linux/kernel/sched/rt.c, branch v6.16

sched/rt: Fix race in push_rt_task

2025-04-08T18:55:55Z

Overview ======== When a CPU chooses to call push_rt_task and picks a task to push to another CPU's runqueue then it will call find_lock_lowest_rq method which would take a double lock on both CPUs' runqueues. If one of the locks aren't readily available, it may lead to dropping the current runqueue lock and reacquiring both the locks at once. During this window it is possible that the task is already migrated and is running on some other CPU. These cases are already handled. However, if the task is migrated and has already been executed and another CPU is now trying to wake it up (ttwu) such that it is queued again on the runqeue (on_rq is 1) and also if the task was run by the same CPU, then the current checks will pass even though the task was migrated out and is no longer in the pushable tasks list. Crashes ======= This bug resulted in quite a few flavors of crashes triggering kernel panics with various crash signatures such as assert failures, page faults, null pointer dereferences, and queue corruption errors all coming from scheduler itself. Some of the crashes: -> kernel BUG at kernel/sched/rt.c:1616! BUG_ON(idx >= MAX_RT_PRIO) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? pick_next_task_rt+0x6e/0x1d0 ? do_error_trap+0x64/0xa0 ? pick_next_task_rt+0x6e/0x1d0 ? exc_invalid_op+0x4c/0x60 ? pick_next_task_rt+0x6e/0x1d0 ? asm_exc_invalid_op+0x12/0x20 ? pick_next_task_rt+0x6e/0x1d0 __schedule+0x5cb/0x790 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb -> BUG: kernel NULL pointer dereference, address: 00000000000000c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? __warn+0x8a/0xe0 ? exc_page_fault+0x3d6/0x520 ? asm_exc_page_fault+0x1e/0x30 ? pick_next_task_rt+0xb5/0x1d0 ? pick_next_task_rt+0x8c/0x1d0 __schedule+0x583/0x7e0 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb -> BUG: unable to handle page fault for address: ffff9464daea5900 kernel BUG at kernel/sched/rt.c:1861! BUG_ON(rq->cpu != task_cpu(p)) -> kernel BUG at kernel/sched/rt.c:1055! BUG_ON(!rq->nr_running) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? dequeue_top_rt_rq+0xa2/0xb0 ? do_error_trap+0x64/0xa0 ? dequeue_top_rt_rq+0xa2/0xb0 ? exc_invalid_op+0x4c/0x60 ? dequeue_top_rt_rq+0xa2/0xb0 ? asm_exc_invalid_op+0x12/0x20 ? dequeue_top_rt_rq+0xa2/0xb0 dequeue_rt_entity+0x1f/0x70 dequeue_task_rt+0x2d/0x70 __schedule+0x1a8/0x7e0 ? blk_finish_plug+0x25/0x40 schedule+0x3c/0xb0 futex_wait_queue_me+0xb6/0x120 futex_wait+0xd9/0x240 do_futex+0x344/0xa90 ? get_mm_exe_file+0x30/0x60 ? audit_exe_compare+0x58/0x70 ? audit_filter_rules.constprop.26+0x65e/0x1220 __x64_sys_futex+0x148/0x1f0 do_syscall_64+0x30/0x80 entry_SYSCALL_64_after_hwframe+0x62/0xc7 -> BUG: unable to handle page fault for address: ffff8cf3608bc2c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? spurious_kernel_fault+0x171/0x1c0 ? exc_page_fault+0x3b6/0x520 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? asm_exc_page_fault+0x1e/0x30 ? _cond_resched+0x15/0x30 ? futex_wait_queue_me+0xc8/0x120 ? futex_wait+0xd9/0x240 ? try_to_wake_up+0x1b8/0x490 ? futex_wake+0x78/0x160 ? do_futex+0xcd/0xa90 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? plist_del+0x6a/0xd0 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? dequeue_pushable_task+0x20/0x70 ? __schedule+0x382/0x7e0 ? asm_sysvec_reschedule_ipi+0xa/0x20 ? schedule+0x3c/0xb0 ? exit_to_user_mode_prepare+0x9e/0x150 ? irqentry_exit_to_user_mode+0x5/0x30 ? asm_sysvec_reschedule_ipi+0x12/0x20 Above are some of the common examples of the crashes that were observed due to this issue. Details ======= Let's look at the following scenario to understand this race. 1) CPU A enters push_rt_task a) CPU A has chosen next_task = task p. b) CPU A calls find_lock_lowest_rq(Task p, CPU Z’s rq). c) CPU A identifies CPU X as a destination CPU (X < Z). d) CPU A enters double_lock_balance(CPU Z’s rq, CPU X’s rq). e) Since X is lower than Z, CPU A unlocks CPU Z’s rq. Someone else has locked CPU X’s rq, and thus, CPU A must wait. 2) At CPU Z a) Previous task has completed execution and thus, CPU Z enters schedule, locks its own rq after CPU A releases it. b) CPU Z dequeues previous task and begins executing task p. c) CPU Z unlocks its rq. d) Task p yields the CPU (ex. by doing IO or waiting to acquire a lock) which triggers the schedule function on CPU Z. e) CPU Z enters schedule again, locks its own rq, and dequeues task p. f) As part of dequeue, it sets p.on_rq = 0 and unlocks its rq. 3) At CPU B a) CPU B enters try_to_wake_up with input task p. b) Since CPU Z dequeued task p, p.on_rq = 0, and CPU B updates B.state = WAKING. c) CPU B via select_task_rq determines CPU Y as the target CPU. 4) The race a) CPU A acquires CPU X’s lock and relocks CPU Z. b) CPU A reads task p.cpu = Z and incorrectly concludes task p is still on CPU Z. c) CPU A failed to notice task p had been dequeued from CPU Z while CPU A was waiting for locks in double_lock_balance. If CPU A knew that task p had been dequeued, it would return NULL forcing push_rt_task to give up the task p's migration. d) CPU B updates task p.cpu = Y and calls ttwu_queue. e) CPU B locks Ys rq. CPU B enqueues task p onto Y and sets task p.on_rq = 1. f) CPU B unlocks CPU Y, triggering memory synchronization. g) CPU A reads task p.on_rq = 1, cementing its assumption that task p has not migrated. h) CPU A decides to migrate p to CPU X. This leads to A dequeuing p from Y's queue and various crashes down the line. Solution ======== The solution here is fairly simple. After obtaining the lock (at 4a), the check is enhanced to make sure that the task is still at the head of the pushable tasks list. If not, then it is anyway not suitable for being pushed out. Testing ======= The fix is tested on a cluster of 3 nodes, where the panics due to this are hit every couple of days. A fix similar to this was deployed on such cluster and was stable for more than 30 days. Co-developed-by: Jon Kohler Signed-off-by: Jon Kohler Co-developed-by: Gauri Patwardhan Signed-off-by: Gauri Patwardhan Co-developed-by: Rahul Chunduru Signed-off-by: Rahul Chunduru Signed-off-by: Harshit Agarwal Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: "Steven Rostedt (Google)" Reviewed-by: Phil Auld Tested-by: Will Ton Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20250225180553.167995-1-harshit@nutanix.com

