linux/kernel/sched, branch v6.17

Merge tag 'sched-urgent-2025-09-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2025-09-26T19:30:23Z

Pull scheduler fixes from Ingo Molnar: "Fix two dl_server regressions: a race that can end up leaving the dl_server stuck, and a dl_server throttling bug causing lag to fair tasks" * tag 'sched-urgent-2025-09-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: sched/deadline: Fix dl_server behaviour sched/deadline: Fix dl_server getting stuck

sched/deadline: Fix dl_server behaviour

2025-09-25T07:51:50Z

John reported undesirable behaviour with the dl_server since commit: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling"). When starving fair tasks on purpose (starting spinning FIFO tasks), his fair workload, which often goes (briefly) idle, would delay fair invocations for a second, running one invocation per second was both unexpected and terribly slow. The reason this happens is that when dl_se->server_pick_task() returns NULL, indicating no runnable tasks, it would yield, pushing any later jobs out a whole period (1 second). Instead simply stop the server. This should restore behaviour in that a later wakeup (which restarts the server) will be able to continue running (subject to the CBS wakeup rules). Notably, this does not re-introduce the behaviour cccb45d7c4295 set out to solve, any start/stop cycle is naturally throttled by the timer period (no active cancel). Fixes: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling") Reported-by: John Stultz Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Peter Zijlstra (Intel) Tested-by: John Stultz

sched/deadline: Fix dl_server getting stuck

2025-09-25T07:51:50Z

John found it was easy to hit lockup warnings when running locktorture on a 2 CPU VM, which he bisected down to: commit cccb45d7c429 ("sched/deadline: Less agressive dl_server handling"). While debugging it seems there is a chance where we end up with the dl_server dequeued, with dl_se->dl_server_active. This causes dl_server_start() to return without enqueueing the dl_server, thus it fails to run when RT tasks starve the cpu. When this happens, dl_server_timer() catches the '!dl_se->server_has_tasks(dl_se)' case, which then calls replenish_dl_entity() and dl_server_stopped() and finally return HRTIMER_NO_RESTART. This ends in no new timer and also no enqueue, leaving the dl_server 'dead', allowing starvation. What should have happened is for the bandwidth timer to start the zero-laxity timer, which in turn would enqueue the dl_server and cause dl_se->server_pick_task() to be called -- which will stop the dl_server if no fair tasks are observed for a whole period. IOW, it is totally irrelevant if there are fair tasks at the moment of bandwidth refresh. This removes all dl_se->server_has_tasks() users, so remove the whole thing. Fixes: cccb45d7c4295 ("sched/deadline: Less agressive dl_server handling") Reported-by: John Stultz Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Peter Zijlstra (Intel) Tested-by: John Stultz

Merge tag 'sched_ext-for-6.17-rc7-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext

2025-09-22T18:28:52Z

Pull sched_ext fix from jun Heo: "This contains a fix for sched_ext idle CPU selection that likely fixes a substantial performance regression. The scx_bpf_select_cpu_dfl/and() kfuncs were incorrectly detecting all tasks as migration-disabled when called outside ops.select_cpu(), causing them to always return -EBUSY instead of finding idle CPUs. The fix properly distinguishes between genuinely migration-disabled tasks vs. the current task whose migration is temporarily disabled by BPF execution" * tag 'sched_ext-for-6.17-rc7-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext: sched_ext: idle: Handle migration-disabled tasks in BPF code

sched_ext: idle: Handle migration-disabled tasks in BPF code

2025-09-22T16:24:44Z

When scx_bpf_select_cpu_dfl()/and() kfuncs are invoked outside of ops.select_cpu() we can't rely on @p->migration_disabled to determine if migration is disabled for the task @p. In fact, migration is always disabled for the current task while running BPF code: __bpf_prog_enter() disables migration and __bpf_prog_exit() re-enables it. To handle this, when @p->migration_disabled == 1, check whether @p is the current task. If so, migration was not disabled before entering the callback, otherwise migration was disabled. This ensures correct idle CPU selection in all cases. The behavior of ops.select_cpu() remains unchanged, because this callback is never invoked for the current task and migration-disabled tasks are always excluded. Example: without this change scx_bpf_select_cpu_and() called from ops.enqueue() always returns -EBUSY; with this change applied, it correctly returns idle CPUs. Fixes: 06efc9fe0b8de ("sched_ext: idle: Handle migration-disabled tasks in idle selection") Cc: stable@vger.kernel.org # v6.16+ Signed-off-by: Andrea Righi Acked-by: Changwoo Min Signed-off-by: Tejun Heo

