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The instructions for the ftrace-bisect.sh script, which is used to find
what function is being traced that is causing a kernel crash, and possibly
a triple fault reboot, uses the old method. In 5.1, a new feature was
added that let the user write in the index into available_filter_functions
that maps to the function a user wants to set in set_ftrace_filter (or
set_ftrace_notrace). This takes O(1) to set, as suppose to writing a
function name, which takes O(n) (where n is the number of functions in
available_filter_functions).
The ftrace-bisect.sh requires setting half of the functions in
available_filter_functions, which is O(n^2) using the name method to enable
and can take several minutes to complete. The number method is O(n) which
takes less than a second to complete. Using the number method for any
kernel 5.1 and after is the proper way to do the bisect.
Update the usage to reflect the new change, as well as using the
/sys/kernel/tracing path instead of the obsolete debugfs path.
Link: https://lkml.kernel.org/r/20230123112252.022003dd@gandalf.local.home
Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Fixes: f79b3f3385 ("ftrace: Allow enabling of filters via index of available_filter_functions")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
135 lines
3.9 KiB
Bash
Executable File
135 lines
3.9 KiB
Bash
Executable File
#!/bin/bash
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# SPDX-License-Identifier: GPL-2.0
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#
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# Here's how to use this:
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#
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# This script is used to help find functions that are being traced by function
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# tracer or function graph tracing that causes the machine to reboot, hang, or
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# crash. Here's the steps to take.
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#
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# First, determine if function tracing is working with a single function:
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#
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# (note, if this is a problem with function_graph tracing, then simply
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# replace "function" with "function_graph" in the following steps).
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#
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# # cd /sys/kernel/tracing
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# # echo schedule > set_ftrace_filter
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# # echo function > current_tracer
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#
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# If this works, then we know that something is being traced that shouldn't be.
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#
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# # echo nop > current_tracer
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#
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# Starting with v5.1 this can be done with numbers, making it much faster:
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#
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# The old (slow) way, for kernels before v5.1.
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#
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# [old-way] # cat available_filter_functions > ~/full-file
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#
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# [old-way] *** Note *** this process will take several minutes to update the
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# [old-way] filters. Setting multiple functions is an O(n^2) operation, and we
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# [old-way] are dealing with thousands of functions. So go have coffee, talk
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# [old-way] with your coworkers, read facebook. And eventually, this operation
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# [old-way] will end.
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#
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# The new way (using numbers) is an O(n) operation, and usually takes less than a second.
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#
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# seq `wc -l available_filter_functions | cut -d' ' -f1` > ~/full-file
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#
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# This will create a sequence of numbers that match the functions in
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# available_filter_functions, and when echoing in a number into the
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# set_ftrace_filter file, it will enable the corresponding function in
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# O(1) time. Making enabling all functions O(n) where n is the number of
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# functions to enable.
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#
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# For either the new or old way, the rest of the operations remain the same.
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#
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# # ftrace-bisect ~/full-file ~/test-file ~/non-test-file
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# # cat ~/test-file > set_ftrace_filter
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#
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# # echo function > current_tracer
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#
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# If it crashes, we know that ~/test-file has a bad function.
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#
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# Reboot back to test kernel.
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#
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# # cd /sys/kernel/tracing
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# # mv ~/test-file ~/full-file
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#
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# If it didn't crash.
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#
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# # echo nop > current_tracer
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# # mv ~/non-test-file ~/full-file
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#
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# Get rid of the other test file from previous run (or save them off somewhere).
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# # rm -f ~/test-file ~/non-test-file
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#
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# And start again:
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#
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# # ftrace-bisect ~/full-file ~/test-file ~/non-test-file
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#
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# The good thing is, because this cuts the number of functions in ~/test-file
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# by half, the cat of it into set_ftrace_filter takes half as long each
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# iteration, so don't talk so much at the water cooler the second time.
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#
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# Eventually, if you did this correctly, you will get down to the problem
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# function, and all we need to do is to notrace it.
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#
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# The way to figure out if the problem function is bad, just do:
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#
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# # echo <problem-function> > set_ftrace_notrace
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# # echo > set_ftrace_filter
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# # echo function > current_tracer
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#
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# And if it doesn't crash, we are done.
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#
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# If it does crash, do this again (there's more than one problem function)
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# but you need to echo the problem function(s) into set_ftrace_notrace before
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# enabling function tracing in the above steps. Or if you can compile the
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# kernel, annotate the problem functions with "notrace" and start again.
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#
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if [ $# -ne 3 ]; then
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echo 'usage: ftrace-bisect full-file test-file non-test-file'
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exit
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fi
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full=$1
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test=$2
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nontest=$3
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x=`cat $full | wc -l`
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if [ $x -eq 1 ]; then
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echo "There's only one function left, must be the bad one"
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cat $full
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exit 0
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fi
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let x=$x/2
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let y=$x+1
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if [ ! -f $full ]; then
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echo "$full does not exist"
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exit 1
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fi
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if [ -f $test ]; then
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echo -n "$test exists, delete it? [y/N]"
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read a
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if [ "$a" != "y" -a "$a" != "Y" ]; then
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exit 1
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fi
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fi
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if [ -f $nontest ]; then
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echo -n "$nontest exists, delete it? [y/N]"
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read a
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if [ "$a" != "y" -a "$a" != "Y" ]; then
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exit 1
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fi
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fi
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sed -ne "1,${x}p" $full > $test
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sed -ne "$y,\$p" $full > $nontest
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