Re: [PATCH v3 0/7] riscv: ftrace: atmoic patching and preempt improvements

Björn Töpel
2024-11-27T21:26:13Z

Adding Steven.

Andy Chiu <andybnac@gmail.com> writes:

> This series makes atmoic code patching possible in riscv ftrace. A
> direct benefit of this is that we can get rid of stop_machine() when
> patching function entries. This also makes it possible to run ftrace
> with full kernel preemption. Before this series, the kernel initializes
> patchable function entries to NOP4 + NOP4. To start tracing, it updates
> entries to AUIPC + JALR while holding other cores in stop_machine.
> stop_machine() is required because it is impossible to update 2
> instructions, and be seen atomically. And preemption must have to be
> prevented, as kernel preemption allows process to be scheduled out while
> executing on one of these instruction pairs.
>
> This series addresses the problem by initializing the first NOP4 to
> AUIPC. So, atmoic patching is possible because the kernel only has to
> update one instruction. As long as the instruction is naturally aligned,
> then it is expected to be updated atomically.
>
> However, the address range of the ftrace trampoline is limited to +-2K
> from ftrace_caller after appplying this series. This issue is expected
> to be solved by Puranjay's CALL_OPS, where it adds 8B naturally align
> data in front of pacthable functions and can use it to direct execution
> out to any custom trampolines.
>
> The series is composed by three parts. The first part cleans up the
> existing issues when the kernel is compiled with clang.The second part
> modifies the ftrace code patching mechanism (2-4) as mentioned above.
> Then prepare ftrace to be able to run with kernel preemption (5,6)
>
> An ongoing fix:
>
> Since there is no room for marking *kernel_text_address as notrace[1] at
> source code level, there is a significant performance regression when
> using function_graph with TRACE_IRQFLAGS enabled. There can be as much as
> 8 graph handler being called in each function-entry. The current
> workaround requires us echo "*kernel_text_address" into
> set_ftrace_notrace before starting the trace. However, we observed that
> the kernel still enables the patch site in some cases even with
> *kernel_text_address properly added in the file While the root cause is
> still under investagtion, we consider that it should not be the reason
> for holding back the code patching, in order to unblock the call_ops
> part.

Maybe Steven knows this from the top of his head!

As Andy points out, "*kernel_text_address" is used in the stack
unwinding on RISC-V. So, if you do a tracing without filtering *and*
TRACE_IRQFLAGS, one will drown in traces.

E.g. the ftrace selftest:
| $ ./ftracetest -vvv test.d/ftrace/fgraph-multi.tc

will generate a lot of traces.

Now, if we add:
--8<--
diff --git a/tools/testing/selftests/ftrace/test.d/ftrace/fgraph-multi.tc b/tools/testing/selftests/ftrace/test.d/ftrace/fgraph-multi.tc
index ff88f97e41fb..4f30a4d81d99 100644
--- a/tools/testing/selftests/ftrace/test.d/ftrace/fgraph-multi.tc
+++ b/tools/testing/selftests/ftrace/test.d/ftrace/fgraph-multi.tc
@@ -84,6 +84,7 @@ cd $INSTANCE2
do_test '*rcu*' 'rcu'
cd $WD
cd $INSTANCE3
+echo '*kernel_text_address' > set_ftrace_notrace
echo function_graph > current_tracer

sleep 1
-->8--

The graph tracer will not honor the "set_ftrace_notrace" in $INSTANCE3,
but still enable the *kernel_text_address traces. (Note that there are
no filters in the test, so *all* ftrace recs will be enabled.)

Are we holding the graph tracer wrong?

Happy thanksgiving!
Björn