linux-stable.git/kernel/Kconfig.preempt, branch v7.2.6

sched_ext: Make SCHED_CLASS_EXT select GENERIC_ALLOCATOR

2026-09-14T11:40:47+00:00

[ Upstream commit 524ab50763af33d65e6e042cf7034cd8f82d437b ]

kernel/sched/ext/arena.c uses the gen_pool allocator, which is built only
when GENERIC_ALLOCATOR is set. SCHED_CLASS_EXT doesn't select it, so on
configs where nothing else does, the build fails to link:

  build_policy.o: undefined reference to `gen_pool_create'
  build_policy.o: undefined reference to `gen_pool_for_each_chunk'
  build_policy.o: undefined reference to `gen_pool_destroy'

Fixes: 9eca087deb0b ("sched_ext: Sub-allocator over kernel-claimed BPF arena pages")
Reported-by: kernel test robot 
Closes: https://lore.kernel.org/oe-kbuild-all/202608151315.tvN3X0Oq-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202608151632.3p91bTQj-lkp@intel.com/
Signed-off-by: Tejun Heo 
Signed-off-by: Sasha Levin

sched: Further restrict the preemption modes

2026-01-08T11:43:57+00:00

The introduction of PREEMPT_LAZY was for multiple reasons:

  - PREEMPT_RT suffered from over-scheduling, hurting performance compared to
    !PREEMPT_RT.

  - the introduction of (more) features that rely on preemption; like
    folio_zero_user() which can do large memset() without preemption checks.

    (Xen already had a horrible hack to deal with long running hypercalls)

  - the endless and uncontrolled sprinkling of cond_resched() -- mostly cargo
    cult or in response to poor to replicate workloads.

By moving to a model that is fundamentally preemptable these things become
managable and avoid needing to introduce more horrible hacks.

Since this is a requirement; limit PREEMPT_NONE to architectures that do not
support preemption at all. Further limit PREEMPT_VOLUNTARY to those
architectures that do not yet have PREEMPT_LAZY support (with the eventual goal
to make this the empty set and completely remove voluntary preemption and
cond_resched() -- notably VOLUNTARY is already limited to !ARCH_NO_PREEMPT.)

This leaves up-to-date architectures (arm64, loongarch, powerpc, riscv, s390,
x86) with only two preemption models: full and lazy.

While Lazy has been the recommended setting for a while, not all distributions
have managed to make the switch yet. Force things along. Keep the patch minimal
in case of hard to address regressions that might pop up.

Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Valentin Schneider 
Link: https://patch.msgid.link/20251219101502.GB1132199@noisy.programming.kicks-ass.net

softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT

2025-09-17T14:25:41+00:00

softirqs are preemptible on PREEMPT_RT. There is synchronisation between
individual sections which disable bottom halves. This in turn means that
a forced threaded interrupt cannot preempt another forced threaded
interrupt. Instead it will PI-boost the other handler and wait for its
completion.

This is required because code within a softirq section is assumed to be
non-preemptible and may expect exclusive access to per-CPU resources
such as variables or pinned timers.

Code with such expectation has been identified and updated to use
local_lock_nested_bh() for locking of the per-CPU resource. This means the
softirq lock can be removed.

Disable the softirq synchronization, but add a new config switch
CONFIG_PREEMPT_RT_NEEDS_BH_LOCK which allows to re-enable the synchronized
behavior in case that there are issues, which haven't been detected yet.

The softirq_ctrl.cnt accounting remains to let the NOHZ code know if
softirqs are currently handled.

Signed-off-by: Sebastian Andrzej Siewior 
Signed-off-by: Thomas Gleixner

sched: No PREEMPT_RT=y for all{yes,mod}config

2024-11-07T14:25:05+00:00

While PREEMPT_RT is undoubtedly totally awesome, it does not, at this
time, make sense to have all{yes,mod}config select it.

Reported-by: Stephen Rothwell 
Fixes: 35772d627b55 ("sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT")
Signed-off-by: Peter Zijlstra (Intel)

sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT

2024-11-05T11:55:38+00:00

In order to enable PREEMPT_DYNAMIC for PREEMPT_RT, remove PREEMPT_RT
from the 'Preemption Model' choice. Strictly speaking PREEMPT_RT is
not a change in how preemption works, but rather it makes a ton more
code preemptible.

Notably, take away NONE and VOLUNTARY options for PREEMPT_RT, they make
no sense (but are techincally possible).

Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Sebastian Andrzej Siewior 
Link: https://lkml.kernel.org/r/20241007075055.441622332@infradead.org

sched: Add Lazy preemption model

2024-11-05T11:55:38+00:00

Change fair to use resched_curr_lazy(), which, when the lazy
preemption model is selected, will set TIF_NEED_RESCHED_LAZY.

This LAZY bit will be promoted to the full NEED_RESCHED bit on tick.
As such, the average delay between setting LAZY and actually
rescheduling will be TICK_NSEC/2.

In short, Lazy preemption will delay preemption for fair class but
will function as Full preemption for all the other classes, most
notably the realtime (RR/FIFO/DEADLINE) classes.

The goal is to bridge the performance gap with Voluntary, such that we
might eventually remove that option entirely.

Suggested-by: Thomas Gleixner 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Sebastian Andrzej Siewior 
Link: https://lkml.kernel.org/r/20241007075055.331243614@infradead.org

sched_ext: Build fix on !CONFIG_STACKTRACE[_SUPPORT]

2024-08-01T17:08:01+00:00

scx_dump_task() uses stack_trace_save_tsk() which is only available when
CONFIG_STACKTRACE. Make CONFIG_SCHED_CLASS_EXT select CONFIG_STACKTRACE if
the support is available and skip capturing stack trace if
!CONFIG_STACKTRACE.

Signed-off-by: Tejun Heo 
Reported-by: kernel test robot 
Closes: https://lore.kernel.org/oe-kbuild-all/202407161844.reewQQrR-lkp@intel.com/
Acked-by: David Vernet 
Signed-off-by: Tejun Heo

sched_ext: add CONFIG_DEBUG_INFO_BTF dependency

2024-06-27T19:06:47+00:00

Without BTF, attempting to load any sched_ext scheduler will result in
an error like the following:

  libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?

This makes sched_ext pretty much unusable, so explicitly depend on
CONFIG_DEBUG_INFO_BTF to prevent these issues.

Signed-off-by: Andrea Righi 
Signed-off-by: Tejun Heo

sched_ext: Documentation: scheduler: Document extensible scheduler class

2024-06-18T20:09:21+00:00

Add Documentation/scheduler/sched-ext.rst which gives a high-level overview
and pointers to the examples.

v6: - Add paragraph explaining debug dump.

v5: - Updated to reflect /sys/kernel interface change. Kconfig options
      added.

v4: - README improved, reformatted in markdown and renamed to README.md.

v3: - Added tools/sched_ext/README.

    - Dropped _example prefix from scheduler names.

v2: - Apply minor edits suggested by Bagas. Caveats section dropped as all
      of them are addressed.

Signed-off-by: Tejun Heo 
Reviewed-by: David Vernet 
Acked-by: Josh Don 
Acked-by: Hao Luo 
Acked-by: Barret Rhoden 
Cc: Bagas Sanjaya

sched_ext: Implement core-sched support

2024-06-18T20:09:20+00:00

The core-sched support is composed of the following parts:

- task_struct->scx.core_sched_at is added. This is a timestamp which can be
  used to order tasks. Depending on whether the BPF scheduler implements
  custom ordering, it tracks either global FIFO ordering of all tasks or
  local-DSQ ordering within the dispatched tasks on a CPU.

- prio_less() is updated to call scx_prio_less() when comparing SCX tasks.
  scx_prio_less() calls ops.core_sched_before() if available or uses the
  core_sched_at timestamp. For global FIFO ordering, the BPF scheduler
  doesn't need to do anything. Otherwise, it should implement
  ops.core_sched_before() which reflects the ordering.

- When core-sched is enabled, balance_scx() balances all SMT siblings so
  that they all have tasks dispatched if necessary before pick_task_scx() is
  called. pick_task_scx() picks between the current task and the first
  dispatched task on the local DSQ based on availability and the
  core_sched_at timestamps. Note that FIFO ordering is expected among the
  already dispatched tasks whether running or on the local DSQ, so this path
  always compares core_sched_at instead of calling into
  ops.core_sched_before().

qmap_core_sched_before() is added to scx_qmap. It scales the
distances from the heads of the queues to compare the tasks across different
priority queues and seems to behave as expected.

v3: Fixed build error when !CONFIG_SCHED_SMT reported by Andrea Righi.

v2: Sched core added the const qualifiers to prio_less task arguments.
    Explicitly drop them for ops.core_sched_before() task arguments. BPF
    enforces access control through the verifier, so the qualifier isn't
    actually operative and only gets in the way when interacting with
    various helpers.

Signed-off-by: Tejun Heo 
Reviewed-by: David Vernet 
Reviewed-by: Josh Don 
Cc: Andrea Righi