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Wed, 12 Aug 2026 05:41:58 +0000 (GMT) Received: from li-7bb28a4c-2dab-11b2-a85c-887b5c60d769.ibm.com.com (unknown [9.124.217.151]) by smtpav04.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 12 Aug 2026 05:41:58 +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, meted@linux.ibm.com, ynorov@nvidia.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, 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 Subject: [PATCH v10 09/12] virt: Introduce steal governor driver Date: Wed, 12 Aug 2026 11:10:30 +0530 Message-ID: <20260812054033.95658-10-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260812054033.95658-1-sshegde@linux.ibm.com> References: <20260812054033.95658-1-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: AW1haW4tMjYwODEyMDA0MSBTYWx0ZWRfX2q1CLil9G7Dp SX9HBOsChoJUri8a1royNzwX3pW32t39LhkS9xJJyLyxyVuEVx/bLh8wKKH5yXpz04HhgfVSTLG ytVjWibQxPc8pF9vAtrdciDUZUdib7powby/ohvBBH4yHxZwRLluCk75LtfkR68ZlDGpy9UhVCC 6RMOAeZGaIAvRPrskDs4pBFHvB6LuEJBFPsfjZtF76b37OYeU6FrQ2VckX7+GZScitL53z04Rk9 ioFjuqkBKg+0wPdjAl3S460Nt5TPGR2DX1N2i754fj9KcwjPWEh54S7WDRfFU0XL4jEXbw0dhnm JvzX7h50Mi1kHdIQ34rXf3+gd0k+0g9J99BDEBB2EarfAY8eBF4ruCZt2n/mKvgYMbo5wLOhZz+ xVEZmYaA/IR5rARbO2E17vbC/A0lbTxhHJ+PPuDupgugP8ITcvhMtX/zSLI3a/5CW2XDPAMCe7a HX1UOE0KxTB8GIf2eeQ== X-Proofpoint-ORIG-GUID: NnR1Q3IblPpoGQrK8Nc0vQk9WW3Wvokn X-Proofpoint-Spam-Info: AW1haW4tMjYwODEyMDA0MSBTYWx0ZWRfXzV7Zsn7VdG1A /iGBWJLIzMMqpIqQWKvKedXVmMeILBEYD7bWK4bSySucpDEM4jqlxoJk5QaxD4Kwc3r/tqL6ckP wtrnym1DjUmFfO3SirBqFN2+H6w+MpI= X-Authority-Analysis: v=2.4 cv=IfK3n2qa c=1 sm=1 tr=0 ts=6a7c07b6 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=U7nrCbtTmkRpXpFmAIza:22 a=pGLkceISAAAA:8 a=zd2uoN0lAAAA:8 a=VnNF1IyMAAAA:8 a=VwQbUJbxAAAA:8 a=8b9GpE9nAAAA:8 a=63rvlg9gmhNNS91pnNMA:9 a=T3LWEMljR5ZiDmsYVIUa:22 X-Proofpoint-GUID: 3p1qfRsmXlNtwW5d3MrZUNBuGR7ItCos X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-12_01,2026-08-10_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 priorityscore=1501 impostorscore=0 bulkscore=0 clxscore=1015 lowpriorityscore=0 adultscore=0 malwarescore=0 spamscore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608120041 Introduce a new driver in virt named steal_governor. This driver will compute the steal time and drive the policy decisions regarding the preferred CPU state. More details can be found in Documentation/driver-api/steal-governor.rst. A new kconfig called STEAL_GOVERNOR is introduced in subsequent patches, which enables this driver. This driver will select CONFIG_PREFERRED_CPU. This makes configs driven by user preference/configuration. When the driver is disabled, preferred CPUs remain the same as active CPUs. The file layout of the driver is kept simple for now. The code is in drivers/virt/steal_governor.c, and the configs are part of drivers/virt/Kconfig. The main structure of the steal governor contains: - work, delay: Deferred periodic work function variables. - steal, time: Used to calculate deltas during periodic work. - interval_ms, high_threshold, low_threshold: Tuning knobs for the steal governor. While there, add MAINTAINERS entry for this new driver. Suggested-by: Yury Norov Suggested-by: K Prateek Nayak Signed-off-by: Shrikanth Hegde --- Documentation/driver-api/index.rst | 1 + Documentation/driver-api/steal-governor.rst | 137 ++++++++++++++++++++ MAINTAINERS | 9 ++ drivers/virt/steal_governor.c | 68 ++++++++++ 4 files changed, 215 insertions(+) create mode 100644 Documentation/driver-api/steal-governor.rst create mode 100644 drivers/virt/steal_governor.c diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst index eaf7161ff957..0a973b59cba3 100644 --- a/Documentation/driver-api/index.rst +++ b/Documentation/driver-api/index.rst @@ -138,6 +138,7 @@ Subsystem-specific APIs sm501 soundwire/index spi + steal-governor surface_aggregator/index switchtec sync_file diff --git a/Documentation/driver-api/steal-governor.rst