sched: Add RT_GROUP WARN checks for non-root task_groups

2025-04-08T18:55:54Z

With CONFIG_RT_GROUP_SCHED but runtime disabling of RT_GROUPs we expect the existence of the root task_group only and all rt_sched_entity'ies should be queued on root's rt_rq. If we get a non-root RT_GROUP something went wrong. Signed-off-by: Michal Koutný Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250310170442.504716-9-mkoutny@suse.com

sched: Do not construct nor expose RT_GROUP_SCHED structures if disabled

2025-04-08T18:55:54Z

Thanks to kernel cmdline being available early, before any cgroup hierarchy exists, we can achieve the RT_GROUP_SCHED boottime disabling goal by simply skipping any creation (and destruction) of RT_GROUP data and its exposure via RT attributes. We can do this thanks to previously placed runtime guards that would redirect all operations to root_task_group's data when RT_GROUP_SCHED disabled. Signed-off-by: Michal Koutný Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250310170442.504716-8-mkoutny@suse.com

sched: Bypass bandwitdh checks with runtime disabled RT_GROUP_SCHED

2025-04-08T18:55:54Z

When RT_GROUPs are compiled but not exposed, their bandwidth cannot be configured (and it is not initialized for non-root task_groups neither). Therefore bypass any checks of task vs task_group bandwidth. This will achieve behavior very similar to setups that have !CONFIG_RT_GROUP_SCHED and attach cpu controller to cgroup v2 hierarchy. (On a related note, this may allow having RT tasks with CONFIG_RT_GROUP_SCHED and cgroup v2 hierarchy.) Signed-off-by: Michal Koutný Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250310170442.504716-7-mkoutny@suse.com