Merge tag 'sched_ext-for-6.17-rc6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext

2025-09-17T20:27:31Z

Pull sched_ext fixes from Tejun Heo: - Fix build failure when !FAIR_GROUP_SCHED && EXT_GROUP_SCHED - Revert "sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()" which was causing issues with per-CPU task scheduling and reenqueuing behavior * tag 'sched_ext-for-6.17-rc6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext: sched_ext, sched/core: Fix build failure when !FAIR_GROUP_SCHED && EXT_GROUP_SCHED Revert "sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()"

sched_ext, sched/core: Fix build failure when !FAIR_GROUP_SCHED && EXT_GROUP_SCHED

2025-09-17T09:07:27Z

While collecting SCX related fields in struct task_group into struct scx_task_group, 6e6558a6bc41 ("sched_ext, sched/core: Factor out struct scx_task_group") forgot update tg->scx_weight usage in tg_weight(), which leads to build failure when CONFIG_FAIR_GROUP_SCHED is disabled but CONFIG_EXT_GROUP_SCHED is enabled. Fix it. Fixes: 6e6558a6bc41 ("sched_ext, sched/core: Factor out struct scx_task_group") Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-kbuild-all/202509170230.MwZsJSWa-lkp@intel.com/ Tested-by: Andrea Righi Signed-off-by: Tejun Heo

Revert "sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()"

2025-09-16T20:15:23Z

scx_bpf_reenqueue_local() can be called from ops.cpu_release() when a CPU is taken by a higher scheduling class to give tasks queued to the CPU's local DSQ a chance to be migrated somewhere else, instead of waiting indefinitely for that CPU to become available again. In doing so, we decided to skip migration-disabled tasks, under the assumption that they cannot be migrated anyway. However, when a higher scheduling class preempts a CPU, the running task is always inserted at the head of the local DSQ as a migration-disabled task. This means it is always skipped by scx_bpf_reenqueue_local(), and ends up being confined to the same CPU even if that CPU is heavily contended by other higher scheduling class tasks. As an example, let's consider the following scenario: $ schedtool -a 0,1, -e yes > /dev/null $ sudo schedtool -F -p 99 -a 0, -e \ stress-ng -c 1 --cpu-load 99 --cpu-load-slice 1000 The first task (SCHED_EXT) can run on CPU0 or CPU1. The second task (SCHED_FIFO) is pinned to CPU0 and consumes ~99% of it. If the SCHED_EXT task initially runs on CPU0, it will remain there because it always sees CPU0 as "idle" in the short gaps left by the RT task, resulting in ~1% utilization while CPU1 stays idle: 0[||||||||||||||||||||||100.0%] 8[ 0.0%] 1[ 0.0%] 9[ 0.0%] 2[ 0.0%] 10[ 0.0%] 3[ 0.0%] 11[ 0.0%] 4[ 0.0%] 12[ 0.0%] 5[ 0.0%] 13[ 0.0%] 6[ 0.0%] 14[ 0.0%] 7[ 0.0%] 15[ 0.0%] PID USER PRI NI S CPU CPU%▽MEM% TIME+ Command 1067 root RT 0 R 0 99.0 0.2 0:31.16 stress-ng-cpu [run] 975 arighi 20 0 R 0 1.0 0.0 0:26.32 yes By allowing scx_bpf_reenqueue_local() to re-enqueue migration-disabled tasks, the scheduler can choose to migrate them to other CPUs (CPU1 in this case) via ops.enqueue(), leading to better CPU utilization: 0[||||||||||||||||||||||100.0%] 8[ 0.0%] 1[||||||||||||||||||||||100.0%] 9[ 0.0%] 2[ 0.0%] 10[ 0.0%] 3[ 0.0%] 11[ 0.0%] 4[ 0.0%] 12[ 0.0%] 5[ 0.0%] 13[ 0.0%] 6[ 0.0%] 14[ 0.0%] 7[ 0.0%] 15[ 0.0%] PID USER PRI NI S CPU CPU%▽MEM% TIME+ Command 577 root RT 0 R 0 100.0 0.2 0:23.17 stress-ng-cpu [run] 555 arighi 20 0 R 1 100.0 0.0 0:28.67 yes It's debatable whether per-CPU tasks should be re-enqueued as well, but doing so is probably safer: the scheduler can recognize re-enqueued tasks through the %SCX_ENQ_REENQ flag, reassess their placement, and either put them back at the head of the local DSQ or let another task attempt to take the CPU. This also prevents giving per-CPU tasks an implicit priority boost, which would otherwise make them more likely to reclaim CPUs preempted by higher scheduling classes. Fixes: 97e13ecb02668 ("sched_ext: Skip per-CPU tasks in scx_bpf_reenqueue_local()") Cc: stable@vger.kernel.org # v6.15+ Signed-off-by: Andrea Righi Acked-by: Changwoo Min Signed-off-by: Tejun Heo

