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There's an absolute timestamp event in the ring buffer, but this only saves 59 bits of the timestamp, as the 5 MSB is used for meta data (stating it is an absolute time stamp). This was never an issue as all the clocks currently in use never used those 5 MSB. But now there's a new clock (TAI) that does. To handle this case, when reading an absolute timestamp, a previous full timestamp is passed in, and the 5 MSB of that timestamp is OR'd to the absolute timestamp (if any of the 5 MSB are set), and then to test for overflow, if the new result is smaller than the passed in previous timestamp, then 1 << 59 is added to it. All the extra processing is done on the reader "slow" path, with the exception of the "too big delta" check, and the reading of timestamps for histograms. Note, libtraceevent will need to be updated to handle this case as well. But this is not a user space regression, as user space was never able to handle any timestamps that used more than 59 bits. Link: https://lore.kernel.org/all/20220426175338.3807ca4f@gandalf.local.home/ Link: https://lkml.kernel.org/r/20220427153339.16c33f75@gandalf.local.home Cc: Tom Zanussi <zanussi@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Kurt Kanzenbach <kurt@linutronix.de> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org> |
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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 Restructured Text 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.