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15.20.9870.016; Thu, 30 Apr 2026 04:35:44 +0000 From: Jiping Ma To: kprateek.nayak@amd.com Cc: bsegall@google.com, dietmar.eggemann@arm.com, jiping.ma2@windriver.com, juri.lelli@redhat.com, linux-kernel@vger.kernel.org, mgorman@suse.de, mingo@redhat.com, peterz@infradead.org, qyousef@layalina.io, rostedt@goodmis.org, vincent.guittot@linaro.org, vschneid@redhat.com Subject: Re: [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 04:35:35 +0000 Message-ID: <20260430043535.523617-1-jiping.ma2@windriver.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: References: Content-Transfer-Encoding: 8bit Content-Type: text/plain X-ClientProxiedBy: BN0PR03CA0043.namprd03.prod.outlook.com (2603:10b6:408:e7::18) 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 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The test results get well after the up command. ./cyclictest -m -N --policy=SCHED_OTHER -t 1 -D 30m -a 3 T: 0 ( 736) P: 0 I:1000 C:1776085 Min: 3135 Act: 53535 Avg: 77421 Max: 6946557 Thanks, Jiping > >That should make preemption faster on v6.6. > >> 6.12 110,861 > >At v6.12, the DELAY_DEQUEUE feature seems to have improved the tail >by decaying lag while keeping the task on the runqueue. You can >still try NO_RUN_TO_PARITY but the delay might still be close to the >granularity of the 1ms tick. > >In v6.12, there is also PREEMPT_SHORT which would selectively disable >RUN_TO_PARITY if a task of shorter slice wakes up - you can also try >playing with that where you align the slice of cyclictest close to >its actual runtime. > >Vincent was working on this exact same problem and there should be >a bunch of fixes lined up in v7.1-rc1 + few more being discussed in >the mailing list. In no particular order: > >https://lore.kernel.org/lkml/177583163838.801717.18173795801708799547.tip-bot2@tip-bot2/ >https://lore.kernel.org/lkml/177521940443.1647592.2249539659928731910.tip-bot2@tip-bot2/ >https://lore.kernel.org/lkml/177736116339.3521451.16868557879769343251.tip-bot2@tip-bot2/ >https://lore.kernel.org/lkml/20260429164102.1388139-1-vincent.guittot@linaro.org/ >https://lore.kernel.org/lkml/20260417171642.3539914-1-vincent.guittot@linaro.org/ > >The commit log from first few patches should have the slice values >that Vincent was testing with. > >> >> 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? > >This is by design but there are workarounds to get around this by >setting a slice value and disabling RUN_TO_PARITY. > >> >> 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] > ^ >Since NEED_RESCHED is only set here, it is safe to assume cal.sh was >likely kept running by RUN_TO_PARITY for 3ms after it was selected which >corresponds to the default base_slice. > >> 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 and Regards, >Prateek >