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DescriptionDaniel Bristot de Oliveira
2021-01-25 15:54:56 UTC
Description of problem:
On large systems, stalld is consuming a noticeable amount of CPU time, mainly when many CPUs are running busy-loop rt-task.
The reason being is that it starts a thread to monitor each CPU that is about to stall, parsing the cpu debug file on each thread independently. While this was a shortcut to make stalld PoC, this is now causing unneeded CPU usage, and we plan to make it single-threaded.
Version-Release number of selected component (if applicable):
1.4
How reproducible:
Always
Steps to Reproduce:
1. Start stalld
2. Make N CPUs busy running busy-loop rt workload
3. Check the number of stalld threads
Actual results:
N+1 threads
Expected results:
one or two threads.
Additional info:
Comment 4Daniel Bristot de Oliveira
2021-07-08 07:37:43 UTC
Hi Fernando,
The main cause for the high CPU usage was the parsing of sched_debug. Previously, it ran at least once every second, and at most M times every second, where M is the number of processors running busy-loop RT tasks (e.g., it was linear to the number of processors).
The granularity option reduced the frequency from "every second" to at every "config granularity," with granularity at five-second by default.
The idle detection reduces the at least "once every granularity" sched_debug parsing to no parse at all if no busy loop task is running.
The single-threaded mode reduced the "M times every granularity" to "once at every granularity" - while boosting only one thread per boost duration. With this, stalld became constant w.r.t the number of processors.
With the boost duration lower than the granularity, the single-threaded was less precise than the multi-threaded in the case of more than one thread starving.
This limitation was removed with the overload control, with a small penalty: instead of being once every granularity, it became min(boost period, granularity) in the worst scenario - still good enough given the default values.
Now stalld is parsing the sched_debug, at least none, and most at min(boost period, granularity). Regarding the min(), the first case happens when the system is running busy loop rt tasks with *more than one starving thread on a CPU*, and the second case when the system has rt busy loop, but with *only one thread starving per cpu*, which seems to be the best trade-off between resource usage and precision.
So, answering your question: yes, it does.
-- Daniel
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (stalld bug fix and enhancement update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHBA-2021:4252