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Currently rcu_barrier() is used to ensure that no readers of the inactive mini_Qdisc buffer remain before it is reused. This waits for any pending RCU callbacks to complete, when all that is actually required is to wait for one RCU grace period to elapse after the buffer was made inactive. This means that using rcu_barrier() may result in unnecessary waits. To improve this, store the current RCU state when a buffer is made inactive and use poll_state_synchronize_rcu() to check whether a full grace period has elapsed before reusing it. If a full grace period has not elapsed, wait for a grace period to elapse, and in the non-RT case use synchronize_rcu_expedited() to hasten it. Since this approach eliminates the RCU callback it is no longer necessary to synchronize_rcu() in the tp_head==NULL case. However, the RCU state should still be saved for the previously active buffer. Before this change I would typically see mini_qdisc_pair_swap() take tens of milliseconds to complete. After this change it typcially finishes in less than 1 ms, and often it takes just a few microseconds. Thanks to Paul for walking me through the options for improving this. Cc: "Paul E. McKenney" <paulmck@kernel.org> Signed-off-by: Seth Forshee <sforshee@digitalocean.com> Link: https://lore.kernel.org/r/20211026130700.121189-1-seth@forshee.me Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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.