When a cfs_rq is to be throttled, its limbo list should be empty and
that's why there is a warn in tg_throttle_down() for non empty
cfs_rq->throttled_limbo_list.

When running a test with the following hierarchy:

          root
        /      \
        A*     ...
     /  |  \   ...
        B
       /  \
      C*

where both A and C have quota settings, that warn on non empty limbo list
is triggered for a cfs_rq of C, let's call it cfs_rq_c(and ignore the cpu
part of the cfs_rq for the sake of simpler representation).

Debug showed it happened like this:
Task group C is created and quota is set, so in tg_set_cfs_bandwidth(),
cfs_rq_c is initialized with runtime_enabled set, runtime_remaining
equals to 0 and *unthrottled*. Before any tasks are enqueued to cfs_rq_c,
*multiple* throttled tasks can migrate to cfs_rq_c (e.g., due to task
group changes). When enqueue_task_fair(cfs_rq_c, throttled_task) is
called and cfs_rq_c is in a throttled hierarchy (e.g., A is throttled),
these throttled tasks are directly placed into cfs_rq_c's limbo list by
enqueue_throttled_task().

Later, when A is unthrottled, tg_unthrottle_up(cfs_rq_c) enqueues these
tasks. The first enqueue triggers check_enqueue_throttle(), and with zero
runtime_remaining, cfs_rq_c can be throttled in throttle_cfs_rq() if it
can't get more runtime and enters tg_throttle_down(), where the warning
is hit due to remaining tasks in the limbo list.

I think it's a chaos to trigger throttle on unthrottle path, the status
of a being unthrottled cfs_rq can be in a mixed state in the end, so fix
this by granting 1ns to cfs_rq in tg_set_cfs_bandwidth(). This ensures
cfs_rq_c has a positive runtime_remaining when initialized as unthrottled
and cannot enter tg_unthrottle_up() with zero runtime_remaining.

Also, update outdated comments in tg_throttle_down() since
unthrottle_cfs_rq() is no longer called with zero runtime_remaining.
While at it, remove a redundant assignment to se in tg_throttle_down().

Fixes: e1fad12dcb66 ("sched/fair: Switch to task based throttle model")
Reviewed-By: Benjamin Segall <bsegall@google.com>
Suggested-by: Benjamin Segall <bsegall@google.com>
Signed-off-by: Aaron Lu <ziqianlu@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Hao Jia <jiahao1@lixiang.com>
Link: https://patch.msgid.link/20251030032755.560-1-ziqianlu@bytedance.com

IBM CI tool reported kernel warning[1] when running a CPU removal
operation through drmgr[2]. i.e "drmgr -c cpu -r -q 1"

WARNING: CPU: 0 PID: 0 at kernel/sched/cpudeadline.c:219 cpudl_set+0x58/0x170
NIP [c0000000002b6ed8] cpudl_set+0x58/0x170
LR [c0000000002b7cb8] dl_server_timer+0x168/0x2a0
Call Trace:
[c000000002c2f8c0] init_stack+0x78c0/0x8000 (unreliable)
[c0000000002b7cb8] dl_server_timer+0x168/0x2a0
[c00000000034df84] __hrtimer_run_queues+0x1a4/0x390
[c00000000034f624] hrtimer_interrupt+0x124/0x300
[c00000000002a230] timer_interrupt+0x140/0x320

Git bisects to: commit 4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")

This happens since:
- dl_server hrtimer gets enqueued close to cpu offline, when
  kthread_park enqueues a fair task.
- CPU goes offline and drmgr removes it from cpu_present_mask.
- hrtimer fires and warning is hit.

Fix it by stopping the dl_server before CPU is marked dead.

[1]: https://lore.kernel.org/all/8218e149-7718-4432-9312-f97297c352b9@linux.ibm.com/
[2]: https://github.com/ibm-power-utilities/powerpc-utils/tree/next/src/drmgr

[sshegde: wrote the changelog and tested it]
Fixes: 4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")
Closes: https://lore.kernel.org/all/8218e149-7718-4432-9312-f97297c352b9@linux.ibm.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>

Pull timer core updates from Thomas Gleixner:

 - Address the inconsistent shutdown sequence of per CPU clockevents on
   CPU hotplug, which only removed it from the core but failed to invoke
   the actual device driver shutdown callback. This kept the timer
   active, which prevented power savings and caused pointless noise in
   virtualization.

 - Encapsulate the open coded access to the hrtimer clock base, which is
   a private implementation detail, so that the implementation can be
   changed without breaking a lot of usage sites.

 - Enhance the debug output of the clocksource watchdog to provide
   better information for analysis.

