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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?OMHHEmM0Uv/3xNOa/EO933wFx0Xq89j2L2RTpyl3ElDW8uo8Hj8mZF4bghj+?= =?us-ascii?Q?1/ASl+4aJQP562/vjHsnj4W0gVWkXuvaw1m9ngkbO7ZVlBBPOT1bBFkJRYV+?= =?us-ascii?Q?QBuXPQy+XPHpxQRkqbCBqEjG0WFRCfcomuVh7DG7yk8YCaUNIostmDVIl+8B?= =?us-ascii?Q?k67Plo33iqRoKnBlGgfOGbhL6yY8ighHooyZMBv82SB4KKefUVVoaWmrSNMt?= =?us-ascii?Q?1lNdwnC8ugBxPm7xdnUqoFyBMblon7HEYvSJJWUNuLvKx1xnta2QgblxHSZY?= =?us-ascii?Q?E0JdS7bvq2A/vTwhZuNHH7KQUgWdD6dIKAukIFM7Hw7hgPBlHldQtVXmuho4?= =?us-ascii?Q?8XEdiCjddhz9KCAC8jykW88eNqg0L3UOV61kDOivz4jhMdmO/tDSvLujtiUR?= =?us-ascii?Q?aJQltg2fEfO8a71tGxQM6zg9A/ZYEDdlmos69md6xA0/QwOe5u2hwWgEzdCn?= =?us-ascii?Q?T4CyXpgN2Y6JTVxxXgf1QUrhDytTbGYZ2+/02ZkzU/xSkYYvZfmS3e2sxwgK?= =?us-ascii?Q?UlGWh3QTL/C4F1Tzx32Wb72xIweir9YxTF5P0zIQiNE9B1/zD7rKoXKKExDO?= =?us-ascii?Q?1KkjdzDdNogsgqiMCySWJBa5RNIBEaxV40bH+MxBX2J6/bqMjCinGEOit++O?= =?us-ascii?Q?D0tR8213Ve6k+0ko42/1bmUlvVA89tVpzPvk646eha25gIAVcsL/XF9MmF23?= =?us-ascii?Q?esG73XohGuY/hvfrsbRLp68GjhfgQpCHQLWifpn8p3T7PjZcV25i1teSUaBx?= =?us-ascii?Q?smAs8FJ0OAYNYF3Wp8B37PRwseos42I3ptSpveX93fq4X5KqQHRtUlN2s4rB?= =?us-ascii?Q?TnC1b7vHVdwRHN9roEVgURhsm5AZt7RuilGeLPklGm2Miu8OxGHQcZhQk3Fb?= =?us-ascii?Q?9ZR0yHtpan1q/V8yPpJOqVLnVow+qbbsYeQEGTMN9Pql5OGX1JpWyYp0GNXC?= =?us-ascii?Q?CyO2jgnMBKOCZBbH5iwhTXfAmxqtc41s+/fp90c08ujltVNQ+H6yM+/kdKd+?= =?us-ascii?Q?5kezH5VFz8tgJQFdCyF0hKHnAv6kKgVpoVUkt2R+bMcaeqmjQTaH6bta8FVG?= =?us-ascii?Q?ObLdoUMbTSy4aGY70rHctXYZDFwgkCgwlO98w20e2/BzLU/ZvpOXOQ1a788Y?= =?us-ascii?Q?G8iVgmB/+dhMEyU5vMatW54htIkW2TFpv7xUPIz0RdrzF0QHfrCGijanrrry?= =?us-ascii?Q?T1vBzXGb/zOSLAGw4HObH0JwDYTYOlMNBYu2Ccr89gTuti5tYXQgxwvJqy/3?= =?us-ascii?Q?L/uvQ8blLlejL6UJuWlfMC9ELKOHzF2Opyvpj0b9mskoEjTy627o1CD8LJIM?= =?us-ascii?Q?n53J451Ji9B8fFXtFjFaAlycagfEcp8gQi46e9ql0ePBmQZ+T2Y2IC2rV2Fg?= =?us-ascii?Q?9bF35V5bHrbpC4Vvn+H5ZPSfpUPp2EbWM17AMUlLR+Rqvyrxi2q24N2qxj9k?= =?us-ascii?Q?+eGOVJ1imFWWGNY4PQ2noY3HTNcb0g1CABdblStxGLiodkwLDCRNfC1YgoJW?= =?us-ascii?Q?amJ0r1RA6ZIWTh91LtYF1QxG8RZgo6gB1J3dG1p0VmG3uQOq8+MFXUFYwlbM?= =?us-ascii?Q?UU1LomVPd/4o4U0/LDHL3hmT5xExEKs5oU6wqmuR?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 03733c6e-22e8-44a9-5278-08dd2145bf3c X-MS-Exchange-CrossTenant-AuthSource: CY5PR12MB6405.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Dec 2024 22:29:20.2776 (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: y9KeD7JOLl18SUTP5Baaqf6vvUzmbkBG3yXprEprToNktU80D/+137OInarJI6wIyIF9P7CWb1IGY7wUF/0YhQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR12MB7569 Hi Changwoo, On Fri, Dec 20, 2024 at 03:20:19PM +0900, Changwoo Min wrote: > Many BPF schedulers (such as scx_central, scx_lavd, scx_rusty, scx_bpfland, > and scx_flash) frequently call bpf_ktime_get_ns() for tracking tasks' runtime > properties. If supported, bpf_ktime_get_ns() eventually reads a hardware > timestamp counter (TSC). However, reading a hardware TSC is not > performant in some hardware platforms, degrading IPC. > > This patchset addresses the performance problem of reading hardware TSC > by leveraging the rq clock in the scheduler core, introducing a > scx_bpf_now_ns() function for BPF schedulers. Whenever the rq clock > is fresh and valid, scx_bpf_now_ns() provides the rq clock, which is > already updated by the scheduler core (update_rq_clock), so it can reduce > reading the hardware TSC. > > When the rq lock is released (rq_unpin_lock), the rq clock is invalidated, > so a subsequent scx_bpf_now_ns() call gets the fresh sched_clock for the caller. > > In addition, scx_bpf_now_ns() guarantees the clock is monotonically > non-decreasing for the same CPU, so the clock cannot go backward > in the same CPU. > > Using scx_bpf_now_ns() reduces the number of reading hardware TSC > by 50-80% (76% for scx_lavd, 82% for scx_bpfland, and 51% for scx_rusty) > for the following benchmark: > > perf bench -f simple sched messaging -t -g 20 -l 6000 I've tested this patch set and I haven't observed any significant performance improvements (but also no regressions), even if the systems I've tested are likely quite efficient at reading the hardware TSC. I'm curious if we'd see a more significant difference in non-hardware virtualized systems (i.e., qemu without kvm). Have you done any testing in such environments already? In any case: Tested-by: Andrea Righi -Andrea