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SAMPLE_OA parameter enables sampling of OA buffer and results in a call to init the OA buffer which initializes the OA unit head/tail pointers. The OA_EXPONENT parameter controls the periodicity of the OA reports in the OA buffer and results in starting a hrtimer. Before gen12, all use cases required the use of the OA buffer and i915 enforced this setting when vetting out the parameters passed. In these platforms the hrtimer was enabled if OA_EXPONENT was passed. This worked fine since it was implied that SAMPLE_OA is always passed. With gen12, this changed. Users can use perf without enabling the OA buffer as in OAR use cases. While an OAR use case should ideally not start the hrtimer, we see that passing an OA_EXPONENT parameter will start the hrtimer even though SAMPLE_OA is not specified. This results in an uninitialized OA buffer, so the head/tail pointers used to track the buffer are zero. This itself does not fail, but if we ran a use-case that SAMPLED the OA buffer previously, then the OA_TAIL register is still pointing to an old value. When the timer callback runs, it ends up calculating a wrong/large number of available reports. Since we do a spinlock_irq_save and start processing a large number of reports, NMI watchdog fires and causes a crash. Start the timer only if SAMPLE_OA is specified. v2: - Drop SAMPLE OA check when appending samples (Ashutosh) - Prevent read if OA buffer is not being sampled Fixes: |
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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.