Re: [PATCH] perf evsel: Improve frame pointer unwinding warning for s390

[sashiko-bot@kernel.org]

> On s390 the kernel uses s390 back chain instead of frame pointers for
> stack tracing of user space since v6.7 commit aa44433ac4ee ("s390: add
> USER_STACKTRACE support").  This is because frame pointers on s390
> cannot be used for stack tracing. [1]
> 
> This requires user space to maintain a s390 back chain.  For instance
> user space to be built with compiler option '-mbackchain' (instead of
> '-fno-omit-frame-pointer' used on other architectures, which should
> better not be used on s390 [1]).
> 
> Only few distributions and users built user space with '-mbackchain'.
> Therefore '--call-graph fp' may not produce the expected results.
> 
> Commit ca76fb67ebdd ("perf evlist: Improve default event for s390")
> added a warning for s390 that wrongly claimed that "Framepointer
> unwinding lacks kernel support".  Change the warning to hint at using
> '--call-graph dwarf' if user space does not maintain a s390 back chain.
> 
> Note that '--call-graph fp' may also be useful for other applications,
> such as OpenJDK maintaining a s390 back chain (does not require JVM
> option '-XX:+PreserveFramePointer' on s390):
> 
>   $ perf record --call-graph fp ... -- \
>     java -XX:+UnlockDiagnosticVMOptions -XX:+DumpPerfMapAtExit ...
> 
> [1]: s390: Stack tracing using Frame Pointer, Back Chain, and SFrame,
>      https://conf.gnu-tools-cauldron.org/opo25/talk/Y3CVHY/
> 
> Fixes: ca76fb67ebdd ("perf evlist: Improve default event for s390")
> Signed-off-by: Jens Remus <jremus@linux.ibm.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260807104913.1784943-1-jremus@linux.ibm.com?part=1