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Wed, 12 Aug 2026 05:41:20 +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 05:41:20 +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 05/12] sched/core: Try to use a preferred CPU in is_cpu_allowed Date: Wed, 12 Aug 2026 11:10:26 +0530 Message-ID: <20260812054033.95658-6-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: AW1haW4tMjYwODEyMDA0MSBTYWx0ZWRfX3TJOJUgymRxJ U4z0Hp4ruUSaYEF+7pMpnxIyspR3rMDgRBHpmSutDFuVV1Zy+QqDg4gh61fTWK/rguez+MIPwD5 v9Nb33dHWKGzn3OQOB8HbT5Xim9CSb9ZRnr1pWYt9glwy/zYTsTwBj5Jp4M90AdQj6htyRHHtGi kicAlSamw0wtGPkxNR+uHWvGo7I2Z9t0+iZRYrqPteVmf854HwMqitiV91SQkGQ69t7/4/tGf59 BsvRArOubwbwUAbmdwnCKs4VzyjvZjlTToTUDDYGLOda/tVQ2yLoYs7v3fMkStfFs/UQtQHxTC+ 4E2XTMdaRfhpi+Z+9Is421yNepuDVPX/LiOLdk9mYTaUk0h5ASIh57mgKEv5+FNmVxISlkXsn+A GiPp3/okNQnj2+fG1psTnrdivT/Ts0bGJxWrVgCNialXDx39G7E/3LiM6kGQcnNnLXTzcLhW93c UzEtN8+9CoVS2hkHgjQ== X-Proofpoint-Spam-Info: AW1haW4tMjYwODEyMDA0MSBTYWx0ZWRfX2yaZ6tRQHiX3 ttBJkrIZvb9VQRT6ORMOKckhyWxKWVDMbyF8YkaqYI+7zKPonEco+d1SuF/pIwQTcM4ubA1WY9K XfYv3+hs1Ea81ZTkBxKSbkucgtI3Xcg= X-Authority-Analysis: v=2.4 cv=RqD16imK c=1 sm=1 tr=0 ts=6a7c078f cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=uAbxVGIbfxUO_5tXvNgY:22 a=VnNF1IyMAAAA:8 a=TBRyIjXBjLUW-Ng_AUEA:9 X-Proofpoint-GUID: KpscK3vHwCeCHyXvco_HNY3LSCneEIZ0 X-Proofpoint-ORIG-GUID: g-Gl2t0XYBoQIM-mxrMhN1kx24tk9fC6 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 suspectscore=0 lowpriorityscore=0 clxscore=1015 adultscore=0 bulkscore=0 malwarescore=0 impostorscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608120041 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 and cpu_preferred_mask intersect or not. If yes, then the task has other preferred CPUs. 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. For the majority of cases, this would still keep select_fallback_rq() as O(N). cpumask_intersects(), 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 | 24 ++++++++++++++++++++++-- 1 file changed, 22 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a45f7c308329..1c90bcad0a75 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2494,6 +2494,18 @@ 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; + + return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask); +} + /* * Per-CPU kthreads are allowed to run on !active && online CPUs, see * __set_cpus_allowed_ptr() and select_fallback_rq(). @@ -2509,8 +2521,12 @@ 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)) @@ -2520,7 +2536,11 @@ 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); } -- 2.47.3