mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2025-01-19 20:12:32 +00:00
12f54c3a84
Currently, there is one no-CBs rcuo kthread per CPU, and these kthreads are divided into groups. The first rcuo kthread to come online in a given group is that group's leader, and the leader both waits for grace periods and invokes its CPU's callbacks. The non-leader rcuo kthreads only invoke callbacks. This works well in the real-time/embedded environments for which it was intended because such environments tend not to generate all that many callbacks. However, given huge floods of callbacks, it is possible for the leader kthread to be stuck invoking callbacks while its followers wait helplessly while their callbacks pile up. This is a good recipe for an OOM, and rcutorture's new callback-flood capability does generate such OOMs. One strategy would be to wait until such OOMs start happening in production, but similar OOMs have in fact happened starting in 2018. It would therefore be wise to take a more proactive approach. This commit therefore features per-CPU rcuo kthreads that do nothing but invoke callbacks. Instead of having one of these kthreads act as leader, each group has a separate rcog kthread that handles grace periods for its group. Because these rcuog kthreads do not invoke callbacks, callback floods on one CPU no longer block callbacks from reaching the rcuc callback-invocation kthreads on other CPUs. This change does introduce additional kthreads, however: 1. The number of additional kthreads is about the square root of the number of CPUs, so that a 4096-CPU system would have only about 64 additional kthreads. Note that recent changes decreased the number of rcuo kthreads by a factor of two (CONFIG_PREEMPT=n) or even three (CONFIG_PREEMPT=y), so this still represents a significant improvement on most systems. 2. The leading "rcuo" of the rcuog kthreads should allow existing scripting to affinity these additional kthreads as needed, the same as for the rcuop and rcuos kthreads. (There are no longer any rcuob kthreads.) 3. A state-machine approach was considered and rejected. Although this would allow the rcuo kthreads to continue their dual leader/follower roles, it complicates callback invocation and makes it more difficult to consolidate rcuo callback invocation with existing softirq callback invocation. The introduction of rcuog kthreads should thus be acceptable. Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.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.