linux.git/kernel/sched, branch v6.18.22 Clone of https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git sched_ext: Fix stale direct dispatch state in ddsp_dsq_id 2026-04-11T12:26:52+00:00 Andrea Righi arighi@nvidia.com 2026-04-08T12:47:41+00:00 5e7b2cc8fae9ec2a5bc53311191d2faaff75a4b5 [ Upstream commit 7e0ffb72de8aa3b25989c2d980e81b829c577010 ] @p->scx.ddsp_dsq_id can be left set (non-SCX_DSQ_INVALID) triggering a spurious warning in mark_direct_dispatch() when the next wakeup's ops.select_cpu() calls scx_bpf_dsq_insert(), such as: WARNING: kernel/sched/ext.c:1273 at scx_dsq_insert_commit+0xcd/0x140 The root cause is that ddsp_dsq_id was only cleared in dispatch_enqueue(), which is not reached in all paths that consume or cancel a direct dispatch verdict. Fix it by clearing it at the right places: - direct_dispatch(): cache the direct dispatch state in local variables and clear it before dispatch_enqueue() on the synchronous path. For the deferred path, the direct dispatch state must remain set until process_ddsp_deferred_locals() consumes them. - process_ddsp_deferred_locals(): cache the dispatch state in local variables and clear it before calling dispatch_to_local_dsq(), which may migrate the task to another rq. - do_enqueue_task(): clear the dispatch state on the enqueue path (local/global/bypass fallbacks), where the direct dispatch verdict is ignored. - dequeue_task_scx(): clear the dispatch state after dispatch_dequeue() to handle both the deferred dispatch cancellation and the holding_cpu race, covering all cases where a pending direct dispatch is cancelled. - scx_disable_task(): clear the direct dispatch state when transitioning a task out of the current scheduler. Waking tasks may have had the direct dispatch state set by the outgoing scheduler's ops.select_cpu() and then been queued on a wake_list via ttwu_queue_wakelist(), when SCX_OPS_ALLOW_QUEUED_WAKEUP is set. Such tasks are not on the runqueue and are not iterated by scx_bypass(), so their direct dispatch state won't be cleared. Without this clear, any subsequent SCX scheduler that tries to direct dispatch the task will trigger the WARN_ON_ONCE() in mark_direct_dispatch(). Fixes: 5b26f7b920f7 ("sched_ext: Allow SCX_DSQ_LOCAL_ON for direct dispatches") Cc: stable@vger.kernel.org # v6.12+ Cc: Daniel Hodges <hodgesd@meta.com> Cc: Patrick Somaru <patsomaru@meta.com> Signed-off-by: Andrea Righi <arighi@nvidia.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
[ Upstream commit 7e0ffb72de8aa3b25989c2d980e81b829c577010 ]

@p->scx.ddsp_dsq_id can be left set (non-SCX_DSQ_INVALID) triggering a
spurious warning in mark_direct_dispatch() when the next wakeup's
ops.select_cpu() calls scx_bpf_dsq_insert(), such as:

 WARNING: kernel/sched/ext.c:1273 at scx_dsq_insert_commit+0xcd/0x140

The root cause is that ddsp_dsq_id was only cleared in dispatch_enqueue(),
which is not reached in all paths that consume or cancel a direct dispatch
verdict.

Fix it by clearing it at the right places:

 - direct_dispatch(): cache the direct dispatch state in local variables
   and clear it before dispatch_enqueue() on the synchronous path. For
   the deferred path, the direct dispatch state must remain set until
   process_ddsp_deferred_locals() consumes them.

 - process_ddsp_deferred_locals(): cache the dispatch state in local
   variables and clear it before calling dispatch_to_local_dsq(), which
   may migrate the task to another rq.

 - do_enqueue_task(): clear the dispatch state on the enqueue path
   (local/global/bypass fallbacks), where the direct dispatch verdict is
   ignored.

 - dequeue_task_scx(): clear the dispatch state after dispatch_dequeue()
   to handle both the deferred dispatch cancellation and the holding_cpu
   race, covering all cases where a pending direct dispatch is
   cancelled.

