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Message-ID: <268f8c2c-57ba-e4e3-59ef-45ed3cd794de@suse.com> Date: Thu, 29 Sep 2022 17:55:50 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.3.0 Subject: Re: [RFC PATCH v1 0/2] Add a new acquire resource to query vcpu statistics Content-Language: en-US To: Matias Ezequiel Vara Larsen Cc: Matias Ezequiel Vara Larsen , Andrew Cooper , Wei Liu , George Dunlap , Julien Grall , Stefano Stabellini , Dario Faggioli , Anthony PERARD , xen-devel@lists.xenproject.org References: From: Jan Beulich In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR3P281CA0176.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:a0::11) To VE1PR04MB6560.eurprd04.prod.outlook.com (2603:10a6:803:122::25) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VE1PR04MB6560:EE_|DB9PR04MB8394:EE_ X-MS-Office365-Filtering-Correlation-Id: 76855dc8-229c-4f54-fbc1-08daa23315c5 X-LD-Processed: f7a17af6-1c5c-4a36-aa8b-f5be247aa4ba,ExtFwd X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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The current mechanism to get those > statistics is by querying the hypervisor. This mechanism relies on a hypercall > and holds the domctl spinlock during its execution. When a pv tool like xcp-rrdd > periodically samples these counters, it ends up affecting other paths that share > that spinlock. By using acquire resources, the pv tool only requires a few > hypercalls to set the shared memory region and samples are got without issuing > any other hypercall. The original idea has been suggested by Andrew Cooper to > which I have been discussing about how to implement the current PoC. You can > find the RFC patch series at [1]. The series is rebased on top of stable-4.15. > > I am currently a bit blocked on 1) what to expose and 2) how to expose it. For > 1), I decided to expose what xcp-rrdd is querying, e.g., XEN_DOMCTL_getvcpuinfo. > More precisely, xcp-rrd gets runstate.time[RUNSTATE_running]. This is a uint64_t > counter. However, the time spent in other states may be interesting too. > Regarding 2), I am not sure if simply using an array of uint64_t is enough or if > a different interface should be exposed. The remaining question is when to get > new values. For the moment, I am updating this counter during > vcpu_runstate_change(). > > The current series includes a simple pv tool that shows how this new interface is > used. This tool maps the counter and periodically samples it. > > Any feedback/help would be appreciated. Before looking more closely - was there perhaps kind-of-review feedback during the summit, which would make it more reasonable to look through this once a v2 has appeared? Jan