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When tasks come and go from a runqueue quickly, this can lead to boost being applied and removed quickly which sometimes means we cannot raise the CPU frequency again when we need to (due to the rate limit on frequency updates). This has proved to be a particular issue for RT tasks and alternative methods have been used in the past to work around it. This is an attempt to solve the issue for all task classes and cpufreq governors by introducing a generic mechanism in schedtune to retain the max boost level from task enqueue for a minimum period - defined here as 50ms. This timeout was determined experimentally and is not configurable. A sched_feat guards the application of this to tasks - in the default configuration, task boosting only applied to tasks which have RT policy. Change SCHEDTUNE_BOOST_HOLD_ALL to true to apply it to all tasks regardless of class. It works like so: Every task enqueue (in an allowed class) stores a cpu-local timestamp. If the task is not a member of an allowed class (all or RT depending upon feature selection), the timestamp is not updated. The boost group will stay active regardless of tasks present until 50ms beyond the last timestamp stored. We also store the timestamp of the active boost group to avoid unneccesarily revisiting the boost groups when checking CPU boost level. If the timestamp is more than 50ms in the past when we check boost then we re-evaluate the boost groups for that CPU, taking into account the timestamps associated with each group. Idea based on rt-boost-retention patches from Joel. Change-Id: I52cc2d2e82d1c5aa03550378c8836764f41630c1 Suggested-by: Joel Fernandes <joelaf@google.com> Reviewed-by: Patrick Bellasi <patrick.bellasi@arm.com> Signed-off-by: Chris Redpath <chris.redpath@arm.com> [forward ported from android-4.9-eas-dev proposal] (cherry picked from commit a485e8b7bf8e95759e600396feeb7bfb400b6e46) [ - Trivial cherry-pick conflicts in include/trace/events/sched.h ] Signed-off-by: Quentin Perret <quentin.perret@arm.com> |
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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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.