b/Documentation/driver-api/steal-governor.rst new file mode 100644 index 000000000000..672eeccabfe8 --- /dev/null +++ b/Documentation/driver-api/steal-governor.rst @@ -0,0 +1,137 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Steal Governor +============== + +:Author: Shrikanth Hegde + +Introduction +============ + +The steal governor is aimed at mitigating the Noisy Neighbour problem +which occurs in paravirtualized environments with CPU overcommit. +The performance of a workload running in one VM gets degraded by +the activity of other VMs on the same host. As a result, all VMs +collectively make slower forward progress. + +In such systems, high utilization in all VMs causes the hypervisor to +frequently preempt vCPUs. This vCPU preemption is expensive. +To mitigate this, the kernel aims to restrict workloads to a subset of +Preferred CPUs to reduce physical CPU contention. +A detailed explanation of Preferred CPUs is available in +``Documentation/scheduler/sched-paravirt.rst``. + +The steal governor selects ``CONFIG_PREFERRED_CPU=y`` which enables the +scheduler core infrastructure to move the tasks to Preferred CPUs where +possible. The driver controls the policy decisions regarding the state of +preferred CPUs. That is, this driver decides which CPUs are preferred +and which CPUs are non-preferred. + +The driver code is available at ``drivers/virt/steal_governor.c``. + +Core idea +========= + +steal time is an indication available today in Guest which shows contention +for underlying physical CPU. Use it as a hint in the guest to fold the +workload to a reduced set of vCPUs. When there is contention, steal time +will show up in all the guests. When each guest honors the hint and folds +the workload to a smaller set of vCPUs (Preferred CPUs), it reduces the +contention and thereby reduces vCPU preemption. +This is achieved without any cross-guest communication. + +Steal governor driver effectively does: + +1. Periodically computes steal ratio across the possible CPUs. + +2. If steal ratio is greater than high threshold, reduce the number of + preferred CPUs by 1 core. Ensure at least one core is left always. + Skip changing the state of offline CPUs in that core. + +3. If steal ratio is less than or equal to low threshold, increase the + number of preferred CPUs by 1 core. If preferred is same as active, + nothing to be done. Skip changing the state of offline CPUs. + This helps to handle cases where few CPUs are offline in a core and + those offline CPUs will not be marked as preferred. + +4. Ensure preferred CPUs is always subset of active CPUs. + On feature disable it is same as active CPUs. + +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. +Those who enable this driver must ensure it is enabled in all VMs. + +Module Parameters +================= + +interval_ms +----------- + +How often steal governor checks for steal time. +Default: 1000 i.e. 1 second. Value should be in between 100ms to 100sec. + +This controls how fast steal governor driver reacts to changes to the +contention of physical CPUs. Since it does a fair amount of work, setting +too low may have overhead. Setting it too high might render it ineffective. + +low_threshold +------------- + +lower threshold value in percentage * 100. +Default: 200, i.e. 2% steal is considered as low threshold. +Can't be higher than high_threshold. + +This determines what values should be considered as nil/no steal values. +When steal governor sees steal ratio is less than or equal to this value, +it will increase the preferred CPUs by 1 core. +Using zero might cause oscillations. + +high_threshold +-------------- + +higher threshold value in percentage * 100 +Default: 500, i.e. 5% steal is considered as high threshold. +Can't be lower than low_threshold. Must be less than 10000. + +This determines what values should be considered as high steal values. +When steal governor sees steal ratio