Merge tag 'sched-core-2026-02-09' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler updates from Ingo Molnar:
 "Scheduler Kconfig space updates:

   - Further consolidate configurable preemption modes (Peter Zijlstra)

     Reduce the number of architectures that are allowed to offer
     PREEMPT_NONE and PREEMPT_VOLUNTARY, reducing the number of
     preemption models from four to just two: 'full' and 'lazy' on
     up-to-date architectures (arm64, loongarch, powerpc, riscv, s390,
     x86).

     None and voluntary are only available as legacy features on
     platforms that don't implement lazy preemption yet, or which don't
     even support preemption.

     The goal is to eventually remove cond_resched() and voluntary
     preemption altogether.

  RSEQ based 'scheduler time slice extension' support (Thomas Gleixner
  and Peter Zijlstra):

  This allows a thread to request a time slice extension when it enters
  a critical section to avoid contention on a resource when the thread
  is scheduled out inside of the critical section.

   - Add fields and constants for time slice extension
   - Provide static branch for time slice extensions
   - Add statistics for time slice extensions
   - Add prctl() to enable time slice extensions
   - Implement sys_rseq_slice_yield()
   - Implement syscall entry work for time slice extensions
   - Implement time slice extension enforcement timer
   - Reset slice extension when scheduled
   - Implement rseq_grant_slice_extension()
   - entry: Hook up rseq time slice extension
   - selftests: Implement time slice extension test
   - Allow registering RSEQ with slice extension
   - Move slice_ext_nsec to debugfs
   - Lower default slice extension
   - selftests/rseq: Add rseq slice histogram script

  Scheduler performance/scalability improvements:

   - Update rq->avg_idle when a task is moved to an idle CPU, which
     improves the scalability of various workloads (Shubhang Kaushik)

   - Reorder fields in 'struct rq' for better caching (Blake Jones)

   - Fair scheduler SMP NOHZ balancing code speedups (Shrikanth Hegde):
      - Move checking for nohz cpus after time check
      - Change likelyhood of nohz.nr_cpus
      - Remove nohz.nr_cpus and use weight of cpumask instead

   - Avoid false sharing for sched_clock_irqtime (Wangyang Guo)

   - Cleanups (Yury Norov):
      - Drop useless cpumask_empty() in find_energy_efficient_cpu()
      - Simplify task_numa_find_cpu()
      - Use cpumask_weight_and() in sched_balance_find_dst_group()

  DL scheduler updates:

   - Add a deadline server for sched_ext tasks (by Andrea Righi and Joel
     Fernandes, with fixes by Peter Zijlstra)

  RT scheduler updates:

   - Skip currently executing CPU in rto_next_cpu() (Chen Jinghuang)

  Entry code updates and performance improvements (Jinjie Ruan)

  This is part of the scheduler tree in this cycle due to inter-
  dependencies with the RSEQ based time slice extension work:

    - Remove unused syscall argument from syscall_trace_enter()
    - Rework syscall_exit_to_user_mode_work() for architecture reuse
    - Add arch_ptrace_report_syscall_entry/exit()
    - Inline syscall_exit_work() and syscall_trace_enter()

  Scheduler core updates (Peter Zijlstra):

   - Rework sched_class::wakeup_preempt() and rq_modified_*()
   - Avoid rq->lock bouncing in sched_balance_newidle()
   - Rename rcu_dereference_check_sched_domain() =>
            rcu_dereference_sched_domain()
   - <linux/compiler_types.h>: Add the __signed_scalar_typeof() helper

  Fair scheduler updates/refactoring (Peter Zijlstra and Ingo Molnar):

   - Fold the sched_avg update
   - Change rcu_dereference_check_sched_domain() to rcu-sched
   - Switch to rcu_dereference_all()
   - Remove superfluous rcu_read_lock()
   - Limit hrtick work
   - Join two #ifdef CONFIG_FAIR_GROUP_SCHED blocks
   - Clean up comments in 'struct cfs_rq'
   - Separate se->vlag from se->vprot
   - Rename cfs_rq::avg_load to cfs_rq::sum_weight
   - Rename cfs_rq::avg_vruntime to ::sum_w_vruntime & helper functions
   - Introduce and use the vruntime_cmp() and vruntime_op() wrappers for
     wrapped-signed aritmetics
   - Sort out 'blocked_load*' namespace noise

