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On PREEMPT_RT enabled kernels hrtimers which are not explicitely marked for hard interrupt expiry mode are moved into soft interrupt context either for latency reasons or because the hrtimer callback takes regular spinlocks or invokes other functions which are not suitable for hard interrupt context on PREEMPT_RT. The hrtimer_sleeper callback is RT compatible in hard interrupt context, but there is a latency concern: Untrusted userspace can spawn many threads which arm timers for the same expiry time on the same CPU. On expiry that causes a latency spike due to the wakeup of a gazillion threads. OTOH, priviledged real-time user space applications rely on the low latency of hard interrupt wakeups. These syscall related wakeups are all based on hrtimer sleepers. If the current task is in a real-time scheduling class, mark the mode for hard interrupt expiry. [ tglx: Split out of a larger combo patch. Added changelog ] Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20190726185753.645792403@linutronix.de |
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| init | ||
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| README | ||
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.