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Wed, 9 Sep 2026 13:57:28 +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, sunlightlinux@gmail.com Subject: [PATCH v13 06/13] sched/core: Try to use a preferred CPU in is_cpu_allowed Date: Wed, 9 Sep 2026 19:26:10 +0530 Message-ID: <20260909135617.871006-7-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260909135617.871006-1-sshegde@linux.ibm.com> References: <20260909135617.871006-1-sshegde@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-doc@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-ORIG-GUID: agan9hkQNmu0nw1T6EVpTUaDLVnDUOOc X-Proofpoint-GUID: 2Zd1QorcqOcY30UiVji6Va9hfgNQC7Te X-Authority-Analysis: v=2.4 cv=DbEnbPtW c=1 sm=1 tr=0 ts=6aa165d9 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=VdqzKS8jKosA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=iQ6ETzBq9ecOQQE5vZCe:22 a=VnNF1IyMAAAA:8 a=xlrYe_syFdJcAglVzgYA:9 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwOTA5MDE1MSBTYWx0ZWRfX0TNNS1TCsXKe RqAe+YObOdBq0YXiGPTBu3yUX8eE/5mcQGcHWY8JDzbLNp3v2UBY1+rLYU3O0S/BRSnAQt1YY32 1yOerU+NJVDnzqg0GZJn8V71+Sg8ikjLpt+ll5WyfPAZNIRs/ivz8Fbu3nZ4k0W9QmkGQUNz2ln J+uRJEu3HEZG6GQEO69sQ0XQtqqAIv/k3Is8rpMtqVLJ22Djauc72iQ1T/kKI+4GrlcfYj60H9w 5d23fqf1+S4QQhbcB9n4ZY9j5gbqiLSjbb4kZpKhbyZTyajwbfbHgu8kXkVbvbYL/lKDq3TqY2g 1/SBSaWSY8SxMh6Ko7GNnKIRKJE2PH5jAA6yA+cpn9vTJLRrQJELTiDwyJidj9s2RniEdx9TDKO QcOw8QReOdrbKfIgxDnUNYd54w+qu08N/Y//xr7ylpkvF/5ZULwFHvQprh9A9I6CuMxy3rBrAfR KSyI6xtK+NkHcadh84Q== X-Proofpoint-Spam-Info: AW1haW4tMjYwOTA5MDE1MSBTYWx0ZWRfXwOTcIE9pzd76 Myrgnn/EZvM+Qqg3RX4OyuVRRoijon3dRLIBq/BJzwnW0Svoy9XFxzjWRhKzcSEqm2rwwd8AG49 gnCDgsfq3MeBimAUXg55nRcC/Ok1v8Q= 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=2026-09-08_03,2026-09-09_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 priorityscore=1501 impostorscore=0 adultscore=0 spamscore=0 clxscore=1015 suspectscore=0 bulkscore=0 malwarescore=0 lowpriorityscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2609090151 When possible, try to choose a preferred CPU. This is essential to maintain user affinities when preferred CPUs change. A task pinned on a non-preferred CPU should continue to run there, since this is a non-user triggered event. If a CPU is non-preferred and the task can run on other CPUs which are currently preferred, then choose a preferred CPU instead. This is decided by checking if cpus_ptr, cpu_preferred_mask and task possible CPUs intersect or not. If yes, then the task has other preferred CPUs. This takes care of tasks with architecture-specific CPU masks (e.g., 32-bit tasks on arm64). The push task mechanism uses a stopper thread which calls select_fallback_rq() and uses this mechanism to pick a preferred CPU. This takes care of the wakeup path for FAIR tasks too. is_cpu_allowed() is called to ensure wakeups happen on preferred CPUs. With that, additional checks in available_idle_cpu() are not necessary. Ignore the preferred CPU state if a task's affinity is changing and its new mask no longer includes the CPU it is currently running on. This ensures migration_cpu_stop() does not abort, preventing the task from being stranded outside its allowed affinity. For the majority of cases, this would still keep select_fallback_rq() as O(N). cpumask_intersects_and(), which is O(N), is called only if !cpu_preferred. The task running there is expected to move out. Subsequently, it should run on a preferred CPU. This becomes O(N**2) only for tasks pinned solely to non-preferred CPUs. That is a rare case. Overhead is minimal when the CPU is preferred. Signed-off-by: Shrikanth Hegde --- kernel/sched/core.c | 29 +++++++++++++++++++++++++++-- 1 file changed, 27 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a689a0cea4eb..b4ef2e92d786 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2504,6 +2504,23 @@ static inline bool rq_has_pinned_tasks(struct rq *rq) return rq->nr_pinned; } +static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p) +{ + if (cpu_preferred(cpu)) + return false; + + /* Only FAIR tasks honor preferred CPU state */ + if (unlikely(p->sched_class != &fair_sched_class)) + return false; + + /* Ignore preferred state if task affinity is changing */ + if (unlikely(!cpumask_test_cpu(task_cpu(p), p->cpus_ptr))) + return false; + + return cpumask_intersects_and(p->cpus_ptr, cpu_preferred_mask, + task_cpu_possible_mask(p)); +} + /* * Per-CPU kthreads are allowed to run on !active && online CPUs, see * __set_cpus_allowed_ptr() and select_fallback_rq(). @@ -2519,8 +2536,12 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu) return cpu_online(cpu); /* Non kernel threads are not allowed during either online or offline. */ - if (!(p->flags & PF_KTHREAD)) + if (!(p->flags & PF_KTHREAD)) { + /* Try to use preferred CPU if task's affinity allows */ + if (task_can_sched_on_preferred(cpu, p)) + return false; return cpu_active(cpu); + } /* KTHREAD_IS_PER_CPU is always allowed. */ if (kthread_is_per_cpu(p)) @@ -2530,7 +2551,11 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu) if (cpu_dying(cpu)) return false; - /* But are allowed during online. */ + /* Try to keep unbound kthreads on a preferred CPU if possible. */ + if (task_can_sched_on_preferred(cpu, p)) + return false; + + /* Otherwise, they are allowed to run on online CPU. */ return cpu_online(cpu); } -- 2.52.0