sched: Fix sched_numa_find_nth_cpu() if mask offline

2025-09-03T16:20:06Z

sched_numa_find_nth_cpu() uses a bsearch to look for the 'closest' CPU in sched_domains_numa_masks and given cpus mask. However they might not intersect if all CPUs in the cpus mask are offline. bsearch will return NULL in that case, bail out instead of dereferencing a bogus pointer. The previous behaviour lead to this bug when using maxcpus=4 on an rk3399 (LLLLbb) (i.e. booting with all big CPUs offline): [ 1.422922] Unable to handle kernel paging request at virtual address ffffff8000000000 [ 1.423635] Mem abort info: [ 1.423889] ESR = 0x0000000096000006 [ 1.424227] EC = 0x25: DABT (current EL), IL = 32 bits [ 1.424715] SET = 0, FnV = 0 [ 1.424995] EA = 0, S1PTW = 0 [ 1.425279] FSC = 0x06: level 2 translation fault [ 1.425735] Data abort info: [ 1.425998] ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000 [ 1.426499] CM = 0, WnR = 0, TnD = 0, TagAccess = 0 [ 1.426952] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0 [ 1.427428] swapper pgtable: 4k pages, 39-bit VAs, pgdp=0000000004a9f000 [ 1.428038] [ffffff8000000000] pgd=18000000f7fff403, p4d=18000000f7fff403, pud=18000000f7fff403, pmd=0000000000000000 [ 1.429014] Internal error: Oops: 0000000096000006 [#1] SMP [ 1.429525] Modules linked in: [ 1.429813] CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-rc4-dirty #343 PREEMPT [ 1.430559] Hardware name: Pine64 RockPro64 v2.1 (DT) [ 1.431012] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) [ 1.431634] pc : sched_numa_find_nth_cpu+0x2a0/0x488 [ 1.432094] lr : sched_numa_find_nth_cpu+0x284/0x488 [ 1.432543] sp : ffffffc084e1b960 [ 1.432843] x29: ffffffc084e1b960 x28: ffffff80078a8800 x27: ffffffc0846eb1d0 [ 1.433495] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000 [ 1.434144] x23: 0000000000000000 x22: fffffffffff7f093 x21: ffffffc081de6378 [ 1.434792] x20: 0000000000000000 x19: 0000000ffff7f093 x18: 00000000ffffffff [ 1.435441] x17: 3030303866666666 x16: 66663d736b73616d x15: ffffffc104e1b5b7 [ 1.436091] x14: 0000000000000000 x13: ffffffc084712860 x12: 0000000000000372 [ 1.436739] x11: 0000000000000126 x10: ffffffc08476a860 x9 : ffffffc084712860 [ 1.437389] x8 : 00000000ffffefff x7 : ffffffc08476a860 x6 : 0000000000000000 [ 1.438036] x5 : 000000000000bff4 x4 : 0000000000000000 x3 : 0000000000000000 [ 1.438683] x2 : 0000000000000000 x1 : ffffffc0846eb000 x0 : ffffff8000407b68 [ 1.439332] Call trace: [ 1.439559] sched_numa_find_nth_cpu+0x2a0/0x488 (P) [ 1.440016] smp_call_function_any+0xc8/0xd0 [ 1.440416] armv8_pmu_init+0x58/0x27c [ 