watchdog: return early in watchdog_hardlockup_check()

Patch series "watchdog/hardlockup: Improvements to hardlockup", v2.

This series addresses limitations in the hardlockup detector
implementations and updates the documentation to reflect actual behavior
and recent changes.

The changes are structured as follows:

Refactoring (Patch 1)
=====================
Patch 1 refactors watchdog_hardlockup_check() to return early if no
lockup is detected. This reduces the indentation level of the main
logic block, serving as a clean base for the subsequent changes.

Hardlockup Detection Improvements (Patches 2 & 4)
=================================================
The hardlockup detector logic relies on updating saved interrupt counts to
determine if the CPU is making progress.

Patch 1 ensures that the saved interrupt count is updated unconditionally
before checking the "touched" flag. This prevents stale comparisons which
can delay detection. This is a logic fix that ensures the detector
remains accurate even when the watchdog is frequently touched.

Patch 3 improves the Buddy detector's timeliness. The current checking
interval (every 3rd sample) causes high variability in detection time (up
to 24s). This patch changes the Buddy detector to check at every hrtimer
interval (4s) with a missed-interrupt threshold of 3, narrowing the
detection window to a consistent 8-12 second range.

Documentation Updates (Patches 3 & 5)
=====================================
The current documentation does not fully capture the variable nature of
detection latency or the details of the Buddy system.

Patch 3 removes the strict "10 seconds" definition of a hardlockup, which
was misleading given the periodic nature of the detector. It adds a
"Detection Overhead" section to the admin guide, using "Best Case" and
"Worst Case" scenarios to illustrate that detection time can vary
significantly (e.g., ~6s to ~20s).

Patch 5 adds a dedicated section for the Buddy detector, which was
previously undocumented. It details the mechanism, the new timing logic,
and known limitations.


This patch (of 5):

Invert the `is_hardlockup(cpu)` check in `watchdog_hardlockup_check()` to
return early when a hardlockup is not detected. This flattens the main
logic block, reducing the indentation level and making the code easier to
read and maintain.

This refactoring serves as a preparation patch for future hardlockup
changes.

Link: https://lkml.kernel.org/r/20260312-hardlockup-watchdog-fixes-v2-0-45bd8a0cc7ed@google.com
Link: https://lkml.kernel.org/r/20260312-hardlockup-watchdog-fixes-v2-1-45bd8a0cc7ed@google.com
Signed-off-by: Mayank Rungta <mrungta@google.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Li Huafei <lihuafei1@huawei.com>
Cc: Max Kellermann <max.kellermann@ionos.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Stephane Erainan <eranian@google.com>
Cc: Wang Jinchao <wangjinchao600@gmail.com>
Cc: Yunhui Cui <cuiyunhui@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>