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

Andy Chiu
2024-12-11T15:38:41Z

Hi Evgenii,

Evgenii Shatokhin <e.shatokhin@yadro.com> 於 2024年12月2日 週一 下午3:58寫道：
>
> Hi,
>
> On 27.11.2024 20:29, Andy Chiu wrote:
> > 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.
> >
> > [1]: https://lore.kernel.org/linux-riscv/20240613093233.0b349ed0@rorschach.local.home/
> >
> > Changes in v3:
> > - Add a fix tag for patch 1
> > - Add a data fence before sending out remote fence.i (6)
> > - Link to v2: https://lore.kernel.org/all/20240628-dev-andyc-dyn-ftrace-v4-v2-0-1e5f4cb1f049@sifive.com/
> >
> > Changes in v2:
> > - Drop patch 1 as it is merged through fixes.
> > - Drop patch 2, which converts kernel_text_address into notrace. As
> > users can prevent tracing it by configuring the tracefs.
> > - Use a more generic way in kconfig to align functions.
> > - Link to v1: https://lore.kernel.org/r/20240613-dev-andyc-dyn-ftrace-v4-v1-0-1a538e12c01e@sifive.com
> >
> >
> > Andy Chiu (7):
> > riscv: ftrace: support fastcc in Clang for WITH_ARGS
> > riscv: ftrace: align patchable functions to 4 Byte boundary
> > riscv: ftrace: prepare ftrace for atomic code patching
> > riscv: ftrace: do not use stop_machine to update code
> > riscv: vector: Support calling schedule() for preemptible Vector
> > riscv: add a data fence for CMODX in the kernel mode
> > riscv: ftrace: support PREEMPT
> >
> > arch/riscv/Kconfig | 4 +-
> > arch/riscv/include/asm/ftrace.h | 11 +++
> > arch/riscv/include/asm/processor.h | 5 ++
> > arch/riscv/include/asm/vector.h | 22 ++++-
> > arch/riscv/kernel/asm-offsets.c | 7 ++
> > arch/riscv/kernel/ftrace.c | 133 ++++++++++++-----------------
> > arch/riscv/kernel/mcount-dyn.S | 25 ++++--
> > arch/riscv/mm/cacheflush.c | 15 +++-
> > 8 files changed, 135 insertions(+), 87 deletions(-)
> > ---
> > base-commit: 0eb512779d642b21ced83778287a0f7a3ca8f2a1
> > --
> > 2.39.3 (Apple Git-145)
>
> I have tested this series in a QEMU VM (-machine virt) with the
> preemptible kernels, CONFIG_PREEMPT=y.
>
> No issues have been revealed so far.
>
> One of the kernels was built by GCC 13.2.0 (with the patch for minimum
> alignment added on top of it), the other - with LLVM 17.0.6.
>
> In both cases, the basic boottime tests for Ftrace passed.
>
> Switching tracers between nop, function, function_graph and blk in a
> loop in parallel with stress-ng and with network load for several hours
> did not reveal any problems either. Kernel crashes happened in this
> scenario a year ago, but now it runs clean, good!
>
> Function redirection via Ftrace seems to work OK too.
>
> The size of .text section increased slightly after this series, by 0.35%
> - 0.38%, probably because of function alignment:
> * 12075 KB => 12121 KB (GCC)
> * 12167 KB => 12209 KB (LLVM/clang)
>
> Not sure, how to test the vector-related part though, "[PATCH v3 5/7]
> riscv: vector: Support calling schedule() for preemptible Vector"

This should be tested as long as both the kernel is compiled with
RISCV_ISA_V and PREEMPT, and the hardware supports v.

And thanks for the extensive testing

>
> For all other patches in the series:
> Tested-by: Evgenii Shatokhin <e.shatokhin@yadro.com>
>
> Regards,
> Evgenii