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Description of problem:
The [scheduler] plugin of the Tuned daemon seems to set CPU affinity to only existing processes and misses newly fork()ed ones.
Version-Release number of selected component (if applicable):
All
How reproducible:
Always
Steps to Reproduce:
1. use the Tuned [scheduler] plugin
2. watch the affinity of existing processes correctly changed
3. in a terminal, change a cpu affinity of a bash process
4. run "sleep 7777777 &"
Actual results:
5. tuned will miss to set the affinity to the newly forked process
Expected results:
5. tuned sets the affinity to the newly forked process correctly
Additional info:
The problem seems to be in the _thread_code() function. An event perf.RECORD_FORK is being missed there.
Comment 1Jaroslav Škarvada
2020-11-06 10:05:45 UTC
Currently, the event filter is catching exec calls and prctl(PR_SET_NAME). Even with such optimized event filter we have received several reports that the perf code overhead is quite big for some performance benchmarks (IIRC up to 30 %). I guess with the event filter extended of the fork events, the number of triggers of the perf code will probably increase by a magnitude. I am afraid it will kill the performance, because the overhead will be bigger. Also I think that the most of the perf triggers will be useless and will just waste CPU time, because usually the child process inherits the parent affinity.
I think we could add configuration switch to the scheduler plugin to add the fork to the event filter, but I think such configuration should be by default off.
There is also possibility to use the cgroups feature of the scheduler plugin which has been introduced in tuned-2.14.0.
Comment 4Jaroslav Škarvada
2020-11-24 20:05:40 UTC
Upstream PR:
https://github.com/redhat-performance/tuned/pull/309
Added `perf_process_fork` parameter. If set to `true` it will also listen to perf.RECORD_FORK events, i.e. it will explicitly process child processes created by the fork system call. Usually, child processes inherit affinity from their parents, thus the `scheduler` plugin doesn't need to explicitly process them, that's why the `perf_process_fork` parameter is by default set to `false` and child processes aren't explicitly processed. This default setting may also save some CPU time especially on loaded systems.
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 (tuned 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-2021:1671