Re: [PATCH -next V7 0/7] riscv: Optimize function trace

From: Evgenii Shatokhin
Date: Mon Jan 16 2023 - 10:14:50 EST

Hi,

On 12.01.2023 12:05, guoren@xxxxxxxxxx wrote:
From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>

The previous ftrace detour implementation fc76b8b8011 ("riscv: Using
PATCHABLE_FUNCTION_ENTRY instead of MCOUNT") contain three problems.

- The most horrible bug is preemption panic which found by Andy [1].
Let's disable preemption for ftrace first, and Andy could continue
the ftrace preemption work.

It seems, the patches #2-#7 of this series do not require "riscv: ftrace: Fixup panic by disabling preemption" and can be used without it.

How about moving that patch out of the series and processing it separately?

As it was pointed out in the discussion of that patch, some other solution to non-atomic changes of the prologue might be needed anyway.

- The "-fpatchable-function-entry= CFLAG" wasted code size
!RISCV_ISA_C.
- The ftrace detour implementation wasted code size.
- When livepatching, the trampoline (ftrace_regs_caller) would not
return to <func_prolog+12> but would rather jump to the new function.
So, "REG_L ra, -SZREG(sp)" would not run and the original return
address would not be restored. The kernel is likely to hang or crash
as a result. (Found by Evgenii Shatokhin [4])

Patches 1,2,3 fixup above problems.

Patches 4,5,6,7 are the features based on reduced detour code
patch, we include them in the series for test and maintenance.

You can directly try it with:
https://github.com/guoren83/linux/tree/ftrace_fixup_v7

Make function graph use ftrace directly [2] (patch 4, 5)
========================================================

In RISC-V architecture, when we enable the ftrace_graph tracer on some
functions, the function tracings on other functions will suffer extra
graph tracing work. In essence, graph_ops isn't limited by its func_hash
due to the global ftrace_graph_[regs]_call label. That should be
corrected.

What inspires me is the commit 0c0593b45c9b ("x86/ftrace: Make function
graph use ftrace directly") that uses graph_ops::func function to
install return_hooker and makes the function called against its
func_hash.

This series of patches makes function graph use ftrace directly for
riscv.

If FTRACE_WITH_REGS isn't defined, ftrace_caller keeps ftrace_graph_call
so that it can be replaced with the calling of prepare_ftrace_return by
the enable/disable helper.

As for defining FTRACE_WITH_REGS, ftrace_caller is adjusted to save the
necessary regs against the pt_regs layout, so it can reasonably call the
graph_ops::func function - ftrace_graph_func. And
ftrace_graph_[regs]_call
and its enable/disable helper aren't needed.

Test log:

The tests generated by CONFIG_FTRACE_STARTUP_TEST have passed in the
local
qemu-system-riscv64 virt machine. The following is the log during
startup.

```
Nov 15 03:07:13 stage4 kernel: Testing tracer function: PASSED
Nov 15 03:07:13 stage4 kernel: Testing dynamic ftrace: PASSED
Nov 15 03:07:13 stage4 kernel: Testing dynamic ftrace ops #1:
Nov 15 03:07:13 stage4 kernel: (1 0 1 0 0)
Nov 15 03:07:13 stage4 kernel: (1 1 2 0 0)
Nov 15 03:07:13 stage4 kernel: (2 1 3 0 365)
Nov 15 03:07:13 stage4 kernel: (2 2 4 0 399)
Nov 15 03:07:13 stage4 kernel: (3 2 4 0 146071)
Nov 15 03:07:13 stage4 kernel: (3 3 5 0 146105) PASSED
Nov 15 03:07:13 stage4 kernel: Testing dynamic ftrace ops #2:
Nov 15 03:07:13 stage4 kernel: (1 0 1 589 0)
Nov 15 03:07:13 stage4 kernel: (1 1 2 635 0)
Nov 15 03:07:13 stage4 kernel: (2 1 3 1 2)
Nov 15 03:07:13 stage4 kernel: (2 2 4 125 126)
Nov 15 03:07:13 stage4 kernel: (3 2 4 146001 146078)
Nov 15 03:07:13 stage4 kernel: (3 3 5 146035 146112) PASSED
Nov 15 03:07:13 stage4 kernel: Testing ftrace recursion: PASSED
Nov 15 03:07:13 stage4 kernel: Testing ftrace recursion safe: PASSED
Nov 15 03:07:13 stage4 kernel: Testing ftrace regs: PASSED
Nov 15 03:07:13 stage4 kernel: Testing tracer nop: PASSED
Nov 15 03:07:13 stage4 kernel: Testing tracer irqsoff: PASSED
Nov 15 03:07:13 stage4 kernel: Testing tracer wakeup:
Nov 15 03:07:13 stage4 kernel: sched: DL replenish lagged too much
Nov 15 03:07:13 stage4 kernel: PASSED
Nov 15 03:07:13 stage4 kernel: Testing tracer wakeup_rt: PASSED
Nov 15 03:07:13 stage4 kernel: Testing tracer wakeup_dl: PASSED
Nov 15 03:07:13 stage4 kernel: Testing tracer function_graph: PASSED
```

