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Tests showed, that under certain conditions, the summary number of jiffies spent on softirq/idle, which are counted by system statistics can be even below 10% of expected value, resulting in false load presentation. The issue was observed on the quad-core Marvell Armada 8k SoC, whose two 10G ports were bound into L2 bridge. Load was controlled by bidirectional UDP traffic, produced by a packet generator. Under such condition, the dominant load is softirq. With 100% single CPU occupation or without any activity (all CPUs 100% idle), total number of jiffies is 10000 (2500 per each core) in 10s interval. Also with other kind of load this was true. However below a saturation threshold it was observed, that with CPU which was occupied almost by softirqs only, the statistic were awkward. See the mpstat output: CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle all 0.00 0.00 0.13 0.00 0.00 0.55 0.00 0.00 0.00 99.32 0 0.00 0.00 0.00 0.00 0.00 23.08 0.00 0.00 0.00 76.92 1 0.00 0.00 0.40 0.00 0.00 0.00 0.00 0.00 0.00 99.60 2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 100.00 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 100.00 Above would mean basically no total load, debug CPU0 occupied in 25%. Raw statistics, printed every 10s from /proc/stat unveiled a root cause - summary idle/softirq jiffies on loaded CPU were below 200, i.e. over 90% samples lost. All problems were gone after enabling fine granulity IRQ time accounting. This patch fixes possible wrong statistics processing by enabling CONFIG_IRQ_TIME_ACCOUNTING for arm64 platfroms, which is by default done on other architectures, e.g. x86 and arm. Tests showed no noticeable performance penalty, nor stability impact. Signed-off-by: Marcin Wojtas <mw@semihalf.com> Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. 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.