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While running in nohz_full mode, a task may enqueue a timer while the tick is stopped. However the only places where the timer wheel, alongside the timer migration machinery's decision, may reprogram the next event accordingly with that new timer's expiry are the idle loop or any IRQ tail. However neither the idle task nor an interrupt may run on the CPU if it resumes busy work in userspace for a long while in full dynticks mode. To solve this, the timer enqueue path raises a self-IPI that will re-evaluate the timer wheel on its IRQ tail. This asynchronous solution avoids potential locking inversion. This is supposed to happen both for local and global timers but commit:b2cf7507e1("timers: Always queue timers on the local CPU") broke the global timers case with removing the ->is_idle field handling for the global base. As a result, global timers enqueue may go unnoticed in nohz_full. Fix this with restoring the idle tracking of the global timer's base, allowing self-IPIs again on enqueue time. Fixes:b2cf7507e1("timers: Always queue timers on the local CPU") Reported-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20240318230729.15497-3-frederic@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.