From: Paul Turner <redacted>

CPU scheduler marks need_resched flag to signal a schedule() on a
particular CPU. But, schedule() may not happen immediately in cases
where the current task is executing in the kernel mode (no
preemption state) for extended periods of time.

This patch adds a warn_on if need_resched is pending for more than the
time specified in sysctl resched_latency_warn_ms. If it goes off, it is
likely that there is a missing cond_resched() somewhere. Monitoring is
done via the tick and the accuracy is hence limited to jiffy scale. This
also means that we won't trigger the warning if the tick is disabled.

This feature is default disabled. It can be toggled on using sysctl
resched_latency_warn_enabled.

Signed-off-by: Paul Turner <redacted>
Signed-off-by: Josh Don <redacted>
---
Delta from v1:
- separate sysctl for enabling/disabling and triggering warn_once
  behavior
- add documentation
- static branch for the enable
 Documentation/admin-guide/sysctl/kernel.rst | 23 ++++++
 include/linux/sched/sysctl.h                |  4 ++
 kernel/sched/core.c                         | 78 ++++++++++++++++++++-
 kernel/sched/debug.c                        | 10 +++
 kernel/sched/sched.h                        | 10 +++
 kernel/sysctl.c                             | 24 +++++++
 6 files changed, 148 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 1d56a6b73a4e..2d4a21d3b79f 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst

@@ -1077,6 +1077,29 @@ ROM/Flash boot loader. Maybe to tell it what to do after
 rebooting. ???
 
 
+resched_latency_warn_enabled
+============================
+
+Enables/disables a warning that will trigger if need_resched is set for
+longer than sysctl ``resched_latency_warn_ms``. This warning likely
+indicates a kernel bug, such as a failure to call cond_resched().
+
+Requires ``CONFIG_SCHED_DEBUG``.
+
+
+resched_latency_warn_ms
+=======================
+
+See ``resched_latency_warn_enabled``.
+
+
+resched_latency_warn_once
+=========================
+
+If set, ``resched_latency_warn_enabled`` will only trigger one warning
+per boot.
+
+
 sched_energy_aware
 ==================