is higher than this value, it will +reduce the preferred CPUs by 1 core. + +Limitations of default values +----------------------------- + +Because of the vast diversity in VM configurations (e.g., highly populated +vs. sparsely populated CPU masks, few offlined CPUs etc), the default +thresholds may not be optimal for all systems. Users may need to tune these +parameters based on the system under test to achieve the best results. + +For example: +Possible CPUs = 128 and Active CPUs = 8 +Steal on online CPUs = 50% +steal ratio: (50% * 8 + 0% * 120) / 128 = 3.125% +This would fall in between and default values won't work. +In this example, if one wants effective 2% and 5% limits, then set, +low_threshold = (2% * 8 + 0% * 120) / 128 = 0.1250% = 12 +high_threshold = (5% * 8 + 0% * 120) / 128 = 0.3125% = 31 + +Using possible CPUs helps to handle spikes during CPU hotplug as the steal +time across possible CPUs is a monotonically increasing value. + +Reasons for CONFIG_STEAL_GOVERNOR=m +=================================== + +Selecting this driver makes CONFIG_PREFERRED_CPU=y. That makes configs +driven by user preference. Though one can have CONFIG_STEAL_GOVERNOR=y, +It is recommended to build CONFIG_STEAL_GOVERNOR=m due to below reasons: + +1. Doing periodic work has additional overheads. Enabling this driver + in systems where steal time cannot happen is of no use. There is no + benefit with additional overheads in such systems. + +2. This works well when all VMs work in co-operative manner. When an + administrative user enables it in one VM, he/she will likely enable + it all VMs. + +3. User can tweak the module parameters by reloading the module. diff --git a/MAINTAINERS b/MAINTAINERS index 15011f5752a9..40d46ba48ecd 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -25914,6 +25914,15 @@ F: rust/helpers/jump_label.c F: rust/kernel/generated_arch_static_branch_asm.rs.S F: rust/kernel/jump_label.rs +STEAL GOVERNOR DRIVER +M: Shrikanth Hegde +R: Yury Norov +L: linux-kernel@vger.kernel.org +S: Maintained +T: git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched/core +F: Documentation/driver-api/steal-governor.rst +F: drivers/virt/steal_governor.c + STI AUDIO (ASoC) DRIVERS M: Arnaud Pouliquen L: linux-sound@vger.kernel.org diff --git a/drivers/virt/steal_governor.c b/drivers/virt/steal_governor.c new file mode 100644 index 000000000000..d427282966d2 --- /dev/null +++ b/drivers/virt/steal_governor.c @@ -0,0 +1,68 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * Steal time governor driver periodically computes steal time. + * Based on the thresholds it either reduce/increase the preferred + * CPUs which can be used by the workload to avoid vCPU preemption + * to an extent possible in paravirtualized environment. + * + * Available with CONFIG_STEAL_GOVERNOR + * + * Copyright (C) 2026 IBM + * Author: Shrikanth Hegde + */ + +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt + +#include +#include +#include +#include +#include +#include +#include +#include +#include + +#if !IS_ENABLED(CONFIG_PREFERRED_CPU) +#error "Steal Governor requires CONFIG_PREFERRED_CPU" +#endif + +struct steal_governor { + ktime_t time; + u64 steal; + unsigned long delay; + unsigned int interval_ms; + unsigned int high_threshold; + unsigned int low_threshold; + struct delayed_work work; +}; + +static struct steal_governor sg_ctx; + +static void restore_preferred_to_active(void) +{ + int cpu; + + guard(cpus_read_lock)(); + for_each_cpu(cpu, cpu_active_mask) + set_cpu_preferred(cpu, true); +} + +static int __init steal_governor_init(void) +{ + pr_info("enabled\n"); + return 0; +} + +static void __exit steal_governor_exit(void) +{ + restore_preferred_to_active(); + pr_info("disabled\n"); +} + +module_init(steal_governor_init); +module_exit(steal_governor_exit); + +MODULE_LICENSE("GPL"); +MODULE_AUTHOR("IBM Corporation"); +MODULE_DESCRIPTION("Virtualization Steal Time Governor"); -- 2.47.3