkajVXZlhWblVncS9G?= =?utf-8?B?RE1UTUQ1MGs1bCtGZmRCeS9mS2R5MDdpYmFFSU8xc2h6eG82aE04a1RITEhT?= =?utf-8?B?aDZucUZJNEpiMVo3UWZXeVRFVHhvUHNOL1dIVUk4dlk3TW82b2RSQmVzOFIy?= =?utf-8?B?OVVyS29EaFA0ZjkyRVI2QU5vWjVmbVBuOU9iRXQvd0hBSk8rVUZXY293MWNR?= =?utf-8?B?ejlnQmM2Y0ZPSXZHNFRiQXQyYTlCdFpDeVZKc21qMzVENVBIZWg3aEVJZ0pQ?= =?utf-8?B?Yit4eUwxWVU2aTJQd0VLQzVmTHpJUmR2Znp1YVJ0RFZSVWJpSWRKMjlwYjg4?= =?utf-8?B?QVhwQThvaXdHcmwyWlY2QkZlOUFPenJnTkg0Q3lhTnFLV3dwZmhBWXBKcUlq?= =?utf-8?B?NDlRejJJVzhIbVVvNVVKTEpvSXo1V000SHd1ejB2Q3ZBVW10eHRjR3dDVzZD?= =?utf-8?B?Nnc9PQ==?= X-MS-Exchange-CrossTenant-Network-Message-Id: 8a88c3ef-aea3-4271-b7bb-08dcfab85611 X-MS-Exchange-CrossTenant-AuthSource: PH0PR11MB4886.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Nov 2024 21:01:20.6593 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: tm6KyfMyAABEuJEcMqE93Zp+qBRbg0Ml0bM2hjj/hQzO9wbs6ziphnt+mX5cyOHgHGmL5QNSnPC7lFLKhybC81vKhdaq0w02ZM0ksgy4fus= X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN9PR11MB5289 X-OriginatorOrg: intel.com On 10/31/2024 11:39 PM, Joe Damato wrote: > On Thu, Oct 31, 2024 at 10:47:05PM -0500, Samudrala, Sridhar wrote: >> >> >> On 10/31/2024 7:48 PM, Joe Damato wrote: >>> Describe irq suspension, the epoll ioctls, and the tradeoffs of using >>> different gro_flush_timeout values. >>> >>> Signed-off-by: Joe Damato >>> Co-developed-by: Martin Karsten >>> Signed-off-by: Martin Karsten >>> Acked-by: Stanislav Fomichev >>> Reviewed-by: Bagas Sanjaya >>> --- >> >> >> >>> + >>> +IRQ suspension >>> +-------------- >>> + >>> +IRQ suspension is a mechanism wherein device IRQs are masked while epoll >>> +triggers NAPI packet processing. >>> + >>> +While application calls to epoll_wait successfully retrieve events, the kernel will >>> +defer the IRQ suspension timer. If the kernel does not retrieve any events >>> +while busy polling (for example, because network traffic levels subsided), IRQ >>> +suspension is disabled and the IRQ mitigation strategies described above are >>> +engaged. >>> + >>> +This allows users to balance CPU consumption with network processing >>> +efficiency. >>> + >>> +To use this mechanism: >>> + >>> + 1. The per-NAPI config parameter ``irq_suspend_timeout`` should be set to the >>> + maximum time (in nanoseconds) the application can have its IRQs >>> + suspended. This is done using netlink, as described above. This timeout >>> + serves as a safety mechanism to restart IRQ driver interrupt processing if >>> + the application has stalled. This value should be chosen so that it covers >>> + the amount of time the user application needs to process data from its >>> + call to epoll_wait, noting that applications can control how much data >>> + they retrieve by setting ``max_events`` when calling epoll_wait. >>> + >>> + 2. The sysfs parameter or per-NAPI config parameters ``gro_flush_timeout`` >>> + and ``napi_defer_hard_irqs`` can be set to low values. They will be used >>> + to defer IRQs after busy poll has found no data. >> >> Is it required to set gro_flush_timeout and napi_defer_hard_irqs when >> irq_suspend_timeout is set? Doesn't it override any smaller >> gro_flush_timeout value? > > It is not required to use gro_flush_timeout or napi_defer_hard_irqs, > but if they are set they will take over when epoll finds no events. > Their usage is recommended. See the Usage section of the cover > letter for details. > > While gro_flush_timeout and napi_defer_hard_irqs are not strictly > required, it is difficult for the polling-based packet delivery loop > to gain control over packet delivery. > > Please see a previous email about this from the RFC for more > details: > > https://lore.kernel.org/netdev/2bb121dd-3dcd-4142-ab87-02ccf4afd469@uwaterloo.ca/ OK. Thanks for the clarification. > > In the cover letter, you can note the difference in performance when > gro_flush_timeout is set to different values. Note the explanation > of suspendX; each suspend case is testing a different > gro_flush_timeout. May be you can also include a test scenario in your perf results where gro_flush_timeout and napi_defer_hard_irqs are not set to show that a non-zero value of gro_flush_timeout and napi_defer_hard_irqs is recommended when using irq_suspend_timeout. > > Let us know if you have any other questions; both Martin and I are > happy to help or further explain anything that is not clear. >