Add WITH_DIRECT_CALLS support [3] (patch 6, 7)
==============================================

This series adds DYNAMIC_FTRACE_WITH_DIRECT_CALLS support for RISC-V.
SAMPLE_FTRACE_DIRECT and SAMPLE_FTRACE_DIRECT_MULTI are also included
here as the samples for testing DIRECT_CALLS related interface.

First, select the DYNAMIC_FTRACE_WITH_DIRECT_CALLS to provide
register_ftrace_direct[_multi] interfaces allowing user to register
the customed trampoline (direct_caller) as the mcount for one or
more target functions. And modify_ftrace_direct[_multi] are also
provided for modify direct_caller.

At the same time, the samples in ./samples/ftrace/ can be built
as kerenl module for testing these interfaces with SAMPLE_FTRACE_DIRECT
and SAMPLE_FTRACE_DIRECT_MULTI selected.

Second, to make the direct_caller and the other ftrace hooks
(eg. function/fgraph tracer, k[ret]probes) co-exist, a temporary
register
are nominated to store the address of direct_caller in
ftrace_regs_caller.
After the setting of the address direct_caller by direct_ops->func and
the RESTORE_REGS in ftrace_regs_caller, direct_caller will be jumped to
by the `jr` inst.

The following tests have been passed in my local qemu-riscv64 virt
machine.

1. tests with CONFIG_FTRACE_STARTUP_TEST
2. tests of samples/ftrace/ftrace*.ko
3. manual tests with any combination of the following hooks
- function/function_graph tracer
- ftrace*.ko
- kprobe/kretprobe

For your reference, here is the log when function tracer, kretprobe and
ftrace-direct-too.ko co-hooks the handle_mm_fault function.

```
[root@stage4 tracing]# echo handle_mm_fault > set_ftrace_filter
[root@stage4 tracing]# echo 'r:myr handle_mm_fault' > kprobe_events
[root@stage4 tracing]# echo function > current_tracer
[root@stage4 tracing]# echo 1 > events/kprobes/myr/enable
[root@stage4 tracing]# insmod /root/ftrace-direct-too.ko
[root@stage4 tracing]#
[root@stage4 tracing]# cat trace | tail
cat-388 [000] ...1. 583.051438: myr:
(do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
cat-388 [000] ...2. 583.057930: handle_mm_fault
<-do_page_fault
cat-388 [000] ..... 583.057990: my_direct_func:
handle mm fault vma=000000002d9fe19c address=ffffffae9b7000 flags=215
cat-388 [000] ...1. 583.058284: myr:
(do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
tail-389 [001] ...2. 583.059062: handle_mm_fault
<-do_page_fault
tail-389 [001] ..... 583.059104: my_direct_func:
handle mm fault vma=0000000017f3c48e address=aaaaaabebf3000 flags=215
tail-389 [001] ...1. 583.059325: myr:
(do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
tail-389 [001] ...2. 583.060371: handle_mm_fault
<-do_page_fault
tail-389 [001] ..... 583.060410: my_direct_func:
handle mm fault vma=0000000017f3c48e address=aaaaaabebf1000 flags=255
tail-389 [001] ...1. 583.060996: myr:
(do_page_fault+0x16c/0x5f2 <- handle_mm_fault)
```
Note1:

The checkpatch.pl will output some warnings on this series, like this

```
WARNING: Prefer using '"%s...", __func__' to using 'my_direct_func2',
this function's name, in a string
111: FILE: samples/ftrace/ftrace-direct-multi-modify.c:48:
+" call my_direct_func2\n"
```

The reason is that checkpatch depends on patch context providing the
function name. In the above warning, my_direct_func2 has some codeline
distance with the changed trunk, so its declaration doesn't come into
the patch, and then the warning jumps out.

