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15.20.9870.016; Thu, 30 Apr 2026 02:56:45 +0000 From: Jiping Ma To: peterz@infradead.org, vincent.guittot@linaro.org Cc: bsegall@google.com, dietmar.eggemann@arm.com, juri.lelli@redhat.com, kprateek.nayak@amd.com, linux-kernel@vger.kernel.org, mgorman@suse.de, mingo@redhat.com, qyousef@layalina.io, rostedt@goodmis.org, vschneid@redhat.com Subject: [Question] Possible scheduler regression: occasional ~3ms wake-up to sched-switch delay in cyclictest on kernel 6.6 and 6.12 Date: Thu, 30 Apr 2026 02:56:38 +0000 Message-ID: <20260430025638.512420-1-jiping.ma2@windriver.com> X-Mailer: git-send-email 2.53.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: SJ0PR13CA0083.namprd13.prod.outlook.com (2603:10b6:a03:2c4::28) To PH7PR11MB6498.namprd11.prod.outlook.com (2603:10b6:510:1f1::21) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: 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Issue: During cyclictest under CPU contention, we occasionally see a ~3ms delay from wake-up to sched-switch ion kernel 6.6 and kernel 6.12. Most of the time the latency is comparable to kernel 5.15, but these sporadic spikes significantly impact the max latency. This behavior is not observed on kernel 5.15 under the same conditions. Test setup: Platform: x86 Kernel type: std (non-RT) Cyclictest command: ./cyclictest -m -N --policy=SCHED_OTHER -t 1 -D 30m -a 3 CPU load: 6 instances of a pure CPU calculation shell script (cal.sh) bound to CPU 3, running in background to create contention cal.sh (CPU stress script): #!/bin/sh x=1 while :; do i=0 while [ $i -lt 100000 ]; do x=$(( (x * 1103515245 + 12345) & 0x7fffffff )) i=$((i + 1)) done done Results (max latency, ns): Kernel cyclictest max (ns) 5.15 53,461 6.6 251,926 6.12 110,861 Key observation: The majority of cyclictest samples show latency similar to kernel 5.15. However, there are occasional spikes where the wake-up to sched-switch gap reaches ~3ms. This suggests the scheduler is intermittently failing to preempt the running tasks promptly when the cyclictest thread wakes up. Since all tasks (cyclictest + 6x cal.sh) are SCHED_OTHER on the same CPU, this may be related to changes in the EEVDF scheduler introduced in kernel 6.6. Questions: 1. Is this occasional preemption delay a known side effect of the EEVDF scheduler replacing CFS in kernel 6.6+? 2. Is this by design, or should it be considered a scheduler bug? 3ms delay kernel trace log: cal.sh-917 [003] d.h2. 1667.138647: sched_waking: comm=cyclictest pid=943 prio=120 target_cpu=003 cal.sh-917 [003] d.h3. 1667.138648: sched_stat_runtime: comm=cal.sh pid=917 runtime=101740 [ns] cal.sh-917 [003] d.h3. 1667.138649: sched_wakeup: comm=cyclictest pid=943 prio=120 target_cpu=003 cal.sh-917 [003] d.h.. 1667.138649: hrtimer_expire_exit: hrtimer=00000000c56c84b4 cal.sh-917 [003] d.h.. 1667.138649: write_msr: 6e0, value 42dd8d2cf32 cal.sh-917 [003] d.h.. 1667.138649: local_timer_exit: vector=236 cal.sh-917 [003] d.h.. 1667.139540: local_timer_entry: vector=236 cal.sh-917 [003] d.h.. 1667.139540: hrtimer_expire_entry: hrtimer=000000007fd337f4 function=tick_sched_timer now=1665335000684 cal.sh-917 [003] d.h1. 1667.139541: notifier_run: pvclock_gtod_notify [kvm] cal.sh-917 [003] d.h.. 1667.139541: rcu_utilization: Start scheduler-tick cal.sh-917 [003] d.h.. 1667.139541: rcu_utilization: End scheduler-tick cal.sh-917 [003] d.h.. 1667.139542: read_msr: e8, value 4892b7c3d38 cal.sh-917 [003] d.h.. 1667.139542: read_msr: e7, value 38088ff481e cal.sh-917 [003] d.h1. 1667.139542: sched_stat_runtime: comm=cal.sh pid=917 runtime=894430 [ns] cal.sh-917 [003] d.h.. 1667.139543: hrtimer_expire_exit: hrtimer=000000007fd337f4 cal.sh-917 [003] d.h1. 