 - The usual set of cleanups and enhancements all over the place

* tag 'timers-core-2025-09-29' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  time: Fix spelling mistakes in comments
  clocksource: Print durations for sync check unconditionally
  LoongArch: Remove clockevents shutdown call on offlining
  tick: Do not set device to detached state in tick_shutdown()
  hrtimer: Reorder branches in hrtimer_clockid_to_base()
  hrtimer: Remove hrtimer_clock_base:: Get_time
  hrtimer: Use hrtimer_cb_get_time() helper
  media: pwm-ir-tx: Avoid direct access to hrtimer clockbase
  ALSA: hrtimer: Avoid direct access to hrtimer clockbase
  lib: test_objpool: Avoid direct access to hrtimer clockbase
  sched/core: Avoid direct access to hrtimer clockbase
  timers/itimer: Avoid direct access to hrtimer clockbase
  posix-timers: Avoid direct access to hrtimer clockbase
  jiffies: Remove obsolete SHIFTED_HZ comment

Pull scheduler updates from Ingo Molnar:
 "Core scheduler changes:

   - Make migrate_{en,dis}able() inline, to improve performance
     (Menglong Dong)

   - Move STDL_INIT() functions out-of-line (Peter Zijlstra)

   - Unify the SCHED_{SMT,CLUSTER,MC} Kconfig (Peter Zijlstra)

  Fair scheduling:

   - Defer throttling to when tasks exit to user-space, to reduce the
     chance & impact of throttle-preemption with held locks and other
     resources (Aaron Lu, Valentin Schneider)

   - Get rid of sched_domains_curr_level hack for tl->cpumask(), as the
     warning was getting triggered on certain topologies (Peter
     Zijlstra)

  Misc cleanups & fixes:

   - Header cleanups (Menglong Dong)

   - Fix race in push_dl_task() (Harshit Agarwal)"

* tag 'sched-core-2025-09-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched: Fix some typos in include/linux/preempt.h
  sched: Make migrate_{en,dis}able() inline
  rcu: Replace preempt.h with sched.h in include/linux/rcupdate.h
  arch: Add the macro COMPILE_OFFSETS to all the asm-offsets.c
  sched/fair: Do not balance task to a throttled cfs_rq
  sched/fair: Do not special case tasks in throttled hierarchy
  sched/fair: update_cfs_group() for throttled cfs_rqs
  sched/fair: Propagate load for throttled cfs_rq
  sched/fair: Get rid of throttled_lb_pair()
  sched/fair: Task based throttle time accounting
  sched/fair: Switch to task based throttle model
  sched/fair: Implement throttle task work and related helpers
  sched/fair: Add related data structure for task based throttle
  sched: Unify the SCHED_{SMT,CLUSTER,MC} Kconfig
  sched: Move STDL_INIT() functions out-of-line
  sched/fair: Get rid of sched_domains_curr_level hack for tl->cpumask()
  sched/deadline: Fix race in push_dl_task()

Pull sched_ext updates from Tejun Heo:

 - Code organization cleanup. Separate internal types and accessors to
   ext_internal.h to reduce the size of ext.c and improve
   maintainability.

 - Prepare for cgroup sub-scheduler support by adding @sch parameter to
   various functions and helpers, reorganizing scheduler instance
   handling, and dropping obsolete helpers like scx_kf_exit() and
   kf_cpu_valid().

 - Add new scx_bpf_cpu_curr() and scx_bpf_locked_rq() BPF helpers to
   provide safer access patterns with proper RCU protection.
   scx_bpf_cpu_rq() is deprecated with warnings due to potential race
   conditions.

 - Improve debugging with migration-disabled counter in error state
   dumps, SCX_EFLAG_INITIALIZED flag, bitfields for warning flags, and
   other enhancements to help diagnose issues.

 - Use cgroup_lock/unlock() for cgroup synchronization instead of
   scx_cgroup_rwsem based synchronization. This is simpler and allows
   enable/disable paths to synchronize against cgroup changes
   independent of the CPU controller.

 - rhashtable_lookup() replacement to avoid redundant RCU locking was
   reverted due to RCU usage warnings. Will be redone once rhashtable is
   updated to use rcu_dereference_all().

 - Other misc updates and fixes including bypass handling improvements,
   scx_task_iter_relock() improvements, tools/sched_ext updates, and
   compatibility helpers.

* tag 'sched_ext-for-6.18' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext: (28 commits)
  Revert "sched_ext: Use rhashtable_lookup() instead of rhashtable_lookup_fast()"
  sched_ext: Misc updates around scx_sched instance pointer
  sched_ext: Drop scx_kf_exit() and scx_kf_error()
  sched_ext: Add the @sch parameter to scx_dsq_insert_preamble/commit()
  sched_ext: Drop kf_cpu_valid()
  sched_ext: Add the @sch parameter to ext_idle helpers
  sched_ext: Add the @sch parameter to __bstr_format()
  sched_ext: Separate out scx_kick_cpu() and add @sch to it
  tools/sched_ext: scx_qmap: Make debug output quieter by default
  sched_ext: Make qmap dump operation non-destructive
  sched_ext: Add SCX_EFLAG_INITIALIZED to indicate successful ops.init()
  sched_ext: Use bitfields for boolean warning flags
  sched_ext: Fix stray scx_root usage in task_can_run_on_remote_rq()
  sched_ext: Improve SCX_KF_DISPATCH comment
  sched_ext: Use rhashtable_lookup() instead of rhashtable_lookup_fast()
  sched_ext: Verify RCU protection in scx_bpf_cpu_curr()
  sched_ext: Add migration-disabled counter to error state dump
  sched_ext: Fix NULL dereference in scx_bpf_cpu_rq() warning
  tools/sched_ext: Add compat helper for scx_bpf_cpu_curr()
  sched_ext: deprecation warn for scx_bpf_cpu_rq()
  ...

