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client-ip=165.204.84.17; helo=satlexmb08.amd.com; pr=C Received: from satlexmb08.amd.com (165.204.84.17) by SJ5PEPF000001ED.mail.protection.outlook.com (10.167.242.201) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9587.10 via Frontend Transport; Wed, 4 Feb 2026 07:48:42 +0000 Received: from satlexmb07.amd.com (10.181.42.216) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.17; Wed, 4 Feb 2026 01:48:42 -0600 Received: from [10.136.38.118] (10.180.168.240) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server id 15.2.2562.17 via Frontend Transport; Tue, 3 Feb 2026 23:48:39 -0800 Message-ID: <257cb7f6-2261-4f04-8a2a-23d6f4adb06c@amd.com> Date: Wed, 4 Feb 2026 13:18:39 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv2] sched/fair: Skip SCHED_IDLE rq for SCHED_IDLE task To: Christian Loehle , , , , CC: , , References: <20260203184939.2138022-1-christian.loehle@arm.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20260203184939.2138022-1-christian.loehle@arm.com> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: SrQePgpgUBT5vUE46zwJt7sTYmGGgSgBfrG+HD7O9sfzJteTi6FqlDS1B+4gW0iqvbg4EacZ6nJo3zmHgqRMGTXXbNX+JBou3fadV4D0exZUkUsbTOimFWbHVq5bMH72YK1AhwM9MV11HWFAYTaGIWGdoqTxJwjOTWsQDdMAhLbe+1jqJpdQIdFEK4R5ydj0TkKlWY/YlJkvFbwUi+P9SSUgk3aANVkzvkm+BYg0q2U2wJPtGiXBi8DvzuwD3z4GwRwHIvtjzg2SnEqECBBimm+W5ZE/75uzoIlovQp4YZsXATqSzmV+dKl9X+MwyfCiFuE0ghQsohGbjB7mANoynHyVAavuBrqUV2NQ5mTxnLxlw8lv9zenfhGGmNj/OhYfHRRjCHW0aAU6/ZbOPCdYfmko3ue85sf6dgF5+hux0yxdoZdSBsNopiyt3OXMxRXS X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 04 Feb 2026 07:48:42.7591 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 9d4e33ae-84f7-4918-48a6-08de63c1d18b X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb08.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ5PEPF000001ED.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW4PR12MB7383 Hello Chris, On 2/4/2026 12:19 AM, Christian Loehle wrote: > CPUs whose rq only have SCHED_IDLE tasks running are considered to be > equivalent to truly idle CPUs during wakeup path. For fork and exec > SCHED_IDLE is even preferred. > This is based on the assumption that the SCHED_IDLE CPU is not in an > idle state and might be in a higher P-state, allowing the task/wakee > to run immediately without sharing the rq. > > However this assumption doesn't hold if the wakee has SCHED_IDLE policy > itself, as it will share the rq with existing SCHED_IDLE tasks. In this > case, we are better off continuing to look for a truly idle CPU. > > On a Intel Xeon 2-socket with 64 logical cores in total this yields > for kernel compilation using SCHED_IDLE: > > +---------+----------------------+----------------------+--------+ > | workers | mainline (seconds) | patch (seconds) | delta% | > +=========+======================+======================+========+ > | 1 | 4384.728 ± 21.085 | 3843.250 ± 16.235 | -12.35 | > | 2 | 2242.513 ± 2.099 | 1971.696 ± 2.842 | -12.08 | > | 4 | 1199.324 ± 1.823 | 1033.744 ± 1.803 | -13.81 | > | 8 | 649.083 ± 1.959 | 559.123 ± 4.301 | -13.86 | > | 16 | 370.425 ± 0.915 | 325.906 ± 4.623 | -12.02 | > | 32 | 234.651 ± 2.255 | 217.266 ± 0.253 | -7.41 | > | 64 | 202.286 ± 1.452 | 197.977 ± 2.275 | -2.13 | > | 128 | 217.092 ± 1.687 | 212.164 ± 1.138 | -2.27 | > +---------+----------------------+----------------------+--------+ I couldn't spot much difference for kernel compilation on my 3rd Generation EPYC system likely due to smaller LLC size. For sched-messaging, I found the following interesting trend when running with SCHED_IDLE: (Normalized runtime [Var%]; %diff - higher the better) tip/sched:core +patch (%diff) 1-group 1.00 [5.00%] 0.88 [10.78%] 11.80% 2-group 1.00 [5.15%] 0.93 [26.06%] 6.99% 4-group 1.00 [5.48%] 0.89 [11.03%] 11.13% 8-group 1.00 [6.62%] 1.21 [12.37%] -21.30% 16-group 1.00 [9.46%] 1.28 [ 9.42%] -27.59% There is a good improvement for lower utilization. Once the system is trending towards overutilized but SIS_UTIL cut-off is still non-zero, we search a little bit longer for a fully idle CPU when the probability for finding one is actually low. I suppose that scenario is rare where we only have SCHED_IDLE tasks that care about throughput on a busy system to actually notice this but it was worth pointing out. Feel free to include: Tested-by: K Prateek Nayak > > Signed-off-by: Christian Loehle -- Thanks and Regards, Prateek