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Tue, 7 Apr 2026 19:20:16 +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, tglx@linutronix.de, yury.norov@gmail.com, gregkh@linuxfoundation.org Cc: sshegde@linux.ibm.com, pbonzini@redhat.com, seanjc@google.com, kprateek.nayak@amd.com, vschneid@redhat.com, iii@linux.ibm.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, joelagnelf@nvidia.com Subject: [PATCH v2 02/17] sched/docs: Document cpu_preferred_mask and Preferred CPU concept Date: Wed, 8 Apr 2026 00:49:35 +0530 Message-ID: <20260407191950.643549-3-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260407191950.643549-1-sshegde@linux.ibm.com> References: <20260407191950.643549-1-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-Spam-Details-Enc: AW1haW4tMjYwNDA3MDE3NSBTYWx0ZWRfX4xz6TKqyrEvE N2bdRTCNYeDiL1bLKKpZwiYkggtUbXPZUxaH7JAjckgLNdu1SikJe8v3stz0VLrgr7kneA2VR/A BgzzYa/jzwWQo68htvwYov1FnvWg1gh6wJttkJutDXwEMVkabr057Aicw7P5r++y4Rk4zzbM/oy IVNXmBD70R7T/nMUhEtq44SErs5Ow1IsRfDuBVree4QXxkQCllGUSaqonvsnqS7ejj0zXNiPc3j CjFvoUBgQ3kI19wKd6TsQQyaKROdwfj3HIw1Ummo4Y5puMTPGIDN83MakPSMqPK4yM4te8toGgH fA54tpGvYu/wcjN2B6bv3RoV++LlPft6f/7hQWM6UeXiyHotnK27cWigyDV58Le5LCt9r+dgWxz vrXQ5ns4HXl0yCcMMG8xr90UPchw9nlaCBArtWxl2IobLjX/5d9bhUUXhSdbtVRWhJpwaeMa85v 3mZYHDTVbAm7AIvnjzA== X-Proofpoint-GUID: V9BR2Nks4Oq-9jXon8MjsiD9fh2EfXyK X-Authority-Analysis: v=2.4 cv=a/wAM0SF c=1 sm=1 tr=0 ts=69d558fb cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=A5OVakUREuEA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VnNF1IyMAAAA:8 a=jh6Gn3DwPfN3ZNOXjfUA:9 X-Proofpoint-ORIG-GUID: 78gY9cAf-4FN6Vu7-MtKlkJMTWl6-4G_ X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-04-07_04,2026-04-07_05,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 clxscore=1015 spamscore=0 impostorscore=0 priorityscore=1501 phishscore=0 lowpriorityscore=0 adultscore=0 malwarescore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2604010000 definitions=main-2604070175 Add documentation for new cpumask called cpu_preferred_mask. This could help users in understanding what this mask and the concept behind it. Signed-off-by: Shrikanth Hegde --- Documentation/scheduler/sched-arch.rst | 48 ++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) diff --git a/Documentation/scheduler/sched-arch.rst b/Documentation/scheduler/sched-arch.rst index ed07efea7d02..2e926c7afc8f 100644 --- a/Documentation/scheduler/sched-arch.rst +++ b/Documentation/scheduler/sched-arch.rst @@ -62,6 +62,54 @@ 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 VM's co-ordinate among themselves and ask for +less CPU 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 online 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, Preffered CPUs +will become same as online CPUs again. This will be done by new scheduler +feature called STEAL_MONITOR. + +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 only a preferred CPU at wakeup. +2. Push the task away from non-preferred CPU at tick. +3. Only select preferred CPUs for load balance. + +This works only for SCHED_RT and SCHED_NORMAL. SCHED_EXT and userspace can +make choices using cpu_preferred_mask. + +/sys/devices/system/cpu/preferred prints the current cpu_preferred_mask in +cpulist format. + +Notes: +1. This feature is available under CONFIG_PARAVIRT. +2. preferred CPUs is same as online CPUs until STEAL_MONITOR is enabled. +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. +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