Re: [PATCH 00/18] [ANNOUNCE] Dynamically created function based events

[Masami Hiramatsu <mhiramat@kernel.org>]

On Fri, 02 Feb 2018 18:04:58 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> 
> At Kernel Summit back in October, we tried to bring up trace markers, which
> would be nops within the kernel proper, that would allow modules to hook
> arbitrary trace events to them. The reaction to this proposal was less than
> favorable. We were told that we were trying to make a work around for a
> problem, and not solving it. The problem in our minds is the notion of a
> "stable trace event".
> 
> There are maintainers that do not want trace events, or more trace events in
> their subsystems. This is due to the fact that trace events post an
> interface to user space, and this interface could become required by some
> tool. This may cause the trace event to become stable where it must not
> break the tool, and thus prevent the code from changing.
> 
> Or, the trace event may just have to add padding for fields that tools
> may require. The "success" field of the sched_wakeup trace event is one such
> instance. There is no more "success" variable, but tools may fail if it were
> to go away, so a "1" is simply added to the trace event wasting ring buffer
> real estate.
> 
> I talked with Linus about this, and he told me that we already have these
> markers in the kernel. They are from the mcount/__fentry__ used by function
> tracing. Have the trace events be created by these, and see if this will
> satisfy most areas that want trace events.
> 
> I decided to implement this idea, and here's the patch set.
> 
> Introducing "function based events". These are created dynamically by a
> tracefs file called "function_events". By writing a pseudo prototype into
> this file, you create an event.

This seems very similar feature of what kprobe-based event does.

Earlier version of kprobe-based event supported Nth argument, but I decided
to drop it because we can not ensure the "function signature" from kernel
binary. It has been passed to "perf probe", so that we can define line-level
granularity. 

Of course if it is easy to support "argN" again as it does if the arch's
calling convention is clearly stated. (and now kprobe is based on ftrace
if it is on the entry of functions)

So my question is, what is the difference of those from the user perspective?
Only event syntax is a problem?
I'm very confusing...

Thank you,

> 
>  # mount -t tracefs nodev /sys/kernel/tracing
>  # cd /sys/kernel/tracing
>  # echo 'do_IRQ(symbol ip[16] | x64[6] irq_stack[16])' > function_events
>  # cat events/functions/do_IRQ/format
> name: do_IRQ
> ID: 1399
> format:
> 	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
> 	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
> 	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
> 	field:int common_pid;	offset:4;	size:4;	signed:1;
> 
> 	field:unsigned long __parent_ip;	offset:8;	size:8;	signed:0;
> 	field:unsigned long __ip;	offset:16;	size:8;	signed:0;
> 	field:symbol ip;	offset:24;	size:8;	signed:0;
> 	field:x64 irq_stack[6];	offset:32;	size:48;	signed:0;
> 
> print fmt: "%pS->%pS(ip=%pS, irq_stack=%llx:%llx:%llx:%llx:%llx:%llx)", REC->__ip, REC->__parent_ip,
> REC->ip, REC->irq_stack[0], REC->irq_stack[1], REC->irq_stack[2], REC->irq_stack[3], REC->irq_stack[4],
> REC->irq_stack[5]
> 
>  # echo 1 > events/functions/do_IRQ/enable
>  # cat trace
>           <idle>-0     [003] d..3  3647.049344: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.049433: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.049672: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.325709: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.325929: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.325993: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.387571: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.387791: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
>           <idle>-0     [003] d..3  3647.387874: ret_from_intr->do_IRQ(ip=cpuidle_enter_state+0xb1/0x330, irq_stack=ffffffff81665db1,10,246,ffffc900006c3e80,18,ffff88011eae9b40)
> 
> And this is much more powerful than just this. We can show strings, and
> index off of structures into other structures.
> 
>   # echo '__vfs_read(symbol read+40[0]+16)' > function_events
> 
>   # echo 1 > events/functions/__vfs_read/enable
>   # cat trace
>          sshd-1343  [005] ...2   199.734752: vfs_read->__vfs_read(read=tty_read+0x0/0xf0)
>          bash-1344  [003] ...2   199.734822: vfs_read->__vfs_read(read=tty_read+0x0/0xf0)
>          sshd-1343  [005] ...2   199.734835: vfs_read->__vfs_read(read=tty_read+0x0/0xf0)
>  avahi-daemon-910   [003] ...2   200.136740: vfs_read->__vfs_read(read=          (null))
>  avahi-daemon-910   [003] ...2   200.136750: vfs_read->__vfs_read(read=          (null))
> 
> And even read user space:
> 
>   # echo 'SyS_openat(int dfd, string path, x32 flags, x16 mode)' > function_events
>   # echo 1 > events/functions/enable
>   # grep task_fork /proc/kallsyms 
> ffffffff810d5a60 t task_fork_fair
> ffffffff810dfc30 t task_fork_dl
>   # cat trace
>             grep-1820  [000] ...2  3926.107603: entry_SYSCALL_64_fastpath->SyS_openat(dfd=-100, path=/proc/kallsyms, flags=100, mode=0)
> 
> These are fully functional events. That is, they work with ftrace,
> trace-cmd, perf, histograms, triggers, and eBPF.
> 
> What's next? I need to rewrite the function graph tracer, and be able to add
> dynamic events on function return.
> 
> I made this work with x86 with a simple function that only returns
> 6 function parameters for x86_64 and 3 for x86_32. But this could easily
> be extended.
> 
> Cheers!
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
> ftrace/dynamic-ftrace-events
> 
> Head SHA1: 30fbdffd5d38bd27b04fb911f7158f10a99be8c4
> 
> 
> Steven Rostedt (VMware) (18):
>       tracing: Add function based events
>       tracing: Add documentation for function based events
>       tracing: Add simple arguments to function based events
>       tracing/x86: Add arch_get_func_args() function
>       tracing: Add hex print for dynamic ftrace based events
>       tracing: Add indirect offset to args of ftrace based events
>       tracing: Add dereferencing multiple fields per arg
>       tracing: Add "unsigned" to function based events
>       tracing: Add indexing of arguments for function based events
>       tracing: Make func_type enums for easier comparing of arg types
>       tracing: Add symbol type to function based events
>       tracing: Add accessing direct address from function based events
>       tracing: Add array type to function based events
>       tracing: Have char arrays be strings for function based events
>       tracing: Add string type for dynamic strings in function based events
>       tracing: Add NULL to skip args for function based events
>       tracing: Add indirect to indirect access for function based events
>       tracing/perf: Allow perf to use function based events
> 
> ----
>  Documentation/trace/function-based-events.rst |  426 ++++++++
>  arch/x86/kernel/ftrace.c                      |   28 +
>  include/linux/trace_events.h                  |    2 +
>  kernel/trace/Kconfig                          |   12 +
>  kernel/trace/Makefile                         |    1 +
>  kernel/trace/trace.h                          |   11 +
>  kernel/trace/trace_event_ftrace.c             | 1440 +++++++++++++++++++++++++
>  kernel/trace/trace_probe.h                    |   11 -
>  8 files changed, 1920 insertions(+), 11 deletions(-)
>  create mode 100644 Documentation/trace/function-based-events.rst
>  create mode 100644 kernel/trace/trace_event_ftrace.c


-- 
Masami Hiramatsu <mhiramat@kernel.org>