Re: [PATCH] ftrace: Fix DIRECT_CALLS to use SAVE_REGS by default

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 10 Jan 2024 12:20:21 +0000
Mark Rutland <mark.rutland@arm.com> wrote:

> 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.

Yeah, it is hard to find without my fprobe on fgraph series because
all probes (kprobe/fprobe) and function-graph tracer uses SAVE_REGS.
Only function tracer hits this issue, but usually ftrace-direct user
will not use it with function tracer. So we were lucky to find it :)

> 
> 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]

Thank you!

> 
> Thanks,
> Mark.
> 
> > ---
> >  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)
> >  {
> > 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>