Re: [PATCH] ftrace: Fix DIRECT_CALLS to use SAVE_REGS by default
From: Mark Rutland <mark.rutland@arm.com>
Date: 2024-01-10 12:20:31
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On Wed, Jan 10, 2024 at 09:13:06AM +0900, Masami Hiramatsu (Google) wrote:
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
The commit 60c8971899f3 ("ftrace: Make DIRECT_CALLS work WITH_ARGS
and !WITH_REGS") changed DIRECT_CALLS to use SAVE_ARGS when there
are multiple ftrace_ops at the same function, but since the x86 only
support to jump to direct_call from ftrace_regs_caller, when we set
the function tracer on the same target function on x86, ftrace-direct
does not work as below (this actually works on arm64.)
At first, insmod ftrace-direct.ko to put a direct_call on
'wake_up_process()'.
# insmod kernel/samples/ftrace/ftrace-direct.ko
# less trace
...
<idle>-0 [006] ..s1. 564.686958: my_direct_func: waking up rcu_preempt-17
<idle>-0 [007] ..s1. 564.687836: my_direct_func: waking up kcompactd0-63
<idle>-0 [006] ..s1. 564.690926: my_direct_func: waking up rcu_preempt-17
<idle>-0 [006] ..s1. 564.696872: my_direct_func: waking up rcu_preempt-17
<idle>-0 [007] ..s1. 565.191982: my_direct_func: waking up kcompactd0-63
Setup a function filter to the 'wake_up_process' too, and enable it.
# cd /sys/kernel/tracing/
# echo wake_up_process > set_ftrace_filter
# echo function > current_tracer
# less trace
...
<idle>-0 [006] ..s3. 686.180972: wake_up_process <-call_timer_fn
<idle>-0 [006] ..s3. 686.186919: wake_up_process <-call_timer_fn
<idle>-0 [002] ..s3. 686.264049: wake_up_process <-call_timer_fn
<idle>-0 [002] d.h6. 686.515216: wake_up_process <-kick_pool
<idle>-0 [002] d.h6. 686.691386: wake_up_process <-kick_pool
Then, only function tracer is shown on x86.
But if you enable 'kprobe on ftrace' event (which uses SAVE_REGS flag)
on the same function, it is shown again.
# echo 'p wake_up_process' >> dynamic_events
# echo 1 > events/kprobes/p_wake_up_process_0/enable
# echo > trace
# less trace
...
<idle>-0 [006] ..s2. 2710.345919: p_wake_up_process_0: (wake_up_process+0x4/0x20)
<idle>-0 [006] ..s3. 2710.345923: wake_up_process <-call_timer_fn
<idle>-0 [006] ..s1. 2710.345928: my_direct_func: waking up rcu_preempt-17
<idle>-0 [006] ..s2. 2710.349931: p_wake_up_process_0: (wake_up_process+0x4/0x20)
<idle>-0 [006] ..s3. 2710.349934: wake_up_process <-call_timer_fn
<idle>-0 [006] ..s1. 2710.349937: my_direct_func: waking up rcu_preempt-17
To fix this issue, use SAVE_REGS flag for multiple ftrace_ops flag of
direct_call by default.
Fixes: 60c8971899f3 ("ftrace: Make DIRECT_CALLS work WITH_ARGS and !WITH_REGS")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Sorry about this; I hadn't realised that x86 only supported direct calls when
SAVE_REGS was requested.
The patch looks good to me. I applied it atop v6.7 and double-checked that this
still works on arm64 as per your examples above, and everything looks good:
# mount -t tracefs none /sys/kernel/tracing/
# insmod ftrace-direct.ko
# echo wake_up_process > /sys/kernel/tracing/set_ftrace_filter
# echo function > /sys/kernel/tracing/current_tracer
# less /sys/kernel/tracing/trace
...
<idle>-0 [007] ..s3. 172.932840: wake_up_process <-process_timeout
<idle>-0 [007] ..s1. 172.932842: my_direct_func: waking up kcompactd0-62
<idle>-0 [007] ..s3. 173.444836: wake_up_process <-process_timeout
<idle>-0 [007] ..s1. 173.444838: my_direct_func: waking up kcompactd0-62
<idle>-0 [001] d.h5. 173.471116: wake_up_process <-kick_pool
<idle>-0 [001] d.h3. 173.471118: my_direct_func: waking up kworker/1:1-58
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]
Thanks,
Mark.
quoted hunk ↗ jump to hunk
--- kernel/trace/ftrace.c | 10 ++++++++++ 1 file changed, 10 insertions(+)diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index b01ae7d36021..c060d5b47910 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c@@ -5325,7 +5325,17 @@ static LIST_HEAD(ftrace_direct_funcs); static int register_ftrace_function_nolock(struct ftrace_ops *ops); +/* + * If there are multiple ftrace_ops, use SAVE_REGS by default, so that direct + * call will be jumped from ftrace_regs_caller. Only if the architecture does + * not support ftrace_regs_caller but direct_call, use SAVE_ARGS so that it + * jumps from ftrace_caller for multiple ftrace_ops. + */ +#ifndef HAVE_DYNAMIC_FTRACE_WITH_REGS #define MULTI_FLAGS (FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_ARGS) +#else +#define MULTI_FLAGS (FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_REGS) +#endif static int check_direct_multi(struct ftrace_ops *ops) {