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Mon, 10 Aug 2026 11:58:18 +0000 (GMT) Received: from [9.111.54.108] (unknown [9.111.54.108]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 10 Aug 2026 11:58:18 +0000 (GMT) Message-ID: <31646728-5af6-4f3d-b461-8992c213e230@linux.ibm.com> Date: Mon, 10 Aug 2026 13:58:17 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 04/11] sched/core: Try to use a preferred CPU in is_cpu_allowed To: Shrikanth Hegde , 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: 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 References: <20260724140732.2683314-1-sshegde@linux.ibm.com> <20260724140732.2683314-5-sshegde@linux.ibm.com> Content-Language: en-US From: Mete Durlu In-Reply-To: <20260724140732.2683314-5-sshegde@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODEwMDEwMSBTYWx0ZWRfX1Eynb2jkVFgv FLjose4SH98cCzumRXzR/Qz3x1hBYbsoOkhBtQYpEjGmlkytCZA/IoUXC9ORrLhSN4GPyF9JR0q 8vu78MZJBK1jrjaHC3fcLaGW8vWeBSFzobeIGNh7WLC1nT3RWvqwwKASctWF1+PfLHroaHJgnY1 xZAHCfRfoOqL9uryDyGPLYkEzHCcdRCvW3QdlE1p8v4fgsj1qBLQnzWKC/B6+X8jNTQ/US0PTZD psSWX8ZXIKDzHc3JDptVNRF+Ry9STnuUE+P6wk8YCyqvDbnH+iDdeDX8Dnf5W/c2C+YEPzqYlkh AURAS8G89Bxkd4+0J0XuAzc6zJnmgkQRRgmwWd0PdcjEm2XeLcfKj/YxnzcEovzcBsThbtlLgAQ eoLG6PNVjqNnY3KjRGG3XqZig+3AlkqNKVVK0JF69XWVd4nVSbpmxRoKs5g9p3rK3hQ2pmm56lx V6dnRtZlZQf+qcACYPw== X-Proofpoint-Spam-Info: AW1haW4tMjYwODEwMDEwMSBTYWx0ZWRfX2lXtHOM8ITsW QbODE56xhDqNfL/tEDsUMfqceRzCKeV/yCpJbjIhvXxAtnWWwJqjLL0ifNPntHNDSKcxT0FH7qs 26Gwvw5am4zLjck/iVUGyxA+k/X0VEg= X-Authority-Analysis: v=2.4 cv=RqD16imK c=1 sm=1 tr=0 ts=6a79bce2 cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=uAbxVGIbfxUO_5tXvNgY:22 a=VnNF1IyMAAAA:8 a=Jl-25cGGCxnz-ZAPWPUA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: lLRUl7BWDQEODsyHjMtR5P-JOLF_rZFr X-Proofpoint-ORIG-GUID: fG0ZwbHXDL9xX-G4gKDb6EynEZarsSBM 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=2026-08-10_02,2026-08-07_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 priorityscore=1501 suspectscore=0 lowpriorityscore=0 clxscore=1011 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-2608100101 Hi, ... > For majority of the cases this would still keep select_fallback_rq > as O(N). task_has_preferred_cpus which is O(N) is called only if > !cpu_preferred. Then task running there is expected to move out. > So subsequent it should run on preferred CPU. This becomes O(N**2) > only for tasks pinned only non preferred CPUs. That is rare case. > > Signed-off-by: Shrikanth Hegde > --- > kernel/sched/core.c | 12 ++++++++++-- > kernel/sched/sched.h | 12 ++++++++++++ > 2 files changed, 22 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index a45f7c308329..9e8eec4451b6 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2509,8 +2509,12 @@ 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 +2524,11 @@ 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); > } > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 26ae13c86b69..6de6366f2faa 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -4230,4 +4230,16 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change) > > #include "ext/ext.h" > > +static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p) Sorry, I glanced to the earlier versions but I couldn't find the answer to my question. Trivial one but still. FWICT, task_can_sched_on_preferred() is only being used in core.c, is there a reason why it is defined in kernel/sched/sched.h and not in core.c ? Thanks. -Mete