Pull copy_process updates from Christian Brauner:
 "This contains the changes to enable support for clone3() on nios2
  which apparently is still a thing.

  The more exciting part of this is that it cleans up the inconsistency
  in how the 64-bit flag argument is passed from copy_process() into the
  various other copy_*() helpers"

[ Fixed up rv ltl_monitor 32-bit support as per Sasha Levin in the merge ]

* tag 'kernel-6.18-rc1.clone3' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  nios2: implement architecture-specific portion of sys_clone3
  arch: copy_thread: pass clone_flags as u64
  copy_process: pass clone_flags as u64 across calltree
  copy_sighand: Handle architectures where sizeof(unsigned long) < sizeof(u64)

For now, migrate_enable and migrate_disable are global, which makes them
become hotspots in some case. Take BPF for example, the function calling
to migrate_enable and migrate_disable in BPF trampoline can introduce
significant overhead, and following is the 'perf top' of FENTRY's
benchmark (./tools/testing/selftests/bpf/bench trig-fentry):

  54.63% bpf_prog_2dcccf652aac1793_bench_trigger_fentry [k]
                 bpf_prog_2dcccf652aac1793_bench_trigger_fentry
  10.43% [kernel] [k] migrate_enable
  10.07% bpf_trampoline_6442517037 [k] bpf_trampoline_6442517037
  8.06% [kernel] [k] __bpf_prog_exit_recur
  4.11% libc.so.6 [.] syscall
  2.15% [kernel] [k] entry_SYSCALL_64
  1.48% [kernel] [k] memchr_inv
  1.32% [kernel] [k] fput
  1.16% [kernel] [k] _copy_to_user
  0.73% [kernel] [k] bpf_prog_test_run_raw_tp

So in this commit, we make migrate_enable/migrate_disable inline to obtain
better performance. The struct rq is defined internally in
kernel/sched/sched.h, and the field "nr_pinned" is accessed in
migrate_enable/migrate_disable, which makes it hard to make them inline.

Alexei Starovoitov suggests to generate the offset of "nr_pinned" in [1],
so we can define the migrate_enable/migrate_disable in
include/linux/sched.h and access "this_rq()->nr_pinned" with
"(void *)this_rq() + RQ_nr_pinned".

The offset of "nr_pinned" is generated in include/generated/rq-offsets.h
by kernel/sched/rq-offsets.c.

Generally speaking, we move the definition of migrate_enable and
migrate_disable to include/linux/sched.h from kernel/sched/core.c. The
calling to __set_cpus_allowed_ptr() is leaved in ___migrate_enable().

The "struct rq" is not available in include/linux/sched.h, so we can't
access the "runqueues" with this_cpu_ptr(), as the compilation will fail
in this_cpu_ptr() -> raw_cpu_ptr() -> __verify_pcpu_ptr():
  typeof((ptr) + 0)

So we introduce the this_rq_raw() and access the runqueues with
arch_raw_cpu_ptr/PERCPU_PTR directly.

The variable "runqueues" is not visible in the kernel modules, and export
it is not a good idea. As Peter Zijlstra advised in [2], we define and
export migrate_enable/migrate_disable in kernel/sched/core.c too, and use
them for the modules.

Before this patch, the performance of BPF FENTRY is:

  fentry         :  113.030 ± 0.149M/s
  fentry         :  112.501 ± 0.187M/s
  fentry         :  112.828 ± 0.267M/s
  fentry         :  115.287 ± 0.241M/s

After this patch, the performance of BPF FENTRY increases to:

  fentry         :  143.644 ± 0.670M/s
  fentry         :  149.764 ± 0.362M/s
  fentry         :  149.642 ± 0.156M/s
  fentry         :  145.263 ± 0.221M/s

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/bpf/CAADnVQ+5sEDKHdsJY5ZsfGDO_1SEhhQWHrt2SMBG5SYyQ+jt7w@mail.gmail.com/ [1]
Link: https://lore.kernel.org/all/20250819123214.GH4067720@noisy.programming.kicks-ass.net/ [2]

Pull sched_ext/for-6.17-fixes to receive:

 55ed11b181c4 ("sched_ext: idle: Handle migration-disabled tasks in BPF code")

which conflicts with the following commit in for-6.18:

 2407bae23d1e ("sched_ext: Add the @sch parameter to ext_idle helpers")

The conflict is a simple context conflict which can be resolved by taking
the updated parts from both commits.

Signed-off-by: Tejun Heo <tj@kernel.org>

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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509170230.MwZsJSWa-lkp@intel.com/
Tested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

The field timer->base->get_time is a private implementation detail and
should not be accessed outside of the hrtimer core.

Switch to the equivalent helper.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20250821-hrtimer-cleanup-get_time-v2-3-3ae822e5bfbd@linutronix.de