  Scheduler debugging code updates:

   - Export hidden tracepoints to modules (Gabriele Monaco)

   - Convert copy_from_user() + kstrtouint() to kstrtouint_from_user()
     (Fushuai Wang)

   - Add assertions to QUEUE_CLASS (Peter Zijlstra)

   - hrtimer: Fix tracing oddity (Thomas Gleixner)

  Misc fixes and cleanups:

   - Re-evaluate scheduling when migrating queued tasks out of throttled
     cgroups (Zicheng Qu)

   - Remove task_struct->faults_disabled_mapping (Christoph Hellwig)

   - Fix math notation errors in avg_vruntime comment (Zhan Xusheng)

   - sched/cpufreq: Use %pe format for PTR_ERR() printing
     (zenghongling)"

* tag 'sched-core-2026-02-09' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (64 commits)
  sched: Re-evaluate scheduling when migrating queued tasks out of throttled cgroups
  sched/cpufreq: Use %pe format for PTR_ERR() printing
  sched/rt: Skip currently executing CPU in rto_next_cpu()
  sched/clock: Avoid false sharing for sched_clock_irqtime
  selftests/sched_ext: Add test for DL server total_bw consistency
  selftests/sched_ext: Add test for sched_ext dl_server
  sched/debug: Fix dl_server (re)start conditions
  sched/debug: Add support to change sched_ext server params
  sched_ext: Add a DL server for sched_ext tasks
  sched/debug: Stop and start server based on if it was active
  sched/debug: Fix updating of ppos on server write ops
  sched/deadline: Clear the defer params
  entry: Inline syscall_exit_work() and syscall_trace_enter()
  entry: Add arch_ptrace_report_syscall_entry/exit()
  entry: Rework syscall_exit_to_user_mode_work() for architecture reuse
  entry: Remove unused syscall argument from syscall_trace_enter()
  sched: remove task_struct->faults_disabled_mapping
  sched: Update rq->avg_idle when a task is moved to an idle CPU
  selftests/rseq: Add rseq slice histogram script
  hrtimer: Fix trace oddity
  ...

2 files changed, 141 insertions, 0 deletions

diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index 6f0235ecc572..a68b1bea57a8 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -21,6 +21,7 @@ System calls
    ebpf/index
    ioctl/index
    mseal
+   rseq
 
