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Thu, 9 Jul 2026 21:56:59 +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, kernel test robot Subject: [PATCH v7 01/12] sched/docs: Document cpu_preferred_mask and Preferred CPU concept Date: Fri, 10 Jul 2026 03:26:37 +0530 Message-ID: <20260709215648.1246821-2-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260709215648.1246821-1-sshegde@linux.ibm.com> References: <20260709215648.1246821-1-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: Qo-xgbweHZWQnFaGhVNFA524YCwY-V-z X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzA5MDIxOCBTYWx0ZWRfX7HjIAIpyZqOn s7m5ASAKhwvQx5IGTqbkpfjUFRv9mSeKbstC/6dNtIexq1YZqbxO+ryP+fpVMAZivuJPiGcUEM+ yScWt1oeoRDV4lg8r/cCHItoQqebeYtLK7cs9ILrKt4mgoNVpj7ynhlBxvQbnpSvNmhDUD3TZU8 F1nEXsOVTthzfushKV8lBWJ/bWIy7CZzYstI+y/NvkcjvEC57/bCTRjEQADGyAmMPaatmJtBAol Jp9UgbFU2ptALBegMzNa283TxwLCI4sEQnWy3biKowdo5wPf1p5LPN/3JyH87KQ9/ereZfy3Tio co0Zjyv3D9ZFYOu2rdFZSf+t4ZTsyx5Zn7rpUsLsxishwJaGiysOZqyrhleW+HRAFKgi24rTnGe WopmCEZCyFqFr4COD+iK5BrLILOIj4R+FHCboBzTaUOn1FvZI3bhgbrmXIJpAK7HvMAy7mOfWsJ 8I4fdOzRWgcUZx/8/vA== X-Proofpoint-ORIG-GUID: CVZnBEzxMStcDing9OGenmHL76GmNlXD X-Authority-Analysis: v=2.4 cv=FqQ1OWrq c=1 sm=1 tr=0 ts=6a501939 cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=U7nrCbtTmkRpXpFmAIza:22 a=VwQbUJbxAAAA:8 a=QyXUC8HyAAAA:8 a=VnNF1IyMAAAA:8 a=d8MiXRUMn4SEn--m04oA:9 X-Proofpoint-Spam-Info: AW1haW4tMjYwNzA5MDIxOCBTYWx0ZWRfX+y8chdtNExgV 8Z0etvGwPxnWaANiKuY9zpk5uFLO9COJ37VWFmKNCnDRshaRQN897qicNCBv3vYeaPReIRmOzw7 bPk6EXRyQhLeLHYGqEzSQ3KGcnGN+0Y= 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=2026-07-09_04,2026-07-09_04,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 impostorscore=0 clxscore=1015 malwarescore=0 lowpriorityscore=0 priorityscore=1501 bulkscore=0 suspectscore=0 spamscore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607090218 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..82bc27c7daac 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. This is done by loading the +steal_monitor driver available at drivers/virt/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 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_monitor driver (CONFIG_STEAL_MONITOR=y). 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 use/not use 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