sched: Skip non-root task_groups with disabled RT_GROUP_SCHED

2025-04-08T18:55:53Z

First, we want to prevent placement of RT tasks on non-root rt_rqs which we achieve in the task migration code that'd fall back to root_task_group's rt_rq. Second, we want to work with only root_task_group's rt_rq when iterating all "real" rt_rqs when RT_GROUP is disabled. To achieve this we keep root_task_group as the first one on the task_groups and break out quickly. Signed-off-by: Michal Koutný Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250310170442.504716-6-mkoutny@suse.com

sched: Always initialize rt_rq's task_group

2025-04-08T18:55:53Z

rt_rq->tg may be NULL which denotes the root task_group. Store the pointer to root_task_group directly so that callers may use rt_rq->tg homogenously. root_task_group exists always with CONFIG_CGROUPS_SCHED, CONFIG_RT_GROUP_SCHED depends on that. This changes root level rt_rq's default limit from infinity to the value of (originally) global RT throttling. Signed-off-by: Michal Koutný Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250310170442.504716-4-mkoutny@suse.com

sched: Remove unneeed macro wrap

2025-04-08T18:55:53Z

rt_entity_is_task has split definitions based on CONFIG_RT_GROUP_SCHED, therefore we can use it always. No functional change intended. Signed-off-by: Michal Koutný Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250310170442.504716-3-mkoutny@suse.com

sched: Convert CONFIG_RT_GROUP_SCHED macros to code conditions

2025-04-08T18:55:52Z

Convert the blocks guarded by macros to regular code so that the RT group code gets more compile validation. Reasoning is in Documentation/process/coding-style.rst 21) Conditional Compilation. With that, no functional change is expected. Signed-off-by: Michal Koutný Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20250310170442.504716-2-mkoutny@suse.com

Merge tag 'timers-cleanups-2025-03-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2025-03-25T17:54:15Z

Pull timer cleanups from Thomas Gleixner: "A treewide hrtimer timer cleanup hrtimers are initialized with hrtimer_init() and a subsequent store to the callback pointer. This turned out to be suboptimal for the upcoming Rust integration and is obviously a silly implementation to begin with. This cleanup replaces the hrtimer_init(T); T->function = cb; sequence with hrtimer_setup(T, cb); The conversion was done with Coccinelle and a few manual fixups. Once the conversion has completely landed in mainline, hrtimer_init() will be removed and the hrtimer::function becomes a private member" * tag 'timers-cleanups-2025-03-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (100 commits) wifi: rt2x00: Switch to use hrtimer_update_function() io_uring: Use helper function hrtimer_update_function() serial: xilinx_uartps: Use helper function hrtimer_update_function() ASoC: fsl: imx-pcm-fiq: Switch to use hrtimer_setup() RDMA: Switch to use hrtimer_setup() virtio: mem: Switch to use hrtimer_setup() drm/vmwgfx: Switch to use hrtimer_setup() drm/xe/oa: Switch to use hrtimer_setup() drm/vkms: Switch to use hrtimer_setup() drm/msm: Switch to use hrtimer_setup() drm/i915/request: Switch to use hrtimer_setup() drm/i915/uncore: Switch to use hrtimer_setup() drm/i915/pmu: Switch to use hrtimer_setup() drm/i915/perf: Switch to use hrtimer_setup() drm/i915/gvt: Switch to use hrtimer_setup() drm/i915/huc: Switch to use hrtimer_setup() drm/amdgpu: Switch to use hrtimer_setup() stm class: heartbeat: Switch to use hrtimer_setup() i2c: Switch to use hrtimer_setup() iio: Switch to use hrtimer_setup() ...

sched/debug: Make CONFIG_SCHED_DEBUG functionality unconditional

2025-03-19T21:20:53Z

All the big Linux distros enable CONFIG_SCHED_DEBUG, because the various features it provides help not just with kernel development, but with system administration and user-space software development as well. Reflect this reality and enable this functionality unconditionally. Signed-off-by: Ingo Molnar Tested-by: Shrikanth Hegde Cc: Peter Zijlstra Cc: Juri Lelli Cc: Vincent Guittot Cc: Dietmar Eggemann Cc: Steven Rostedt Cc: Ben Segall Cc: Mel Gorman Cc: Valentin Schneider Cc: Linus Torvalds Link: https://lore.kernel.org/r/20250317104257.3496611-4-mingo@kernel.org