Re: [PATCH v4 07/18] tracing/remotes: selftests: Add a test for the dmesg tracefs file

Adding Sashiko review: https://sashiko.dev/#/patchset/20260804140333.797210-2-vdonnefort%40google.com


> --- Patch [7]: [PATCH v4 07/18] tracing/remotes: selftests: Add a test for the dmesg tracefs file ---
> commit e79aa4baf215905a3bfd330e32c4f3d3b06f9a1c
> Author: Vincent Donnefort <[email protected]>
> 
> tracing/remotes: selftests: Add a test for the dmesg tracefs file
> 
> This commit adds a new ftrace selftest for the dmesg tracefs file. It
> exercises the dmesg redirection by toggling it on and off, and verifies
> that events are correctly logged to or withheld from the kernel log.
> 
> > diff --git a/tools/testing/selftests/ftrace/test.d/remotes/dmesg.tc b/tools/testing/selftests/ftrace/test.d/remotes/dmesg.tc
> > new file mode 100644
> > --- /dev/null
> > +++ b/tools/testing/selftests/ftrace/test.d/remotes/dmesg.tc
> 
> [ ... ]
> 
> > +test_dmesg()
> > +{
> > +    echo 0 > tracing_on
> > +    assert_unloaded
> > +
> > +    #
> > +    # Test dmesg on/off when tracing is disabled
> > +    #
> > +    echo 1 > dmesg
> > +    test $(cat dmesg) -eq 1
> > +    assert_loaded
> > +
> > +    echo 0 > dmesg
> > +    test $(cat dmesg) -eq 0
> > +    assert_unloaded
> > +
> > +    #
> > +    # Test events are logged to dmesg
> > +    #
> > +    dmesg -c > /dev/null
> 
> Does this indiscriminately clear the system-wide kernel ring buffer?
> 
> Flushing the global log buffer destroys all previous kernel log history.
> In automated testing frameworks, this can suppress earlier warnings, oopses,
> or bug reports that occurred during the test run or kernel boot, making
> failure triage much more difficult.
> 
> Could this be adjusted to isolate the test output without permanently
> wiping the global dmesg history?

I agree, tests should not clear dmesg history.

-- Steve

> 
> > +
> > +    echo 1 > tracing_on
> > +    assert_loaded
> > +    echo 1 > dmesg
> > +    test $(cat dmesg) -eq 1