linux-stable.git/kernel/sched/stop_task.c, branch v6.18.51

sched: Rework prev_balance() to avoid stale prev references

2026-09-11T09:49:40+00:00

[ Upstream commit 7a3a6bfbd62a2ba3e0ef1e92d6b71abb66890825 ]

Historically, the prev value from __schedule() was the rq->curr.
This prev value is passed down through numerous functions, and
used in the class scheduler implementations. The fact that
prev was on_cpu until the end of __schedule(), meant it was
stable across the rq lock drops that the class->balance()
implementations often do.

However, with proxy-exec, the prev passed to functions called
by __schedule() is rq->donor, which may not be the same as
rq->curr and may not be on_cpu, this makes the prev value
potentially unstable across rq lock drops.

A recently found issue with proxy-exec, is when we begin doing
return migration from try_to_wake_up(), its possible we may be
waking up the rq->donor.  When we do this, we proxy_resched_idle()
to put_prev_set_next() setting the rq->donor to rq->idle, allowing
the rq->donor to be return migrated and allowed to run.

This however runs into trouble, as on another cpu we might be in
the middle of calling __schedule(). Conceptually the rq lock is
held for the majority of the time, but in calling prev_balance()
its possible the class->balance() handler call may briefly drop the rq lock.
This opens a window for try_to_wake_up() to wake and return migrate the
rq->donor before the class logic reacquires the rq lock.

Unfortunately prev_balance() pass in a prev argument, to which we pass
rq->donor. However this prev value can now become stale and incorrect across a
rq lock drop.

So, to correct this, rework the prev_balance() call so that it does not take a
"prev" argument.

Signed-off-by: John Stultz 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20260512025635.2840817-2-jstultz@google.com

Backport adaptation for 6.18:
Keep the existing sched_class::pick_next_task callbacks, pick_task
interfaces, SCX balancing setup and proxy-execution flow. The later
upstream scheduler refactors and lock annotations are not prerequisites
for this dependency.

Convert all six balance callbacks, including the fair and SCX callbacks
still present here, to the new signature. Read rq->donor at the existing
selection and balance call sites, and refresh prev after new-idle
balancing in pick_next_task_fair() because that path still drops the rq
lock inside the stable pick_next_task implementation.

This preserves the existing functions and supplies the selection context
needed for f3629c63a4af (sched/core: Make core-sched flips wait for
in-flight selections) to apply without changes.

Stable-dep-of: f3629c63a4af ("sched/core: Make core-sched flips wait for in-flight selections")
Signed-off-by: Sasha Levin 
Signed-off-by: Greg Kroah-Hartman

sched/smp: Use the SMP version of the stop-CPU scheduling class

2025-06-13T06:47:21+00:00

Simplify the scheduler by making CONFIG_SMP=y code in the stop-CPU
scheduling class unconditional.

Signed-off-by: Ingo Molnar 
Acked-by: Peter Zijlstra 
Cc: Dietmar Eggemann 
Cc: Juri Lelli 
Cc: Linus Torvalds 
Cc: Mel Gorman 
Cc: Sebastian Andrzej Siewior 
Cc: Shrikanth Hegde 
Cc: Steven Rostedt 
Cc: Valentin Schneider 
Cc: Vincent Guittot 
Link: https://lore.kernel.org/r/20250528080924.2273858-36-mingo@kernel.org

sched: Make clangd usable

2025-06-11T09:20:53+00:00

Due to the weird Makefile setup of sched the various files do not
compile as stand alone units. The new generation of editors are trying
to do just this -- mostly to offer fancy things like completions but
also better syntax highlighting and code navigation.

Specifically, I've been playing around with neovim and clangd.

Setting up clangd on the kernel source is a giant pain in the arse
(this really should be improved), but once you do manage, you run into
dumb stuff like the above.

Fix up the scheduler files to at least pretend to work.

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Ingo Molnar 
Tested-by: Juri Lelli 
Link: https://lkml.kernel.org/r/20250523164348.GN39944@noisy.programming.kicks-ass.net

sched: Add put_prev_task(.next)

2024-09-03T13:26:32+00:00

In order to tell the previous sched_class what the next task is, add
put_prev_task(.next).

