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Thu, 14 May 2026 15:23:45 +0000 (GMT) Received: from li-7bb28a4c-2dab-11b2-a85c-887b5c60d769.ibm.com.com (unknown [9.124.213.185]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTP; Thu, 14 May 2026 15:23:45 +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 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, mgorman@suse.de, bsegall@google.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 Subject: [PATCH v3 06/20] sched/core: allow only preferred CPUs in is_cpu_allowed Date: Thu, 14 May 2026 20:51:50 +0530 Message-ID: <20260514152204.481115-7-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260514152204.481115-1-sshegde@linux.ibm.com> References: <20260514152204.481115-1-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-Authority-Analysis: v=2.4 cv=KbvidwYD c=1 sm=1 tr=0 ts=6a05e90d cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=NGcC8JguVDcA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VnNF1IyMAAAA:8 a=h5kyaPOYuMV5y32J0wcA:9 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTE0MDE1MyBTYWx0ZWRfX7WLnvIG23hDa 0CzwJWnSzbQ/eEKxs54nSwRXFkgD1AturMYU8OyD0DAtAp2804fJ2TfGJYN37PyWaxcnfTYMyKq zmE11gGOWOwBhUn4X4ZKpJiW+h1/eOpBSS8BnKwQS0QCq9G0Lbz+XalQYAH1KceQ1Z7kMgnpvwn BIquikwrR+iFRAC3OpOaife29hByHejLC6IkJ42hwvToOaXXj7N9W5Gr9tXaHXRce0BUO936Nu2 pCnoOlhakMVpDB5YD3UEUBGUXhdbu0+hsw/zBjYn4Zgsyxq6uS2/LYpSHq6rUxIU7MkUiYVhWhr 1D8u4sPdeZeOt59dw/QXN1byHa8HLrZGK8Pcq2ImYFmDJNwJoGUrI9vlNmKYqlM6oJH11/1/FOL MstwHr+euZT3qKSTov1767NKOKhrPf1TUFYCh7NqbkSfbpy9q0f5X0lt8C3R2oz5qWCua4ql+aH 5pJSGghZac3gwHzSMOg== X-Proofpoint-GUID: uvgmOl2ivjrBKjLTYr6_0gvMn-K81C8K X-Proofpoint-ORIG-GUID: qhvfmHTNm6wXsjofDRfkwNSN2Y1k8qQ0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-05-14_03,2026-05-13_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 bulkscore=0 impostorscore=0 malwarescore=0 lowpriorityscore=0 phishscore=0 spamscore=0 suspectscore=0 clxscore=1015 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2605050000 definitions=main-2605140153 When possible, choose a preferred CPUs to pick. Push task mechanism uses stopper thread which going to call select_fallback_rq and use this mechanism to pick only a preferred CPU. When task is affined only to non-preferred CPUs it should continue to run there. Detect that by checking if cpus_ptr and cpu_preferred_mask intersect or not. Signed-off-by: Shrikanth Hegde --- kernel/sched/core.c | 27 +++++++++++++++++++++++++-- kernel/sched/sched.h | 5 +++++ 2 files changed, 30 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 3ae5f19c1b7e..292d4e7db0fd 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2468,6 +2468,8 @@ static inline bool rq_has_pinned_tasks(struct rq *rq) */ static inline bool is_cpu_allowed(struct task_struct *p, int cpu) { + bool task_has_preferred_cpu = false; + /* When not in the task's cpumask, no point in looking further. */ if (!task_allowed_on_cpu(p, cpu)) return false; @@ -2476,9 +2478,26 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu) if (is_migration_disabled(p)) return cpu_online(cpu); + /* + * This is essential to maintain user affinities when preferred + * CPUs change. A task pinned on non-preferred CPU should continue + * to run there, since this is non-user triggered. + * + * 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. + */ + task_has_preferred_cpu = !cpu_preferred(cpu) && task_has_preferred_cpus(p); + /* Non kernel threads are not allowed during either online or offline. */ - if (!(p->flags & PF_KTHREAD)) - return cpu_active(cpu); + if (!(p->flags & PF_KTHREAD)) { + if (!cpu_active(cpu)) + return false; + if (task_has_preferred_cpu) + return false; + } /* KTHREAD_IS_PER_CPU is always allowed. */ if (kthread_is_per_cpu(p)) @@ -2488,6 +2507,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu) if (cpu_dying(cpu)) return false; + /* Try on preferred CPU first if possible*/ + if (task_has_preferred_cpu) + return false; + /* But are allowed during online. */ return cpu_online(cpu); } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index ffe77b2b6296..faf36bc7bd12 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -4130,4 +4130,9 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change) #include "ext.h" +static inline bool task_has_preferred_cpus(struct task_struct *p) +{ + return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask); +} + #endif /* _KERNEL_SCHED_SCHED_H */ -- 2.47.3