 - scx_disable_task(): clear the direct dispatch state when
   transitioning a task out of the current scheduler. Waking tasks may
   have had the direct dispatch state set by the outgoing scheduler's
   ops.select_cpu() and then been queued on a wake_list via
   ttwu_queue_wakelist(), when SCX_OPS_ALLOW_QUEUED_WAKEUP is set. Such
   tasks are not on the runqueue and are not iterated by scx_bypass(),
   so their direct dispatch state won't be cleared. Without this clear,
   any subsequent SCX scheduler that tries to direct dispatch the task
   will trigger the WARN_ON_ONCE() in mark_direct_dispatch().

Fixes: 5b26f7b920f7 ("sched_ext: Allow SCX_DSQ_LOCAL_ON for direct dispatches")
Cc: stable@vger.kernel.org # v6.12+
Cc: Daniel Hodges <hodgesd@meta.com>
Cc: Patrick Somaru <patsomaru@meta.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
sched_ext: Refactor do_enqueue_task() local and global DSQ paths 2026-04-11T12:26:52+00:00 Tejun Heo tj@kernel.org 2026-04-08T12:47:40+00:00 d5776a31c16579935855c631bce09eb07dc7da23 [ Upstream commit 3546119f18647d7ddbba579737d8a222b430cb1c ] The local and global DSQ enqueue paths in do_enqueue_task() share the same slice refill logic. Factor out the common code into a shared enqueue label. This makes adding new enqueue cases easier. No functional changes. Reviewed-by: Andrea Righi <arighi@nvidia.com> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> Signed-off-by: Tejun Heo <tj@kernel.org> Stable-dep-of: 7e0ffb72de8a ("sched_ext: Fix stale direct dispatch state in ddsp_dsq_id") Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
[ Upstream commit 3546119f18647d7ddbba579737d8a222b430cb1c ]

The local and global DSQ enqueue paths in do_enqueue_task() share the same
slice refill logic. Factor out the common code into a shared enqueue label.
This makes adding new enqueue cases easier. No functional changes.

Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Stable-dep-of: 7e0ffb72de8a ("sched_ext: Fix stale direct dispatch state in ddsp_dsq_id")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
sched_ext: Fix is_bpf_migration_disabled() false negative on non-PREEMPT_RCU 2026-04-11T12:26:47+00:00 Changwoo Min changwoo@igalia.com 2026-04-02T02:31:50+00:00 72c43eb2e334febe93018cfb68ae828f55c6e49e commit 0c4a59df370bea245695c00aaae6ae75747139bd upstream. Since commit 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c"), the BPF prolog (__bpf_prog_enter) calls migrate_disable() only when CONFIG_PREEMPT_RCU is enabled, via rcu_read_lock_dont_migrate(). Without CONFIG_PREEMPT_RCU, the prolog never touches migration_disabled, so migration_disabled == 1 always means the task is truly migration-disabled regardless of whether it is the current task. The old unconditional p == current check was a false negative in this case, potentially allowing a migration-disabled task to be dispatched to a remote CPU and triggering scx_error in task_can_run_on_remote_rq(). Only apply the p == current disambiguation when CONFIG_PREEMPT_RCU is enabled, where the ambiguity with the BPF prolog still exists. Fixes: 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c") Cc: stable@vger.kernel.org # v6.18+ Link: https://lore.kernel.org/lkml/20250821090609.42508-8-dongml2@chinatelecom.cn/ Signed-off-by: Changwoo Min <changwoo@igalia.com> Reviewed-by: Andrea Righi <arighi@nvidia.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit 0c4a59df370bea245695c00aaae6ae75747139bd upstream.

Since commit 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for
trampoline.c"), the BPF prolog (__bpf_prog_enter) calls migrate_disable()
only when CONFIG_PREEMPT_RCU is enabled, via rcu_read_lock_dont_migrate().
Without CONFIG_PREEMPT_RCU, the prolog never touches migration_disabled,
so migration_disabled == 1 always means the task is truly
migration-disabled regardless of whether it is the current task.

The old unconditional p == current check was a false negative in this
case, potentially allowing a migration-disabled task to be dispatched to
a remote CPU and triggering scx_error in task_can_run_on_remote_rq().

Only apply the p == current disambiguation when CONFIG_PREEMPT_RCU is
enabled, where the ambiguity with the BPF prolog still exists.

