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Wed, 17 Jun 2026 17:42:37 +0000 (GMT) Received: from li-7bb28a4c-2dab-11b2-a85c-887b5c60d769.ibm.com.com (unknown [9.39.28.77]) by smtpav06.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 17 Jun 2026 17:42:37 +0000 (GMT) From: Shrikanth Hegde To: linux-kernel@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, yury.norov@gmail.com, kprateek.nayak@amd.com, iii@linux.ibm.com Cc: sshegde@linux.ibm.com, tglx@kernel.org, gregkh@linuxfoundation.org, pbonzini@redhat.com, seanjc@google.com, vschneid@redhat.com, huschle@linux.ibm.com, rostedt@goodmis.org, dietmar.eggemann@arm.com, mgorman@suse.de, bsegall@google.com, maddy@linux.ibm.com, srikar@linux.ibm.com, hdanton@sina.com, chleroy@kernel.org, vineeth@bitbyteword.org, frederic@kernel.org, arighi@nvidia.com, pauld@redhat.com, christian.loehle@arm.com, tj@kernel.org, tommaso.cucinotta@gmail.com, maz@kernel.org, rafael@kernel.org Subject: [PATCH v4 02/20] sched/docs: Document cpu_preferred_mask and Preferred CPU concept Date: Wed, 17 Jun 2026 23:11:21 +0530 Message-ID: <20260617174139.155540-3-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260617174139.155540-1-sshegde@linux.ibm.com> References: <20260617174139.155540-1-sshegde@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-GUID: qXPhXE4HbZZI4helQRqGhQXQeimX7gl8 X-Authority-Analysis: v=2.4 cv=Le0MLDfi c=1 sm=1 tr=0 ts=6a32dc99 cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=FelO9ux0wxsA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=VnNF1IyMAAAA:8 a=f0vDFc08jm7PY2BpABoA:9 X-Proofpoint-Spam-Info: AW1haW4tMjYwNjE3MDE2NiBTYWx0ZWRfX2ZqoJO59vKNU UnOovXlwRr+Or12mjqM1r8ohJvamkwGrIosnJq6gh0RhEMQyzMqa6YnsJFUuOG1WpVtgX1Wh7k2 djAynu1KDXLw+8fWAfWYZH/G+rguvMo= X-Proofpoint-ORIG-GUID: WgZDIEmfCKXVirVWoSl7tShFD3kMA4v3 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNjE3MDE2NiBTYWx0ZWRfX0I4nW359NQJv DFckvIX6n5COjRl3ZtnI2e39SkGwVdtHilH7cn0HFk2YnI1zeZtklq5i5IYjV5NLc7MfPNhVW/l ZzpVV+AjL5uQLZU6ktEG5y6yjX0buQKiYgB3WUqx9noKlfBLFE5bOOS+5SUFdErM0YRA35bGFE2 NrSpW4X+PWml8mhHSahR5RllutiCi4WSsg8MJZ4ThwJH2cZ+flK8ZO+L6pTCJNKe70uMSfQp9Xi 4GfR1TxfPLNBI1BMVGZTgS0pmoRSwXJP+nZmw39VPOwRS4nRi9Mwu8e09F2+uRlG22N9D29Oook UxeEhcq7/9J3zi84Le98f0eeCqn8SjMkg55AG53FZEw4/Q06LgceppYgmoML4sRBHjHVkRAE9wv xFUbLRBesMzdbqD7xP5/HyXj7R51+8RnrEb4pcZJgFnFI2kFHrxh4Vw6hX2KGnmJrZps67HJn9/ tq1fwKSx7lNsOn+Tzpw== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.125,FMLib:17.12.100.49 definitions=2026-06-17_02,2026-06-17_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 priorityscore=1501 malwarescore=0 spamscore=0 suspectscore=0 impostorscore=0 phishscore=0 lowpriorityscore=0 bulkscore=0 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2606170166 Add documentation for new cpumask called cpu_preferred_mask. This could help users in understanding what this mask is and the concept behind it. Document how to enable it and implementation aspects of it. Signed-off-by: Shrikanth Hegde --- v3->v4: - update docs to reflect preferred is subset of active. Documentation/scheduler/sched-arch.rst | 49 ++++++++++++++++++++++++++ 1 file changed, 49 insertions(+) diff --git a/Documentation/scheduler/sched-arch.rst b/Documentation/scheduler/sched-arch.rst index ed07efea7d02..f17c54f44421 100644 --- a/Documentation/scheduler/sched-arch.rst +++ b/Documentation/scheduler/sched-arch.rst @@ -62,6 +62,55 @@ Your cpu_idle routines need to obey the following rules: arch/x86/kernel/process.c has examples of both polling and sleeping idle functions. +Preferred CPUs +============== + +In virtualised environments it is possible to overcommit CPU resources. +i.e sum of virtual CPU(vCPU) of all VM's is greater than number of physical +CPUs(pCPU). Under such conditions when all or many VM's have high utilization, +hypervisor won't be able to satisfy the CPU requirement and has to context +switch within or across VM. i.e hypervisor need to preempt one vCPU to run +another. This is called vCPU preemption. This is more expensive compared to +task context switch within a vCPU. + +In such cases it is better that combined vCPU ask from all VM is reduced +by not using some of the vCPUs. vCPUs where workload can be safely +scheduled which won't increase any contention for pCPU are called as +"Preferred CPUs". + +In most cases preferred CPUs will be same as active CPUs, when there is pCPU +contention, Preferred CPUs will reduce based on the amount of steal time. +When the pCPU contention goes away as indicated by steal time, Preferred CPUs +will become same as active CPUs again. One has to enable the feature by +writing 1 to /sys/kernel/debug/sched/steal_monitor/enable + +One of the design construct is preferred CPUs is always subset of active CPUs. +With CONFIG_PREFERRED_CPU=n, it is same as active CPUs. + +For scheduling decisions such as wakeup, pushing the task etc, needs this +CPU state info. This is maintained in cpu_preferred_mask. + +vCPUs which are not in cpu_preferred_mask should be treated as vCPUs which +should not be used at this moment provided it doesn't break user affinity. +This is achieved by +1. Selecting a preferred CPU at wakeup. +2. Push the task away from non-preferred CPU at tick. +3. Only select preferred CPUs for load balance. + +/sys/devices/system/cpu/preferred prints the current cpu_preferred_mask in +cpulist format. + +Notes: +1. This feature is available under CONFIG_PREFERRED_CPU +2. This feature works for FAIR class only. +3. A task pinned, which can't be moved to preferred CPUs will continue + to run based on its affinity. But no load balancing happens +4. If needed, steal time based governors/arch dependent method + could be used to cater to different types of cpu numbers. + Arch can do so by implementing its own hooks. +5. Decision to use/not use is driven by kernel. Hence it shouldn't + break user affinities. One of the main reason why CPU hotplug + or Isolated cpuset partitions was not a solution. Possible arch/ problems ======================= -- 2.47.3