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Fri, 24 Jul 2026 14:08:15 +0000 (GMT) Received: from li-7bb28a4c-2dab-11b2-a85c-887b5c60d769.ibm.com.com (unknown [9.39.20.178]) by smtpav06.fra02v.mail.ibm.com (Postfix) with ESMTP; Fri, 24 Jul 2026 14:08:15 +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, corbet@lwn.net 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, 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, rdunlap@infradead.org, kernellwp@gmail.com, linux-doc@vger.kernel.org, jgross@suse.com, virtualization@lists.linux.dev, kernel test robot Subject: [PATCH v9 01/11] sched/docs: Document cpu_preferred_mask and Preferred CPU concept Date: Fri, 24 Jul 2026 19:37:22 +0530 Message-ID: <20260724140732.2683314-2-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260724140732.2683314-1-sshegde@linux.ibm.com> References: <20260724140732.2683314-1-sshegde@linux.ibm.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev 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-ORIG-GUID: zfJtBqWYy4RPBgg9Ra0grRIQS4B_uzAN X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzI0MDEyNCBTYWx0ZWRfX5Nm9c30R9/aR pbK1I1UvrcHZs1wiX2nrgLD8D7CI02N4EL3iISTPnQkEvxAeg+slKx0zwr5762UzumzSXKoBT3R lCUrhefislCRYIgHh5EldQCZCj25NkYQB3JyiGAIAvplUECaXp6ff0mUC5uegoAx3srwys6J801 FMtRa2bvy0doUQUiI+rF+A533Q85dYhKY2haOJkQ+VkcUt/+sMeWx84qykSo80dFIKyIMb59dpW KW8e6wW6seUdaWpr5zm2X2m/4raxnt6gBj1YjUOBpdEPlBMPahiE8jm/EflIbnT5qALiN08KVeY omdh7egm0FLrqhFHHJVlKS+uKzZzr+ZJiPi0E/mgVIZNi7IxUQNxcIgf6w6N4NO1jI49TLnqXJk Qp9L61D8wBVz3aFMKxMDHlr5wfPB35kq2C/T0pL5bnKGvZA52Gl9M9dsgFOgSJuARTIkOk9DF8K UJv/cfA6jAIg4q4PwDw== X-Proofpoint-Spam-Info: AW1haW4tMjYwNzI0MDEyNCBTYWx0ZWRfXxi4KmnWVBiEV vZ73AyK8OZeD9gElBftOQf7mL46f+BAK6BHA2lT2rUqin5AhYgaXY5UOqb0giT96wPGl5bygDCG 6oUtHpNfdtuuNkesYTU+dYnkEmUdlZI= X-Proofpoint-GUID: SkJLJ8YmHLcpd8gHNSfk7wC9QoY1eToF X-Authority-Analysis: v=2.4 cv=SM5ykuvH c=1 sm=1 tr=0 ts=6a6371dd cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=VwQbUJbxAAAA:8 a=QyXUC8HyAAAA:8 a=VnNF1IyMAAAA:8 a=d8MiXRUMn4SEn--m04oA:9 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-07-24_03,2026-07-24_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 lowpriorityscore=0 bulkscore=0 suspectscore=0 adultscore=0 spamscore=0 malwarescore=0 priorityscore=1501 phishscore=0 impostorscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607240124 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. Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-kbuild-all/202606180717.yNM0yb41-lkp@intel.com/ Signed-off-by: Shrikanth Hegde --- Documentation/scheduler/sched-arch.rst | 58 ++++++++++++++++++++++++++ 1 file changed, 58 insertions(+) diff --git a/Documentation/scheduler/sched-arch.rst b/Documentation/scheduler/sched-arch.rst index ed07efea7d02..0a0a8eadbfd6 100644 --- a/Documentation/scheduler/sched-arch.rst +++ b/Documentation/scheduler/sched-arch.rst @@ -62,6 +62,64 @@ 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. +the sum of virtual CPUs (vCPUs) of all VMs is greater than number of physical +CPUs (pCPUs). Under such conditions when all or many VMs have high utilization, +hypervisor won't be able to satisfy the CPU requirement and has to context +switch within or across VMs. The hypervisor needs 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 VMs is reduced +by not using some of the vCPUs in each VM. vCPUs where workload can be safely +scheduled which won't increase any contention for pCPU are called as +"Preferred CPUs". + +Main design construct is preferred CPUs are always a subset of active 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. These policy decisions are taken by +steal_governor driver available at drivers/virt/steal_governor.c + +Scheduling decisions such as wakeup, pushing the task etc, need 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 using fallback mechanism. +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. It is selected + by steal_governor driver (CONFIG_STEAL_GOVERNOR). On enabling the + driver, CPU preferred state can change based on steal time. Without that + driver, preferred CPUs is same as active CPUs. + +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. Decision to change the preferred CPU state is driven by kernel. + Hence it shouldn't break user affinities. One of the main reasons why + CPU hotplug or Isolated cpuset partitions was not a solution. + +5. This feature works best only when all the VMs enable the feature as + it is a co-operative scheme. If a specific VM doesn't enable this feature + it may end up with more CPUs than others, still should lead to better + performance when seen from system view. + Users who enable this driver must ensure it is enabled in all VMs. Possible arch/ problems ======================= -- 2.47.3