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Wed, 17 Jun 2026 17:43:08 +0000 (GMT) Received: from li-7bb28a4c-2dab-11b2-a85c-887b5c60d769.ibm.com.com (unknown [9.39.28.77]) by smtpav06.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 17 Jun 2026 17:43:07 +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 v4 06/20] sched/core: allow only preferred CPUs in is_cpu_allowed Date: Wed, 17 Jun 2026 23:11:25 +0530 Message-ID: <20260617174139.155540-7-sshegde@linux.ibm.com> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260617174139.155540-1-sshegde@linux.ibm.com> References: <20260617174139.155540-1-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=OcSoyBTY c=1 sm=1 tr=0 ts=6a32dcb8 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=FelO9ux0wxsA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VnNF1IyMAAAA:8 a=eC0SA1m0-JWMKL2UKjMA:9 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNjE3MDE2NiBTYWx0ZWRfX7+y3BtkiVk6n Xfh5MYAyduI40zXpnLd5DZjdH1w4tluqd0FQl8KMSUqE0XXKJbOKi4sFleoZv/8NcFsZP0VnWUw VYa6jri6zYUqjgh0kJPpHvwAG0r7PaLBNExf8/qCSWPcfN4sP7w3qUpRiSRmtvYfU5eluYBF6pH BtM6OBSk9jD3MC7z8aPBz53ci8coP0YJgBZEDVTwgzwQpzHDI5bj2A76bC7ThfkAvKb4Qyxm4G+ JZdo21x46nNr0bv2lfyU7Vb/akbkMcIW7FXdb/PN7KDnh5cDcg5edshW4K8DOFZX+KYtiyXEFBR 1wcvOQ4HaC6s0wnuW6ioChibNrCsNvjNW9Sm6LqvmhoKeaTILrn+nYwU+Zc/sjV1+HHIFDME7EL RazzV5leRJz2y2O7OdD5X4NtMGZxm0t+nyq/IIXK3gLScETqacShnip5/5UFqHs5gWFFSBJ/86L wgZeoDliipjS+16iQLQ== X-Proofpoint-GUID: GBpl5VnabJBQeDLDdhf9R2hQKVL9w3-p X-Proofpoint-Spam-Info: AW1haW4tMjYwNjE3MDE2NiBTYWx0ZWRfX8cXWbx1Z7b/O 3gEc3pF5xanUStRjG4LQY62HDPTg6GIYeh94cuWDf2oQcmNAWw1aXgpHkzLOWfv7ccr1tzrnXgM Vwv9w8UIX9yNvYdz568MirDF3Yb7KMM= X-Proofpoint-ORIG-GUID: rvDwPFYYXvnlNiNhFnDM0F-IVpanYSJM X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.125,FMLib:17.12.100.49 definitions=2026-06-17_02,2026-06-17_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 phishscore=0 bulkscore=0 suspectscore=0 lowpriorityscore=0 impostorscore=0 malwarescore=0 spamscore=0 adultscore=0 clxscore=1015 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2606170166 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. Since is_cpu_allowed can be called directly or repeatedly in select_fallback_rq, encode the info in task_struct->has_preferred_cpu_state if the path is via select_fallback_rq or not. This helps to avoid N**2 complexity for the rare cases. Signed-off-by: Shrikanth Hegde --- v3->v4: - Missing case of PF_KTHREAD is avoided. - Add a new field in task_struct which encodes intersection of tasks affinity and preferred CPUs and path its coming from. include/linux/sched.h | 1 + kernel/sched/core.c | 34 ++++++++++++++++++++++++++++++++-- kernel/sched/sched.h | 18 ++++++++++++++++++ 3 files changed, 51 insertions(+), 2 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index fc6ecb3869dd..2d0b1a6d50ac 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1657,6 +1657,7 @@ struct task_struct { #ifdef CONFIG_UNWIND_USER struct unwind_task_info unwind_info; #endif + int has_preferred_cpu_state; /* CPU-specific state of this task: */ struct thread_struct thread; diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 9e16946c9d62..714816cfa975 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2500,6 +2500,8 @@ static inline bool rq_has_pinned_tasks(struct rq *rq) */ static inline bool is_cpu_allowed(struct task_struct *p, int cpu) { + bool task_check_preferred_cpu = false; + /* When not in the task's cpumask, no point in looking further. */ if (!task_allowed_on_cpu(p, cpu)) return false; @@ -2508,9 +2510,22 @@ 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. + * + * If CPU is non-preferred and task can run on other CPUs which are + * currently preferred, then choose those other CPUs instead + */ + task_check_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)) + if (!(p->flags & PF_KTHREAD)) { + if (task_check_preferred_cpu) + return false; return cpu_active(cpu); + } /* KTHREAD_IS_PER_CPU is always allowed. */ if (kthread_is_per_cpu(p)) @@ -2520,6 +2535,10 @@ 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_check_preferred_cpu) + return false; + /* But are allowed during online. */ return cpu_online(cpu); } @@ -3549,6 +3568,14 @@ static int select_fallback_rq(int cpu, struct task_struct *p) enum { cpuset, possible, fail } state = cpuset; int dest_cpu; + /* + * Cache value whether task's affinity spans preferred CPUs. + * This helps to avoid repeating the same for each CPU + * later in the loop. Encode call to is_cpu_allowed coming + * via select_fallback_rq. + */ + p->has_preferred_cpu_state = task_has_preferred_cpus(p) << 8 | 0x1; + /* * If the node that the CPU is on has been offlined, cpu_to_node() * will return -1. There is no CPU on the node, and we should @@ -3560,7 +3587,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p) /* Look for allowed, online CPU in same node. */ for_each_cpu(dest_cpu, nodemask) { if (is_cpu_allowed(p, dest_cpu)) - return dest_cpu; + goto clear_and_return; } } @@ -3604,6 +3631,8 @@ static int select_fallback_rq(int cpu, struct task_struct *p) } } +clear_and_return: + p->has_preferred_cpu_state = 0; return dest_cpu; } @@ -4612,6 +4641,7 @@ static void __sched_fork(u64 clone_flags, struct task_struct *p) init_numa_balancing(clone_flags, p); p->wake_entry.u_flags = CSD_TYPE_TTWU; p->migration_pending = NULL; + p->has_preferred_cpu_state = 0; init_sched_mm(p); } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index c7c2dea65edd..38fd84b0b8f8 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -4213,4 +4213,22 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change) #include "ext.h" +/* + * has_preferred_cpu_state is encoding two bits of information. + * First Byte is to encode where the call to is_cpu_allowed coming from. + * Second Byte is to encode the intersection of task affinity + * and cpu_preferred_mask. + * + * If 1st Byte is set, call to is_cpu_allowed coming from select_fallback_rq. + * That helps to avoid repeated calculation keeping time complexity same. + */ +static inline bool task_has_preferred_cpus(struct task_struct *p) +{ + int cached_value = p->has_preferred_cpu_state; + + if (cached_value & 0x1) + return p->has_preferred_cpu_state >> 8; + else + return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask); +} #endif /* _KERNEL_SCHED_SCHED_H */ -- 2.47.3