[PATCH 2/4] doc: watchdog: Clarify hardlockup detection timing

[Mayank Rungta via B4 Relay]

From: Mayank Rungta <mrungta@xxxxxxxxxx>

The current documentation implies that a hardlockup is strictly defined
as looping for "more than 10 seconds." However, the detection mechanism
is periodic (based on `watchdog_thresh`), meaning detection time varies
significantly depending on when the lockup occurs relative to the NMI
perf event.

Update the definition to remove the strict "more than 10 seconds"
constraint in the introduction and defer details to the Implementation
section.

Additionally, add a "Detection Overhead" section illustrating the
Best Case (~6s) and Worst Case (~20s) detection scenarios to provide
administrators with a clearer understanding of the watchdog's
latency.

Signed-off-by: Mayank Rungta <mrungta@xxxxxxxxxx>
---
 Documentation/admin-guide/lockup-watchdogs.rst | 41 +++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/lockup-watchdogs.rst b/Documentation/admin-guide/lockup-watchdogs.rst
index 3e09284a8b9bef75c0ac1607a1809ac3b8a4c1ea..1b374053771f676d874716b3210cade55ae89b28 100644
--- a/Documentation/admin-guide/lockup-watchdogs.rst
+++ b/Documentation/admin-guide/lockup-watchdogs.rst
@@ -16,7 +16,7 @@ details), and a compile option, "BOOTPARAM_SOFTLOCKUP_PANIC", are
 provided for this.
 
 A 'hardlockup' is defined as a bug that causes the CPU to loop in
-kernel mode for more than 10 seconds (see "Implementation" below for
+kernel mode for several seconds (see "Implementation" below for
 details), without letting other interrupts have a chance to run.
 Similarly to the softlockup case, the current stack trace is displayed
 upon detection and the system will stay locked up unless the default
@@ -64,6 +64,45 @@ administrators to configure the period of the hrtimer and the perf
 event. The right value for a particular environment is a trade-off
 between fast response to lockups and detection overhead.
 
+Detection Overhead
+------------------
+
+The hardlockup detector checks for lockups using a periodic NMI perf
+event. This means the time to detect a lockup can vary depending on
+when the lockup occurs relative to the NMI check window.
+
+**Best Case:**
+In the best case scenario, the lockup occurs just before the first
+heartbeat is due. The detector will notice the missing hrtimer
+interrupt almost immediately during the next check.
+
+::
+
+  Time 100.0: cpu 1 heartbeat
+  Time 100.1: hardlockup_check, cpu1 stores its state
+  Time 103.9: Hard Lockup on cpu1
+  Time 104.0: cpu 1 heartbeat never comes
+  Time 110.1: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
+
+  Time to detection: ~6 seconds
+
+**Worst Case:**
+In the worst case scenario, the lockup occurs shortly after a valid
+interrupt (heartbeat) which itself happened just after the NMI check.
+The next NMI check sees that the interrupt count has changed (due to
+that one heartbeat), assumes the CPU is healthy, and resets the
+baseline. The lockup is only detected at the subsequent check.
+
+::
+
+  Time 100.0: hardlockup_check, cpu1 stores its state
+  Time 100.1: cpu 1 heartbeat
+  Time 100.2: Hard Lockup on cpu1
+  Time 110.0: hardlockup_check, cpu1 stores its state (misses lockup as state changed)
+  Time 120.0: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
+
+  Time to detection: ~20 seconds
+
 By default, the watchdog runs on all online cores.  However, on a
 kernel configured with NO_HZ_FULL, by default the watchdog runs only
 on the housekeeping cores, not the cores specified in the "nohz_full"

-- 
2.53.0.273.g2a3d683680-goog