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Testing showed that rcu_pending() can return 1 when offloaded callbacks are ready to execute. This invokes RCU core processing, for example, by raising RCU_SOFTIRQ, eventually resulting in a call to rcu_core(). However, rcu_core() explicitly avoids in any way manipulating offloaded callbacks, which are instead handled by the rcuog and rcuoc kthreads, which work independently of rcu_core(). One exception to this independence is that rcu_core() invokes do_nocb_deferred_wakeup(), however, rcu_pending() also checks rcu_nocb_need_deferred_wakeup() in order to correctly handle this case, invoking rcu_core() when needed. This commit therefore avoids needlessly invoking RCU core processing by checking rcu_segcblist_ready_cbs() only on non-offloaded CPUs. This reduces overhead, for example, by reducing softirq activity. This change passed 30 minute tests of TREE01 through TREE09 each. On TREE08, there is at most 150us from the time that rcu_pending() chose not to invoke RCU core processing to the time when the ready callbacks were invoked by the rcuoc kthread. This provides further evidence that there is no need to invoke rcu_core() for offloaded callbacks that are ready to invoke. Cc: Neeraj Upadhyay <neeraju@codeaurora.org> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Signed-off-by: Paul E. McKenney <paulmck@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 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.