1.440770] armv8_cortex_a72_pmu_init+0x20/0x2c [ 1.441199] arm_pmu_device_probe+0x1e4/0x5e8 [ 1.441603] armv8_pmu_device_probe+0x1c/0x28 [ 1.442007] platform_probe+0x5c/0xac [ 1.442347] really_probe+0xbc/0x298 [ 1.442683] __driver_probe_device+0x78/0x12c [ 1.443087] driver_probe_device+0xdc/0x160 [ 1.443475] __driver_attach+0x94/0x19c [ 1.443833] bus_for_each_dev+0x74/0xd4 [ 1.444190] driver_attach+0x24/0x30 [ 1.444525] bus_add_driver+0xe4/0x208 [ 1.444874] driver_register+0x60/0x128 [ 1.445233] __platform_driver_register+0x24/0x30 [ 1.445662] armv8_pmu_driver_init+0x28/0x4c [ 1.446059] do_one_initcall+0x44/0x25c [ 1.446416] kernel_init_freeable+0x1dc/0x3bc [ 1.446820] kernel_init+0x20/0x1d8 [ 1.447151] ret_from_fork+0x10/0x20 [ 1.447493] Code: 90022e21 f000e5f5 910de2b5 2a1703e2 (f8767803) [ 1.448040] ---[ end trace 0000000000000000 ]--- [ 1.448483] note: swapper/0[1] exited with preempt_count 1 [ 1.449047] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b [ 1.449741] SMP: stopping secondary CPUs [ 1.450105] Kernel Offset: disabled [ 1.450419] CPU features: 0x000000,00080000,20002001,0400421b [ 1.450935] Memory Limit: none [ 1.451217] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]--- Yury: with the fix, the function returns cpu == nr_cpu_ids, and later in smp_call_function_any -> smp_call_function_single -> generic_exec_single we test the cpu for '>= nr_cpu_ids' and return -ENXIO. So everything is handled correctly. Fixes: cd7f55359c90 ("sched: add sched_numa_find_nth_cpu()") Cc: stable@vger.kernel.org Signed-off-by: Christian Loehle Signed-off-by: Yury Norov (NVIDIA)

sched/deadline: Don't count nr_running for dl_server proxy tasks

2025-08-26T08:46:01Z

On CPU offline the kernel stalled with below call trace: INFO: task kworker/0:1:11 blocked for more than 120 seconds. cpuhp hold the cpu hotplug lock endless and stalled vmstat_shepherd. This is because we count nr_running twice on cpuhp enqueuing and failed the wait condition of cpuhp: enqueue_task_fair() // pick cpuhp from idle, rq->nr_running = 0 dl_server_start() [...] add_nr_running() // rq->nr_running = 1 add_nr_running() // rq->nr_running = 2 [switch to cpuhp, waiting on balance_hotplug_wait()] rcuwait_wait_event(rq->nr_running == 1 && ...) // failed, rq->nr_running=2 schedule() // wait again It doesn't make sense to count the dl_server towards runnable tasks, since it runs other tasks. Fixes: 63ba8422f876 ("sched/deadline: Introduce deadline servers") Signed-off-by: Yicong Yang Signed-off-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20250627035420.37712-1-yangyicong@huawei.com