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Hardware only fetches descriptors on cachelines of 8, essentially ignoring the lower 3 bits of the tail register. Thus, it is pointless to bump tail by an unaligned access as the hardware will ignore some of the new descriptors we allocated. Thus, it's ideal if we can ensure tail writes are always aligned to 8. At first, it seems like we'd already do this, since we allocate descriptors in batches which are a multiple of 8. Since we'd always increment by a multiple of 8, it seems like the value should always be aligned. However, this ignores allocation failures. If we fail to allocate a buffer, our tail register will become unaligned. Once it has become unaligned it will essentially be stuck unaligned until a buffer allocation happens to fail at the exact amount necessary to re-align it. We can do better, by simply rounding down the number of buffers we're about to allocate (cleaned_count) such that "next_to_clean + cleaned_count" is rounded to the nearest multiple of 8. We do this by calculating how far off that value is and subtracting it from the cleaned_count. This essentially defers allocation of buffers if they're going to be ignored by hardware anyways, and re-aligns our next_to_use and tail values after a failure to allocate a descriptor. This calculation ensures that we always align the tail writes in a way the hardware expects and don't unnecessarily allocate buffers which won't be fetched immediately. Signed-off-by: Jacob Keller <jacob.e.keller@intel.com> Tested-by: Andrew Bowers <andrewx.bowers@intel.com> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.