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This is a tool that is intended to work around the fact that the preemptoff, irqsoff, and preemptirqsoff tracers only work in overwrite mode. The idea is to act randomly in such a way that we do not systematically lose any latencies, so that if enough testing is done, all latencies will be captured. If the same burst of latencies is repeated, then sooner or later we will have captured all the latencies. It also works with the wakeup_dl, wakeup_rt, and wakeup tracers. However, in that case it is probably not useful to use the random sleep functionality. The reason why it may be desirable to catch all latencies with a long test campaign is that for some organizations, it's necessary to test the kernel in the field and not practical for developers to work iteratively with field testers. Because of cost and project schedules it is not possible to start a new test campaign every time a latency problem has been fixed. It uses inotify to detect changes to /sys/kernel/tracing/trace. When a latency is detected, it will either sleep or print immediately, depending on a function that act as an unfair coin toss. If immediate print is chosen, it means that we open /sys/kernel/tracing/trace and thereby cause a blackout period that will hide any subsequent latencies. If sleep is chosen, it means that we wait before opening /sys/kernel/tracing/trace, by default for 1000 ms, to see if there is another latency during this period. If there is, then we will lose the previous latency. The coin will be tossed again with a different probability, and we will either print the new latency, or possibly a subsequent one. The probability for the unfair coin toss is chosen so that there is equal probability to obtain any of the latencies in a burst. However, this assumes that we make an assumption of how many latencies there can be. By default the program assumes that there are no more than 2 latencies in a burst, the probability of immediate printout will be: 1/2 and 1 Thus, the probability of getting each of the two latencies will be 1/2. If we ever find that there is more than one latency in a series, meaning that we reach the probability of 1, then the table will be expanded to: 1/3, 1/2, and 1 Thus, we assume that there are no more than three latencies and each with a probability of 1/3 of being captured. If the probability of 1 is reached in the new table, that is we see more than two closely occurring latencies, then the table will again be extended, and so on. On my systems, it seems like this scheme works fairly well, as long as the latencies we trace are long enough, 300 us seems to be enough. This userspace program receive the inotify event at the end of a latency, and it has time until the end of the next latency to react, that is to open /sys/kernel/tracing/trace. Thus, if we trace latencies that are >300 us, then we have at least 300 us to react. The minimum latency will of course not be 300 us on all systems, it will depend on the hardware, kernel version, workload and configuration. Example usage: In one shell, give the following command: sudo latency-collector -rvv -t preemptirqsoff -s 2000 -a 3 This will trace latencies > 2000us with the preemptirqsoff tracer, using random sleep with maximum verbosity, with a probability table initialized to a size of 3. In another shell, generate a few bursts of latencies: root@host:~# modprobe preemptirq_delay_test delay=3000 test_mode=alternate burst_size=3 root@host:~# echo 1 > /sys/kernel/preemptirq_delay_test/trigger root@host:~# echo 1 > /sys/kernel/preemptirq_delay_test/trigger root@host:~# echo 1 > /sys/kernel/preemptirq_delay_test/trigger root@host:~# echo 1 > /sys/kernel/preemptirq_delay_test/trigger If all goes well, you should be getting stack traces that shows all the different latencies, i.e. you should see all the three functions preemptirqtest_0, preemptirqtest_1, preemptirqtest_2 in the stack traces. Link: https://lkml.kernel.org/r/20210212134421.172750-2-Viktor.Rosendahl@bmw.de Signed-off-by: Viktor Rosendahl <Viktor.Rosendahl@bmw.de> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> |
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| kernel | ||
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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.