 Security-related interfaces
 ===========================
diff --git a/Documentation/userspace-api/rseq.rst b/Documentation/userspace-api/rseq.rst
new file mode 100644
index 000000000000..3cd27a3c7c7e
--- /dev/null
+++ b/Documentation/userspace-api/rseq.rst
@@ -0,0 +1,140 @@
+=====================
+Restartable Sequences
+=====================
+
+Restartable Sequences allow to register a per thread userspace memory area
+to be used as an ABI between kernel and userspace for three purposes:
+
+ * userspace restartable sequences
+
+ * quick access to read the current CPU number, node ID from userspace
+
+ * scheduler time slice extensions
+
+Restartable sequences (per-cpu atomics)
+---------------------------------------
+
+Restartable sequences allow userspace to perform update operations on
+per-cpu data without requiring heavyweight atomic operations. The actual
+ABI is unfortunately only available in the code and selftests.
+
+Quick access to CPU number, node ID
+-----------------------------------
+
+Allows to implement per CPU data efficiently. Documentation is in code and
+selftests. :(
+
+Scheduler time slice extensions
+-------------------------------
+
+This allows a thread to request a time slice extension when it enters a
+critical section to avoid contention on a resource when the thread is
+scheduled out inside of the critical section.
+
+The prerequisites for this functionality are:
+
+    * Enabled in Kconfig
+
+    * Enabled at boot time (default is enabled)
+
+    * A rseq userspace pointer has been registered for the thread
+
+The thread has to enable the functionality via prctl(2)::
+
+    prctl(PR_RSEQ_SLICE_EXTENSION, PR_RSEQ_SLICE_EXTENSION_SET,
+          PR_RSEQ_SLICE_EXT_ENABLE, 0, 0);
+
+prctl() returns 0 on success or otherwise with the following error codes:
+
+========= ==============================================================
+Errorcode Meaning
+========= ==============================================================
+EINVAL	  Functionality not available or invalid function arguments.
+          Note: arg4 and arg5 must be zero
+ENOTSUPP  Functionality was disabled on the kernel command line
+ENXIO	  Available, but no rseq user struct registered
+========= ==============================================================
+
+The state can be also queried via prctl(2)::
+
+  prctl(PR_RSEQ_SLICE_EXTENSION, PR_RSEQ_SLICE_EXTENSION_GET, 0, 0, 0);
+
+prctl() returns ``PR_RSEQ_SLICE_EXT_ENABLE`` when it is enabled or 0 if
+disabled. Otherwise it returns with the following error codes:
+
+========= ==============================================================
+Errorcode Meaning
+========= ==============================================================
+EINVAL	  Functionality not available or invalid function arguments.
+          Note: arg3 and arg4 and arg5 must be zero
+========= ==============================================================
+
+The availability and status is also exposed via the rseq ABI struct flags
+field via the ``RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT`` and the
+``RSEQ_CS_FLAG_SLICE_EXT_ENABLED_BIT``. These bits are read-only for user
+space and only for informational purposes.
+
+If the mechanism was enabled via prctl(), the thread can request a time
+slice extension by setting rseq::slice_ctrl::request to 1. If the thread is
+interrupted and the interrupt results in a reschedule request in the
+kernel, then the kernel can grant a time slice extension and return to
+userspace instead of scheduling out. The length of the extension is
+determined by debugfs:rseq/slice_ext_nsec. The default value is 5 usec; which
+is the minimum value. It can be incremented to 50 usecs, however doing so
+can/will affect the minimum scheduling latency.
+
+Any proposed changes to this default will have to come with a selftest and
+rseq-slice-hist.py output that shows the new value has merrit.
+
+The kernel indicates the grant by clearing rseq::slice_ctrl::request and
+setting rseq::slice_ctrl::granted to 1. If there is a reschedule of the
+thread after granting the extension, the kernel clears the granted bit to
+indicate that to userspace.
+
+If the request bit is still set when the leaving the critical section,
+userspace can clear it and continue.
+
+If the granted bit is set, then userspace invokes rseq_slice_yield(2) when
+leaving the critical section to relinquish the CPU. The kernel enforces
+this by arming a timer to prevent misbehaving userspace from abusing this
+mechanism.
+
+If both the request bit and the granted bit are false when leaving the
+critical section, then this indicates that a grant was revoked and no
+further action is required by userspace.
+
+The required code flow is as follows::
+
+    rseq->slice_ctrl.request = 1;
+    barrier();  // Prevent compiler reordering
+    critical_section();
+    barrier();  // Prevent compiler reordering
+    rseq->slice_ctrl.request = 0;
+    if (rseq->slice_ctrl.granted)
+        rseq_slice_yield();
+
+As all of this is strictly CPU local, there are no atomicity requirements.
+Checking the granted state is racy, but that cannot be avoided at all::
+
+    if (rseq->slice_ctrl.granted)
+      -> Interrupt results in schedule and grant revocation
+        rseq_slice_yield();
+
+So there is no point in pretending that this might be solved by an atomic
+operation.
+
+If the thread issues a syscall other than rseq_slice_yield(2) within the
+granted timeslice extension, the grant is also revoked and the CPU is
+relinquished immediately when entering the kernel. This is required as
+syscalls might consume arbitrary CPU time until they reach a scheduling
+point when the preemption model is either NONE or VOLUNTARY and therefore
+might exceed the grant by far.
+
+The preferred solution for user space is to use rseq_slice_yield(2) which
+is side effect free. The support for arbitrary syscalls is required to
+support onion layer architectured applications, where the code handling the
+critical section and requesting the time slice extension has no control
+over the code within the critical section.
+
+The kernel enforces flag consistency and terminates the thread with SIGSEGV
+if it detects a violation.