linux.git/kernel/sched/debug.c, branch v7.2-rc1

sched/fair: Remove task_group->se pointer array

2026-06-02T10:26:11+00:00

Now that struct sched_entity is co-located with struct cfs_rq for non-root task
groups, the task_group->se pointer array is redundant. The associated
sched_entity can be loaded directly from the cfs_rq.

This patch performs the access conversion with the helpers:

 - is_root_task_group(tg): checks if a task group is the root task group. It
   compares the task group's address with the global root_task_group variable.

 - tg_se(tg, cpu): retrieves the cfs_rq and returns the address of the
   co-located se. This function checks if tg is the root task group to ensure
   behaving the same of previous tg->se[cpu]. Replaces all accesses that use
   the tg->se[cpu] pointer array with calls to the new tg_se(tg, cpu) accessor.

 - cfs_rq_se(cfs_rq): simplifies access paths like cfs_rq->tg->se[...] to use
   the co-located sched_entity. This function also checks if tg is the root
   task group to ensure same behavior.

Since tg_se is not in very hot code paths, and the branch is a register
comparison with an immediate value (`&root_task_group`), the performance impact
is expected to be negligible.

Signed-off-by: Zecheng Li 
Signed-off-by: Zecheng Li 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: K Prateek Nayak 
Reviewed-by: Josh Don 
Link: https://patch.msgid.link/20260522141623.600235-3-zli94@ncsu.edu

sched/deadline: Reject debugfs dl_server writes for offline CPUs

2026-05-29T10:43:15+00:00

Writing runtime or period via the per-CPU dl_server debugfs files
(/sys/kernel/debug/sched/{fair,ext}_server/cpu*/{runtime,period}) on an
offline CPU can trigger two distinct kernel issues:

1) Divide-by-zero in dl_server_apply_params():

  Oops: divide error: 0000 [#1] SMP NOPTI
  RIP: 0010:dl_server_apply_params+0x239/0x3a0
  Call Trace:
   sched_server_write_common.isra.0+0x21a/0x3c0
   full_proxy_write+0x78/0xd0
   vfs_write+0xe7/0x6e0

  Both __dl_sub() and __dl_add() divide by cpus internally, which can be
  0 once the CPU has been removed from any active root-domain span (this
  has been latent since the debugfs interface was introduced).

2) WARN_ON_ONCE in dl_server_start():

  WARNING: kernel/sched/deadline.c:1805 at dl_server_start+0x232/0x270

  Commit ee6e44dfe6e5 ("sched/deadline: Stop dl_server before CPU goes
  offline") added this check to catch enqueueing the server on an
  offline rq.

There's no meaningful semantics for re-configuring the per-CPU dl_server
bandwidth while the CPU is offline, so simply reject the write with
-EBUSY so userspace gets a clear error.

Closes: https://lore.kernel.org/all/20260526092228.3B6891F00A3A@smtp.kernel.org/
Fixes: d741f297bcea ("sched/fair: Fair server interface")
Reported-by: Sashiko 
Signed-off-by: Andrea Righi 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Juri Lelli 
Tested-by: abaci-kreproducer 
Link: https://patch.msgid.link/20260526100502.575774-1-arighi@nvidia.com

sched/debug: Collapse subsequent CONFIG_SCHED_CLASS_EXT sections

2026-05-26T11:53:13+00:00

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20260511120627.281160085@infradead.org

sched: Use {READ,WRITE}_ONCE() for preempt_dynamic_mode

2026-05-26T11:53:13+00:00

Robots figured out you can read and write this concurrently and got
'upset'. Gemini even noted sched_dynamic_show() can generate
'confusing' output if it observed different values during the
printing.

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20260511120627.176946327@infradead.org

sched/debug: Use char * instead of char (*)[]

2026-05-26T11:53:12+00:00

Some of the fancy AI robots are getting 'upset'.

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/20260511120627.065013766@infradead.org

sched/cache: Fix race condition during sched domain rebuild

2026-05-18T19:33:17+00:00

sched_cache_active_set_unlocked() checks hardware support without
locks:
static void sched_cache_active_set(bool locked)
{
        /* hardware does not support */
        if (!static_branch_likely(&sched_cache_present)) {
                _sched_cache_active_set(false, locked);
                return;
        }
    ...
If build_sched_domains() runs concurrently during CPU hotplug,
it can disable sched_cache_present under sched_domains_mutex
and the CPU hotplug lock. If a debugfs write thread evaluates
sched_cache_present as true right before that, and then blocks
or gets preempted, it might proceed to enable sched_cache_active
after the hardware support has been marked as absent. Make it
safer by acquiring cpus_read_lock() and sched_domains_mutex_lock()
when the user changes sched_cache_active via debugfs.

