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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 net 4/7] net/sched: taprio: get corrected value of cycle_time and interval Message-ID: <20231109111146.qrnekz6ykyzrcpbd@skbuf> References: <20231107112023.676016-1-faizal.abdul.rahim@linux.intel.com> <20231107112023.676016-1-faizal.abdul.rahim@linux.intel.com> <20231107112023.676016-5-faizal.abdul.rahim@linux.intel.com> <20231107112023.676016-5-faizal.abdul.rahim@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20231107112023.676016-5-faizal.abdul.rahim@linux.intel.com> <20231107112023.676016-5-faizal.abdul.rahim@linux.intel.com> X-ClientProxiedBy: VI1PR08CA0246.eurprd08.prod.outlook.com (2603:10a6:803:dc::19) To AM0PR04MB6452.eurprd04.prod.outlook.com (2603:10a6:208:16d::21) 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: AM0PR04MB6452:EE_|AM8PR04MB7395:EE_ X-MS-Office365-Filtering-Correlation-Id: bdcdb38a-5bee-488b-9ff5-08dbe114ab85 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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This field is required due > to specific flow like find_entry_to_transmit() -> get_interval_end_time() > which retrieves the interval for each entry. During positive cycle > time correction, it's known that the last entry interval requires > correction. However, for negative correction, the affected entry > is unknown, which is why this new field is necessary. I agree with the motivation, but I'm not sure if the chosen solution is correct. static u32 get_interval(const struct sched_entry *entry, const struct sched_gate_list *oper) { if (entry->correction_active) return entry->interval + oper->cycle_time_correction; return entry->interval; } What if the schedule looks like this: sched-entry S 0x01 125000000 sched-entry S 0x02 125000000 sched-entry S 0x04 125000000 sched-entry S 0x08 125000000 sched-entry S 0x10 125000000 sched-entry S 0x20 125000000 sched-entry S 0x40 125000000 sched-entry S 0x80 125000000 and the calculated cycle_time_correction is -200000000? That would eliminate the entire last sched-entry (0x80), and the previous one (0x40) would run for just 75000000 ns. But your calculation would say that its interval is −75000000 ns (actually reported as an u32 positive integer, so it would be a completely bogus value). So not only is the affected entry unknown, but also the amount of cycle time correction that applies to it is unknown. I'm looking at where we need get_interval(), and it's from: taprio_enqueue_one() -> is_valid_interval() -> find_entry_to_transmit() -> get_interval_end_time() -> get_packet_txtime() -> find_entry_to_transmit() I admit it's a part of taprio which I don't understand too well. Why do we perform such complex calculations in get_interval_end_time() when we should have struct sched_entry :: end_time precomputed and available for this purpose (although it was primarily inteded for advance_sched() and not for enqueue())? Vinicius, do you know?