Fixes: 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c")
Cc: stable@vger.kernel.org # v6.18+
Link: https://lore.kernel.org/lkml/20250821090609.42508-8-dongml2@chinatelecom.cn/
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
sched_ext: Fix inconsistent NUMA node lookup in scx_select_cpu_dfl() 2026-04-11T12:26:34+00:00 Cheng-Yang Chou yphbchou0911@gmail.com 2026-03-21T10:54:58+00:00 b49bf41b4114842addf1f609c1a30c2f5e8925b8 commit db08b1940f4beb25460b4a4e9da3446454f2e8fe upstream. In the WAKE_SYNC path of scx_select_cpu_dfl(), waker_node was computed with cpu_to_node(), while node (for prev_cpu) was computed with scx_cpu_node_if_enabled(). When scx_builtin_idle_per_node is disabled, idle_cpumask(waker_node) is called with a real node ID even though per-node idle tracking is disabled, resulting in undefined behavior. Fix by using scx_cpu_node_if_enabled() for waker_node as well, ensuring both variables are computed consistently. Fixes: 48849271e6611 ("sched_ext: idle: Per-node idle cpumasks") Cc: stable@vger.kernel.org # v6.15+ Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com> Reviewed-by: Andrea Righi <arighi@nvidia.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit db08b1940f4beb25460b4a4e9da3446454f2e8fe upstream.

In the WAKE_SYNC path of scx_select_cpu_dfl(), waker_node was computed
with cpu_to_node(), while node (for prev_cpu) was computed with
scx_cpu_node_if_enabled(). When scx_builtin_idle_per_node is disabled,
idle_cpumask(waker_node) is called with a real node ID even though
per-node idle tracking is disabled, resulting in undefined behavior.

Fix by using scx_cpu_node_if_enabled() for waker_node as well, ensuring
both variables are computed consistently.

Fixes: 48849271e6611 ("sched_ext: idle: Per-node idle cpumasks")
Cc: stable@vger.kernel.org # v6.15+
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
sched/fair: Fix zero_vruntime tracking fix 2026-04-11T12:26:32+00:00 Peter Zijlstra peterz@infradead.org 2026-04-01T13:20:20+00:00 87573883c30f1a8555ff720836bb6ea231058539 [ Upstream commit 1319ea57529e131822bab56bf417c8edc2db9ae8 ] John reported that stress-ng-yield could make his machine unhappy and managed to bisect it to commit b3d99f43c72b ("sched/fair: Fix zero_vruntime tracking"). The combination of yield and that commit was specific enough to hypothesize the following scenario: Suppose we have 2 runnable tasks, both doing yield. Then one will be eligible and one will not be, because the average position must be in between these two entities. Therefore, the runnable task will be eligible, and be promoted a full slice (all the tasks do is yield after all). This causes it to jump over the other task and now the other task is eligible and current is no longer. So we schedule. Since we are runnable, there is no {de,en}queue. All we have is the __{en,de}queue_entity() from {put_prev,set_next}_task(). But per the fingered commit, those two no longer move zero_vruntime. All that moves zero_vruntime are tick and full {de,en}queue. This means, that if the two tasks playing leapfrog can reach the critical speed to reach the overflow point inside one tick's worth of time, we're up a creek. Additionally, when multiple cgroups are involved, there is no guarantee the tick will in fact hit every cgroup in a timely manner. Statistically speaking it will, but that same statistics does not rule out the possibility of one cgroup not getting a tick for a significant amount of time -- however unlikely. Therefore, just like with the yield() case, force an update at the end of every slice. This ensures the update is never more than a single slice behind and the whole thing is within 2 lag bounds as per the comment on entity_key(). Fixes: b3d99f43c72b ("sched/fair: Fix zero_vruntime tracking") Reported-by: John Stultz <jstultz@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com> Tested-by: John Stultz <jstultz@google.com> Link: https://patch.msgid.link/20260401132355.081530332@infradead.org Signed-off-by: Sasha Levin <sashal@kernel.org> 
[ Upstream commit 1319ea57529e131822bab56bf417c8edc2db9ae8 ]

John reported that stress-ng-yield could make his machine unhappy and
managed to bisect it to commit b3d99f43c72b ("sched/fair: Fix
zero_vruntime tracking").

The combination of yield and that commit was specific enough to
hypothesize the following scenario:

Suppose we have 2 runnable tasks, both doing yield. Then one will be
eligible and one will not be, because the average position must be in
between these two entities.

Therefore, the runnable task will be eligible, and be promoted a full
slice (all the tasks do is yield after all). This causes it to jump over
the other task and now the other task is eligible and current is no
longer. So we schedule.