1667.139543: hrtimer_start: hrtimer=000000007fd337f4 function=tick_sched_timer expires=1665336000000 softexpires=1665336000000 mode=ABS cal.sh-917 [003] d.h.. 1667.139543: write_msr: 6e0, value 42dd8fc30e4 cal.sh-917 [003] d.h.. 1667.139543: local_timer_exit: vector=236 cal.sh-917 [003] d.h.. 1667.140540: local_timer_entry: vector=236 cal.sh-917 [003] d.h.. 1667.140540: hrtimer_expire_entry: hrtimer=000000007fd337f4 function=tick_sched_timer now=1665336000925 cal.sh-917 [003] d.h1. 1667.140541: notifier_run: pvclock_gtod_notify [kvm] cal.sh-917 [003] d.h.. 1667.140541: rcu_utilization: Start scheduler-tick cal.sh-917 [003] d.h.. 1667.140541: rcu_utilization: End scheduler-tick cal.sh-917 [003] d.h.. 1667.140542: read_msr: e8, value 4892bb18743 cal.sh-917 [003] d.h.. 1667.140542: read_msr: e7, value 380892863df cal.sh-917 [003] d.h1. 1667.140542: sched_stat_runtime: comm=cal.sh pid=917 runtime=1000134 [ns] cal.sh-917 [003] d.h.. 1667.140543: hrtimer_expire_exit: hrtimer=000000007fd337f4 cal.sh-917 [003] d.h1. 1667.140543: hrtimer_start: hrtimer=000000007fd337f4 function=tick_sched_timer expires=1665337000000 softexpires=1665337000000 mode=ABS cal.sh-917 [003] d.h.. 1667.140543: write_msr: 6e0, value 42dd92592a6 cal.sh-917 [003] d.h.. 1667.140544: local_timer_exit: vector=236 cal.sh-917 [003] d.h.. 1667.141540: local_timer_entry: vector=236 cal.sh-917 [003] d.h.. 1667.141540: hrtimer_expire_entry: hrtimer=000000007fd337f4 function=tick_sched_timer now=1665337000748 cal.sh-917 [003] d.h1. 1667.141541: notifier_run: pvclock_gtod_notify [kvm] cal.sh-917 [003] d.h.. 1667.141541: rcu_utilization: Start scheduler-tick cal.sh-917 [003] d.h.. 1667.141541: softirq_raise: vec=9 [action=RCU] cal.sh-917 [003] d.h.. 1667.141541: rcu_utilization: End scheduler-tick cal.sh-917 [003] d.h.. 1667.141542: read_msr: e8, value 4892be6cf1e cal.sh-917 [003] d.h.. 1667.141542: read_msr: e7, value 38089517df9 cal.sh-917 [003] dNh1. 1667.141542: sched_stat_runtime: comm=cal.sh pid=917 runtime=999991 [ns] cal.sh-917 [003] dNh.. 1667.141543: hrtimer_expire_exit: hrtimer=000000007fd337f4 cal.sh-917 [003] dNh1. 1667.141543: hrtimer_start: hrtimer=000000007fd337f4 function=tick_sched_timer expires=1665338000000 softexpires=1665338000000 mode=ABS cal.sh-917 [003] dNh.. 1667.141543: write_msr: 6e0, value 42dd94ef44f cal.sh-917 [003] dNh.. 1667.141543: local_timer_exit: vector=236 cal.sh-917 [003] .Ns.. 1667.141544: softirq_entry: vec=9 [action=RCU] cal.sh-917 [003] .Ns.. 1667.141544: rcu_utilization: Start RCU core cal.sh-917 [003] dNs1. 1667.141544: rcu_grace_period: rcu_preempt 589121 cpuend cal.sh-917 [003] dNs.. 1667.141544: rcu_batch_start: rcu_preempt CBs=1 bl=10 cal.sh-917 [003] dNs.. 1667.141545: rcu_segcb_stats: SegCbDequeued seglen: (DONE=0, WAIT=0, NEXT_READY=0, NEXT=0) gp_seq: (DONE=0, WAIT=589124, NEXT_READY=589124, NEXT=0) cal.sh-917 [003] .Ns.. 1667.141545: rcu_invoke_callback: rcu_preempt rhp=00000000be314949 func=file_free_rcu cal.sh-917 [003] .Ns.. 1667.141545: kmem_cache_free: call_site=file_free_rcu+0x35/0x60 ptr=00000000be314949 name=kmalloc-cg-256 cal.sh-917 [003] dNs.. 1667.141545: rcu_batch_end: rcu_preempt CBs-invoked=1 idle=.S.. cal.sh-917 [003] .Ns.. 1667.141545: rcu_utilization: End RCU core cal.sh-917 [003] .Ns.. 1667.141546: softirq_exit: vec=9 [action=RCU] cal.sh-917 [003] dN.1. 1667.141546: rcu_utilization: Start context switch cal.sh-917 [003] dN.1. 1667.141546: rcu_utilization: End context switch cal.sh-917 [003] dN.2. 1667.141546: sched_stat_runtime: comm=cal.sh pid=917 runtime=4083 [ns] cal.sh-917 [003] d..2. 1667.141547: sched_switch: prev_comm=cal.sh prev_pid=917 prev_prio=120 prev_state=R ==> next_comm=cyclictest next_pid=943 next_prio=120 Thanks, Jiping Ma