This bug was reported by sashiko.

Fixes: 067a31358143 ("sched/cache: Allow the user space to turn on and off cache aware scheduling")
Signed-off-by: Chen Yu 
Co-developed-by: Tim Chen 
Signed-off-by: Tim Chen 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/9afddf439687f04bb56b46625bd9f153eb8abad5.1778703694.git.tim.c.chen@linux.intel.com

sched/cache: Add user control to adjust the aggressiveness of cache-aware scheduling

2026-05-18T19:33:15+00:00

Introduce a set of debugfs knobs to control how aggressively the
cache aware scheduling does the task aggregation.

(1) aggr_tolerance
With sched_cache enabled, the scheduler uses a process's footprint
as a proxy for its LLC footprint to determine if aggregating tasks
on the preferred LLC could cause cache contention. If the footprint
exceeds the LLC size, aggregation is skipped. Since the kernel
cannot efficiently track per-task cache usage (resctrl is
user-space only), userspace can provide a more accurate hint.

Introduce /sys/kernel/debug/sched/llc_balancing/aggr_tolerance to
let users control how strictly footprint limits aggregation. Values
range from 0 to 100:
- 0: Cache-aware scheduling is disabled.
- 1: Strict; tasks with footprint larger than LLC size are skipped.
- >=100: Aggressive; tasks are aggregated regardless of footprint.
For example, with a 32MB L3 cache:

- aggr_tolerance=1 -> tasks with footprint > 32MB are skipped.
- aggr_tolerance=99 -> tasks with footprint > 784GB are skipped
(784GB = (1 + (99 - 1) * 256) * 32MB).
Similarly, /sys/kernel/debug/sched/llc_balancing/aggr_tolerance also
controls how strictly the number of active threads is considered when
doing cache aware load balance. The number of SMTs is also considered.
High SMT counts reduce the aggregation capacity, preventing excessive
task aggregation on SMT-heavy systems like Power10/Power11.

Yangyu suggested introducing separate aggregation controls for the
number of active threads and memory footprint checks. Since there are
plans to add per-process/task group controls, fine-grained tunables are
deferred to that implementation.

(2) epoch_period, epoch_affinity_timeout,
imb_pct, overaggr_pct are also turned into tunables.

Suggested-by: K Prateek Nayak
Suggested-by: Madadi Vineeth Reddy
Suggested-by: Shrikanth Hegde
Suggested-by: Tingyin Duan
Suggested-by: Jianyong Wu
Suggested-by: Yangyu Chen
Signed-off-by: Chen Yu
Co-developed-by: Tim Chen
Signed-off-by: Tim Chen
Signed-off-by: Peter Zijlstra (Intel)
Tested-by: Tingyin Duan
Link: https://patch.msgid.link/1c62cc060ba2b33d7b1f0ed98b3390128edbae93.1778703694.git.tim.c.chen@linux.intel.com

sched/cache: Allow the user space to turn on and off cache aware scheduling

2026-04-09T13:49:52+00:00

Provide a debugfs directory llc_balancing, and a knob named
"enabled" under it to allow the user to turn off and on the
cache aware scheduling at runtime.

Suggested-by: Peter Zijlstra (Intel) 
Signed-off-by: Chen Yu 
Signed-off-by: Tim Chen 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/0aa56f7fc48db2f8f700cd1aa34dedd0ec88351b.1775065312.git.tim.c.chen@linux.intel.com

Merge branch 'sched/urgent' into sched/core, to resolve conflicts

2026-04-02T13:04:09+00:00

The following fix in sched/urgent:

  e08d007f9d81 ("sched/debug: Fix avg_vruntime() usage")

is in conflict with this pending commit in sched/core:

  4823725d9d1d ("sched/fair: Increase weight bits for avg_vruntime")

Both modify the same variable definition and initialization blocks,
resolve it by merging the two.

 Conflicts:
	kernel/sched/debug.c

Signed-off-by: Ingo Molnar

sched/debug: Fix avg_vruntime() usage

2026-04-02T11:42:43+00:00

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 commit in question changes avg_vruntime() from a function that is
a pure reader, to a function that updates variables. This turns an
unlocked sched/debug usage of this function from a minor mistake into
a data corruptor.

Fixes: af4cf40470c2 ("sched/fair: Add cfs_rq::avg_vruntime")
Fixes: b3d99f43c72b ("sched/fair: Fix zero_vruntime tracking")
Reported-by: John Stultz 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Vincent Guittot 
Tested-by: K Prateek Nayak 
Tested-by: John Stultz 
Link: https://patch.msgid.link/20260401132355.196370805@infradead.org