Since we are runnable, there is no {de,en}queue. All we have is the
__{en,de}queue_entity() from {put_prev,set_next}_task(). But per the
fingered commit, those two no longer move zero_vruntime.

All that moves zero_vruntime are tick and full {de,en}queue.

This means, that if the two tasks playing leapfrog can reach the
critical speed to reach the overflow point inside one tick's worth of
time, we're up a creek.

Additionally, when multiple cgroups are involved, there is no guarantee
the tick will in fact hit every cgroup in a timely manner. Statistically
speaking it will, but that same statistics does not rule out the
possibility of one cgroup not getting a tick for a significant amount of
time -- however unlikely.

Therefore, just like with the yield() case, force an update at the end
of every slice. This ensures the update is never more than a single
slice behind and the whole thing is within 2 lag bounds as per the
comment on entity_key().

Fixes: b3d99f43c72b ("sched/fair: Fix zero_vruntime tracking")
Reported-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260401132355.081530332@infradead.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
sched_ext: Use WRITE_ONCE() for the write side of dsq->seq update 2026-04-02T11:23:00+00:00 zhidao su soolaugust@gmail.com 2026-03-04T05:37:30+00:00 91608747a8d2b65b2dadf328e606e1891d48a3b0 [ Upstream commit 7a8464555d2e5f038758bb19e72ab4710b79e9cd ] bpf_iter_scx_dsq_new() reads dsq->seq via READ_ONCE() without holding any lock, making dsq->seq a lock-free concurrently accessed variable. However, dispatch_enqueue(), the sole writer of dsq->seq, uses a plain increment without the matching WRITE_ONCE() on the write side: dsq->seq++; ^^^^^^^^^^^ plain write -- KCSAN data race The KCSAN documentation requires that if one accessor uses READ_ONCE() or WRITE_ONCE() on a variable to annotate lock-free access, all other accesses must also use the appropriate accessor. A plain write leaves the pair incomplete and will trigger KCSAN warnings. Fix by using WRITE_ONCE() for the write side of the update: WRITE_ONCE(dsq->seq, dsq->seq + 1); This is consistent with bpf_iter_scx_dsq_new() and makes the concurrent access annotation complete and KCSAN-clean. Signed-off-by: zhidao su <suzhidao@xiaomi.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> 
[ Upstream commit 7a8464555d2e5f038758bb19e72ab4710b79e9cd ]

bpf_iter_scx_dsq_new() reads dsq->seq via READ_ONCE() without holding
any lock, making dsq->seq a lock-free concurrently accessed variable.
However, dispatch_enqueue(), the sole writer of dsq->seq, uses a plain
increment without the matching WRITE_ONCE() on the write side:

    dsq->seq++;
    ^^^^^^^^^^^
    plain write -- KCSAN data race

The KCSAN documentation requires that if one accessor uses READ_ONCE()
or WRITE_ONCE() on a variable to annotate lock-free access, all other
accesses must also use the appropriate accessor. A plain write leaves
the pair incomplete and will trigger KCSAN warnings.

Fix by using WRITE_ONCE() for the write side of the update:

    WRITE_ONCE(dsq->seq, dsq->seq + 1);

This is consistent with bpf_iter_scx_dsq_new() and makes the
concurrent access annotation complete and KCSAN-clean.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
sched: idle: Consolidate the handling of two special cases 2026-03-25T10:10:41+00:00 Rafael J. Wysocki rafael.j.wysocki@intel.com 2026-03-13T12:25:41+00:00 34c979e29bf680b70ed633a3c2eadac34348eb0e [ Upstream commit f4c31b07b136839e0fb3026f8a5b6543e3b14d2f ] There are two special cases in the idle loop that are handled inconsistently even though they are analogous. The first one is when a cpuidle driver is absent and the default CPU idle time power management implemented by the architecture code is used. In that case, the scheduler tick is stopped every time before invoking default_idle_call(). The second one is when a cpuidle driver is present, but there is only one idle state in its table. In that case, the scheduler tick is never stopped at all. Since each of these approaches has its drawbacks, reconcile them with the help of one simple heuristic. Namely, stop the tick if the CPU has been woken up by it in the previous iteration of the idle loop, or let it tick otherwise. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Reviewed-by: Christian Loehle <christian.loehle@arm.com> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Reviewed-by: Qais Yousef <qyousef@layalina.io> Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com> Fixes: ed98c3491998 ("sched: idle: Do not stop the tick before cpuidle_idle_call()") [ rjw: Added Fixes tag, changelog edits ] Link: https://patch.msgid.link/4741364.LvFx2qVVIh@rafael.j.wysocki Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Signed-off-by: Sasha Levin <sashal@kernel.org> 
[ Upstream commit f4c31b07b136839e0fb3026f8a5b6543e3b14d2f ]

There are two special cases in the idle loop that are handled
inconsistently even though they are analogous.