Notable SCX will use this to:

 1) determine the next task will leave the SCX sched class and push
    the current task to another CPU if possible.
 2) statistics on how often and which other classes preempt it

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lore.kernel.org/r/20240813224016.367421076@infradead.org

sched: Rework pick_next_task()

2024-09-03T13:26:31+00:00

The current rule is that:

  pick_next_task() := pick_task() + set_next_task(.first = true)

And many classes implement it directly as such. Change things around
to make pick_next_task() optional while also changing the definition to:

  pick_next_task(prev) := pick_task() + put_prev_task() + set_next_task(.first = true)

The reason is that sched_ext would like to have a 'final' call that
knows the next task. By placing put_prev_task() right next to
set_next_task() (as it already is for sched_core) this becomes
trivial.

As a bonus, this is a nice cleanup on its own.

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lore.kernel.org/r/20240813224016.051225657@infradead.org

sched: Allow sched_class::dequeue_task() to fail

2024-08-17T09:06:41+00:00

Change the function signature of sched_class::dequeue_task() to return
a boolean, allowing future patches to 'fail' dequeue.

Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Valentin Schneider 
Tested-by: Valentin Schneider 
Link: https://lkml.kernel.org/r/20240727105028.864630153@infradead.org

sched: Unify runtime accounting across classes

2023-11-15T08:57:48+00:00

All classes use sched_entity::exec_start to track runtime and have
copies of the exact same code around to compute runtime.

Collapse all that.

Signed-off-by: Peter Zijlstra (Intel) 
Signed-off-by: Daniel Bristot de Oliveira 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Phil Auld 
Reviewed-by: Valentin Schneider 
Reviewed-by: Steven Rostedt (Google) 
Link: https://lkml.kernel.org/r/54d148a144f26d9559698c4dd82d8859038a7380.1699095159.git.bristot@kernel.org

sched/fair: Rename check_preempt_curr() to wakeup_preempt()

2023-09-19T08:40:10+00:00

The name is a bit opaque - make it clear that this is about wakeup
preemption.

Also rename the ->check_preempt_curr() methods similarly.

Signed-off-by: Ingo Molnar 
Acked-by: Peter Zijlstra (Intel)

sched: Add update_current_exec_runtime helper

2022-08-26T22:05:35+00:00

Wrap repeated code in helper function update_current_exec_runtime for
update the exec time of the current.

Signed-off-by: Shang XiaoJing 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lkml.kernel.org/r/20220824082856.15674-1-shangxiaojing@huawei.com

sched/headers: Introduce kernel/sched/build_utility.c and build multiple .c files there

2022-02-23T09:58:33+00:00

Collect all utility functionality source code files into a single kernel/sched/build_utility.c file,
via #include-ing the .c files:

    kernel/sched/clock.c
    kernel/sched/completion.c
    kernel/sched/loadavg.c
    kernel/sched/swait.c
    kernel/sched/wait_bit.c
    kernel/sched/wait.c

CONFIG_CPU_FREQ:
    kernel/sched/cpufreq.c

CONFIG_CPU_FREQ_GOV_SCHEDUTIL:
    kernel/sched/cpufreq_schedutil.c

CONFIG_CGROUP_CPUACCT:
    kernel/sched/cpuacct.c

CONFIG_SCHED_DEBUG:
    kernel/sched/debug.c

CONFIG_SCHEDSTATS:
    kernel/sched/stats.c

CONFIG_SMP:
   kernel/sched/cpupri.c
   kernel/sched/stop_task.c
   kernel/sched/topology.c

CONFIG_SCHED_CORE:
   kernel/sched/core_sched.c

CONFIG_PSI:
   kernel/sched/psi.c

CONFIG_MEMBARRIER:
   kernel/sched/membarrier.c

CONFIG_CPU_ISOLATION:
   kernel/sched/isolation.c

CONFIG_SCHED_AUTOGROUP:
   kernel/sched/autogroup.c

The goal is to amortize the 60+ KLOC header bloat from over a dozen build units into
a single build unit.

The build time of build_utility.c also roughly matches the build time of core.c and
fair.c - allowing better load-balancing of scheduler-only rebuilds.

Signed-off-by: Ingo Molnar 
Reviewed-by: Peter Zijlstra