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Kunkun Jiang reports that for a workload involving the simultaneous startup of a large number of VMs (for a total of about 200 vcpus), a lot of CPU time gets spent on spinning on the tmp_mask_lock that exists as a static raw spinlock in irq_do_set_affinity(). This lock protects a global cpumask (tmp_mask) that is used as a temporary variable to compute the resulting affinity. While this is triggered by KVM issuing a irq_set_affinity() call each time a vcpu is about to execute, it is obvious that having a single global resource is not very scalable. Since a cpumask can be a fairly large structure on systems with a high core count, a stack allocation is not really appropriate. Instead, turn the global cpumask into a per-CPU variable, removing the need for locking altogether as the code is executed with preemption and interrupts disabled. [ tglx: Moved the per CPU variable declaration outside of the function ] Reported-by: Kunkun Jiang <jiangkunkun@huawei.com> Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Marc Zyngier <maz@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Kunkun Jiang <jiangkunkun@huawei.com> Link: https://lore.kernel.org/all/20240826080618.3886694-1-maz@kernel.org Link: https://lore.kernel.org/all/a7fc58e4-64c2-77fc-c1dc-f5eb78dbbb01@huawei.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 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.