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When a CPU is slow to provide a quiescent state for a given grace period, RCU takes steps to encourage that CPU to get with the quiescent-state program in a more timely fashion. These steps include these flags in the rcu_data structure: 1. ->rcu_urgent_qs, which causes the scheduling-clock interrupt to request an otherwise pointless context switch from the scheduler. 2. ->rcu_need_heavy_qs, which causes both cond_resched() and RCU's context-switch hook to do an immediate momentary quiscent state. 3. ->rcu_need_heavy_qs, which causes the scheduler-clock tick to be enabled even on nohz_full CPUs with only one runnable task. These flags are of course cleared once the corresponding CPU has passed through a quiescent state. Unless that quiescent state is the CPU going offline, which means that when the CPU comes back online, it will needlessly consume additional CPU time and incur additional latency, which constitutes a minor but very real performance bug. This commit therefore adds the call to rcu_disable_urgency_upon_qs() that clears these flags to the CPU-hotplug offlining code path. Signed-off-by: Zqiang <qiang1.zhang@intel.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
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.