The first one is when a cpuidle driver is absent and the default CPU
idle time power management implemented by the architecture code is used.
In that case, the scheduler tick is stopped every time before invoking
default_idle_call().

The second one is when a cpuidle driver is present, but there is only
one idle state in its table.  In that case, the scheduler tick is never
stopped at all.

Since each of these approaches has its drawbacks, reconcile them with
the help of one simple heuristic.  Namely, stop the tick if the CPU has
been woken up by it in the previous iteration of the idle loop, or let
it tick otherwise.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Qais Yousef <qyousef@layalina.io>
Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Fixes: ed98c3491998 ("sched: idle: Do not stop the tick before cpuidle_idle_call()")
[ rjw: Added Fixes tag, changelog edits ]
Link: https://patch.msgid.link/4741364.LvFx2qVVIh@rafael.j.wysocki
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
sched_ext: Disable preemption between scx_claim_exit() and kicking helper work 2026-03-25T10:10:33+00:00 Tejun Heo tj@kernel.org 2026-03-17T17:21:52+00:00 522acaae34aa7e05859260056b39c7c030592a0c [ Upstream commit 83236b2e43dba00bee5b82eb5758816b1a674f6a ] scx_claim_exit() atomically sets exit_kind, which prevents scx_error() from triggering further error handling. After claiming exit, the caller must kick the helper kthread work which initiates bypass mode and teardown. If the calling task gets preempted between claiming exit and kicking the helper work, and the BPF scheduler fails to schedule it back (since error handling is now disabled), the helper work is never queued, bypass mode never activates, tasks stop being dispatched, and the system wedges. Disable preemption across scx_claim_exit() and the subsequent work kicking in all callers - scx_disable() and scx_vexit(). Add lockdep_assert_preemption_disabled() to scx_claim_exit() to enforce the requirement. Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class") Cc: stable@vger.kernel.org # v6.12+ Reviewed-by: Andrea Righi <arighi@nvidia.com> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
[ Upstream commit 83236b2e43dba00bee5b82eb5758816b1a674f6a ]

scx_claim_exit() atomically sets exit_kind, which prevents scx_error() from
triggering further error handling. After claiming exit, the caller must kick
the helper kthread work which initiates bypass mode and teardown.

If the calling task gets preempted between claiming exit and kicking the
helper work, and the BPF scheduler fails to schedule it back (since error
handling is now disabled), the helper work is never queued, bypass mode
never activates, tasks stop being dispatched, and the system wedges.

Disable preemption across scx_claim_exit() and the subsequent work kicking
in all callers - scx_disable() and scx_vexit(). Add
lockdep_assert_preemption_disabled() to scx_claim_exit() to enforce the
requirement.

Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Cc: stable@vger.kernel.org # v6.12+
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
sched_ext: Simplify breather mechanism with scx_aborting flag 2026-03-25T10:10:33+00:00 Tejun Heo tj@kernel.org 2026-03-17T17:21:51+00:00 c1b8245c0a38787533dd7da0c4e0e68f89a623c0 [ Upstream commit a69040ed57f50156e5452474d25c79b9e62075d0 ] The breather mechanism was introduced in 62dcbab8b0ef ("sched_ext: Avoid live-locking bypass mode switching") and e32c260195e6 ("sched_ext: Enable the ops breather and eject BPF scheduler on softlockup") to prevent live-locks by injecting delays when CPUs are trapped in dispatch paths. Currently, it uses scx_breather_depth (atomic_t) and scx_in_softlockup (unsigned long) with separate increment/decrement and cleanup operations. The breather is only activated when aborting, so tie it directly to the exit mechanism. Replace both variables with scx_aborting flag set when exit is claimed and cleared after bypass is enabled. Introduce scx_claim_exit() to consolidate exit_kind claiming and breather enablement. This eliminates scx_clear_softlockup() and simplifies scx_softlockup() and scx_bypass(). The breather mechanism will be replaced by a different abort mechanism in a future patch. This simplification prepares for that change. Reviewed-by: Dan Schatzberg <schatzberg.dan@gmail.com> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com> Acked-by: Andrea Righi <arighi@nvidia.com> Signed-off-by: Tejun Heo <tj@kernel.org> Stable-dep-of: 83236b2e43db ("sched_ext: Disable preemption between scx_claim_exit() and kicking helper work") Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
[ Upstream commit a69040ed57f50156e5452474d25c79b9e62075d0 ]

