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62eec753caChristian Brauner <brauner@kernel.org> says: As atomic_inc_not_zero() is implemented with a try_cmpxchg() loop it has O(N^2) behaviour under contention with N concurrent operations and it is in a hot path in __fget_files_rcu(). The rcuref infrastructures remedies this problem by using an unconditional increment relying on safe- and dead zones to make this work and requiring rcu protection for the data structure in question. This not just scales better it also introduces overflow protection. However, in contrast to generic rcuref, files require a memory barrier and thus cannot rely on *_relaxed() atomic operations and also require to be built on atomic_long_t as having massive amounts of reference isn't unheard of even if it is just an attack. As suggested by Linus, add a file specific variant instead of making this a generic library. I've been testing this with will-it-scale using a multi-threaded fstat() on the same file descriptor on a machine that Jens gave me access (thank you very much!): processor : 511 vendor_id : AuthenticAMD cpu family : 25 model : 160 model name : AMD EPYC 9754 128-Core Processor and I consistently get a 3-5% improvement on workloads with 256+ and more threads comparing v6.12-rc1 as base with and without these patches applied. * patches from https://lore.kernel.org/r/20241007-brauner-file-rcuref-v2-0-387e24dc9163@kernel.org: fs: port files to file_ref fs: add file_ref fs: protect backing files with rcu Link: https://lore.kernel.org/r/20241007-brauner-file-rcuref-v2-0-387e24dc9163@kernel.org Signed-off-by: Christian Brauner <brauner@kernel.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 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.