You may notice the location of `my_ip` variable changes in the 2nd
patch. I did that for reducing the warnings to some extent. But killing
all the warnings will makes the patch less readable, so I stopped here.

[1] https://lpc.events/event/16/contributions/1171/
[2] https://lore.kernel.org/lkml/20221120084230.910152-1-suagrfillet@xxxxxxxxx/
[3] https://lore.kernel.org/linux-riscv/20221123142025.1504030-1-suagrfillet@xxxxxxxxx/
[4] https://lore.kernel.org/linux-riscv/d7d5730b-ebef-68e5-5046-e763e1ee6164@xxxxxxxxx/

Changes in v7:
- Fixup RESTORE_ABI_REGS by remove PT_T0(sp) overwrite.
- Add FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY
- Fixup kconfig with HAVE_SAMPLE_FTRACE_DIRECT &
HAVE_SAMPLE_FTRACE_DIRECT_MULTI

Changes in v6:
https://lore.kernel.org/linux-riscv/20230107133549.4192639-1-guoren@xxxxxxxxxx/
- Replace 8 with MCOUNT_INSN_SIZE
- Replace "REG_L a1, PT_RA(sp)" with "mv a1, ra"
- Add Evgenii Shatokhin comment

Changes in v5:
https://lore.kernel.org/linux-riscv/20221208091244.203407-1-guoren@xxxxxxxxxx/
- Sort Kconfig entries in alphabetical order.

Changes in v4:
https://lore.kernel.org/linux-riscv/20221129033230.255947-1-guoren@xxxxxxxxxx/
- Include [3] for maintenance. [Song Shuai]

Changes in V3:
https://lore.kernel.org/linux-riscv/20221123153950.2911981-1-guoren@xxxxxxxxxx/
- Include [2] for maintenance. [Song Shuai]

Changes in V2:
https://lore.kernel.org/linux-riscv/20220921034910.3142465-1-guoren@xxxxxxxxxx/
- Add Signed-off for preemption fixup.

Changes in V1:
https://lore.kernel.org/linux-riscv/20220916103817.9490-1-guoren@xxxxxxxxxx/

Andy Chiu (1):
riscv: ftrace: Fixup panic by disabling preemption

Guo Ren (2):
riscv: ftrace: Remove wasted nops for !RISCV_ISA_C
riscv: ftrace: Reduce the detour code size to half

Song Shuai (4):
riscv: ftrace: Add ftrace_graph_func
riscv: ftrace: Add DYNAMIC_FTRACE_WITH_DIRECT_CALLS support
samples: ftrace: Add riscv support for SAMPLE_FTRACE_DIRECT[_MULTI]
riscv : select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY

arch/riscv/Kconfig | 6 +-
arch/riscv/Makefile | 6 +-
arch/riscv/include/asm/ftrace.h | 71 ++++++--
arch/riscv/kernel/ftrace.c | 91 ++++------
arch/riscv/kernel/mcount-dyn.S | 179 +++++++++++++-------
samples/ftrace/ftrace-direct-modify.c | 33 ++++
samples/ftrace/ftrace-direct-multi-modify.c | 37 ++++
samples/ftrace/ftrace-direct-multi.c | 22 +++
samples/ftrace/ftrace-direct-too.c | 26 +++
samples/ftrace/ftrace-direct.c | 22 +++
10 files changed, 356 insertions(+), 137 deletions(-)

--
2.36.1



I tested this series a bit on top of 6.2-rc4, with https://github.com/sugarfillet/linux/commit/9539a80dc6e7d1137ec7a96ebef2ab912a694bd7.patch added.

The kernel was built with CONFIG_DYNAMIC_FTRACE_WITH_REGS=y and CONFIG_RISCV_ISA_C=y.

Here are some results:

* The kernel was built without visible issues.

* Most of the Ftrace startup tests ran and passed. Certain event-specific tests caused soft lockups in one of the test runs but they are likely unrelated to this patchset and could happen without it too.

* "function" and "function_graph" tracers worked in my simple use cases.

* "ipmodify" functionality worked.

* kprobe sample modules worked OK, which is good, because they actually use Ftrace: they plant the probes at the beginning of the resp. functions.

* ftrace-direct-multi and ftrace-direct-multi-modify sample modules reported possibly invalid data. More info - in my reply to
"[PATCH -next V7 6/7] samples: ftrace: Add riscv support for SAMPLE_FTRACE_DIRECT[_MULTI]"

Regards,
Evgenii