The breather mechanism was introduced in 62dcbab8b0ef ("sched_ext: Avoid
live-locking bypass mode switching") and e32c260195e6 ("sched_ext: Enable the
ops breather and eject BPF scheduler on softlockup") to prevent live-locks by
injecting delays when CPUs are trapped in dispatch paths.

Currently, it uses scx_breather_depth (atomic_t) and scx_in_softlockup
(unsigned long) with separate increment/decrement and cleanup operations. The
breather is only activated when aborting, so tie it directly to the exit
mechanism. Replace both variables with scx_aborting flag set when exit is
claimed and cleared after bypass is enabled. Introduce scx_claim_exit() to
consolidate exit_kind claiming and breather enablement. This eliminates
scx_clear_softlockup() and simplifies scx_softlockup() and scx_bypass().

The breather mechanism will be replaced by a different abort mechanism in a
future patch. This simplification prepares for that change.

Reviewed-by: Dan Schatzberg <schatzberg.dan@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Stable-dep-of: 83236b2e43db ("sched_ext: Disable preemption between scx_claim_exit() and kicking helper work")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
sched_ext: Fix starvation of scx_enable() under fair-class saturation 2026-03-25T10:10:33+00:00 Tejun Heo tj@kernel.org 2026-03-17T17:29:35+00:00 c44198f25fdfecc0ec0fe366bf8a47fe17d8e229 [ Upstream commit b06ccbabe2506fd70b9167a644978b049150224a ] During scx_enable(), the READY -> ENABLED task switching loop changes the calling thread's sched_class from fair to ext. Since fair has higher priority than ext, saturating fair-class workloads can indefinitely starve the enable thread, hanging the system. This was introduced when the enable path switched from preempt_disable() to scx_bypass() which doesn't protect against fair-class starvation. Note that the original preempt_disable() protection wasn't complete either - in partial switch modes, the calling thread could still be starved after preempt_enable() as it may have been switched to ext class. Fix it by offloading the enable body to a dedicated system-wide RT (SCHED_FIFO) kthread which cannot be starved by either fair or ext class tasks. scx_enable() lazily creates the kthread on first use and passes the ops pointer through a struct scx_enable_cmd containing the kthread_work, then synchronously waits for completion. The workfn runs on a different kthread from sch->helper (which runs disable_work), so it can safely flush disable_work on the error path without deadlock. Fixes: 8c2090c504e9 ("sched_ext: Initialize in bypass mode") Cc: stable@vger.kernel.org # v6.12+ Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
[ Upstream commit b06ccbabe2506fd70b9167a644978b049150224a ]

During scx_enable(), the READY -> ENABLED task switching loop changes the
calling thread's sched_class from fair to ext. Since fair has higher
priority than ext, saturating fair-class workloads can indefinitely starve
the enable thread, hanging the system. This was introduced when the enable
path switched from preempt_disable() to scx_bypass() which doesn't protect
against fair-class starvation. Note that the original preempt_disable()
protection wasn't complete either - in partial switch modes, the calling
thread could still be starved after preempt_enable() as it may have been
switched to ext class.

Fix it by offloading the enable body to a dedicated system-wide RT
(SCHED_FIFO) kthread which cannot be starved by either fair or ext class
tasks. scx_enable() lazily creates the kthread on first use and passes the
ops pointer through a struct scx_enable_cmd containing the kthread_work,
then synchronously waits for completion.

The workfn runs on a different kthread from sch->helper (which runs
disable_work), so it can safely flush disable_work on the error path
without deadlock.

Fixes: 8c2090c504e9 ("sched_ext: Initialize in bypass mode")
Cc: stable@vger.kernel.org # v6.12+
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>