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A typical set of messages that gets printed as a result of the clocksource watchdog finding the TSC unstable usually does not contain messages indicating CPUs being ahead of or behind the CPU from which the check is carried out. That fact suggests that the TSC does not experience time skew between CPUs (if the clocksource.verify_n_cpus parameter is set to a negative value) but quantitative information is missing. The cs_nsec_max value printed by the "CPU %d check durations" message actually provides a worst case estimate of the time skew. If all CPUs have been checked, the cs_nsec_max value multiplied by 2 is the maximum possible time skew between the TSCs of any two CPUs on the system. The worst case estimate is derived from two boundary cases: 1. No time is consumed to execute instructions between csnow_begin and csnow_mid while all the cs_nsec_max time is consumed by the code between csnow_mid and csnow_end. In this case, the maximum undetectable time skew of a CPU being ahead would be cs_nsec_max. 2. All the cs_nsec_max time is consumed to execute instructions between csnow_begin and csnow_mid while no time is consumed by the code between csnow_mid and csnow_end. In this case, the maximum undetectable time skew of a CPU being behind would be cs_nsec_max. The worst case estimate assumes a system experiencing a corner case consisting of the two boundary cases. Always print the "CPU %d check durations" message so that the maximum possible time skew measured by the TSC sync check can be compared to the time skew measured by the clocksource watchdog. Signed-off-by: Jiri Wiesner <jwiesner@suse.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/all/aIuXXfdITXdI0lLp@incl |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.