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Whenever CPU hotplug state callbacks are registered, the startup callback is invoked on CPUs that have already reached the provided state in order of ascending CPU IDs. In freeze_secondary_cpus() the teardown of CPUs happens in the same are invoked in the same order. This is known to make a difference is the current implementation of these callbacks in arch/x86/events/intel/uncore.c: - uncore_event_cpu_online() designates the first CPU it is invoked for on each package as the uncore event collector for that package - uncore_event_cpu_offline() if the CPU being offlined is the event collector for its package, transfers that responsibility over to the next (by ascending CPU id) one in the same package With the current order of CPU teardowns in freeze_secondary_cpus(), the latter ends up doing the ownership transfer work on every single CPU. That work involves a synchronize_rcu() call, ultimately unnecessarily degrading the performance of CPU offlining. To address this make freeze_secondary_cpus() iterate through the CPUs in reverse order, so that the teardown happens in order of descending CPU IDs. [ tglx: Massage change log ] Signed-off-by: Stanislav Spassov <stanspas@amazon.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20240524160449.48594-1-stanspas@amazon.de
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.