Re: [PATCH V7 1/3] tracing: add a possibility of exporting function trace to other places instead of ring buffer only

[Chunyan Zhang]

On 14 November 2016 at 23:59, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Fri, 21 Oct 2016 20:13:13 +0800
> Chunyan Zhang <zhang.chunyan@xxxxxxxxxx> wrote:
>
>> On 18 October 2016 at 23:44, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>> > On Tue, 18 Oct 2016 16:08:58 +0800
>> > Chunyan Zhang <zhang.chunyan@xxxxxxxxxx> wrote:
>> >
>> >> Currently Function traces can be only exported to ring buffer, this
>> >> patch added trace_export concept which can process traces and export
>> >> them to a registered destination as an addition to the current only
>> >> one output of Ftrace - i.e. ring buffer.
>> >>
>> >> In this way, if we want Function traces to be sent to other destination
>> >> rather than ring buffer only, we just need to register a new trace_export
>> >> and implement its own .write() function for writing traces to storage.
>> >>
>> >> With this patch, only Function trace (trace type is TRACE_FN)
>> >> is supported.
>> >
>> > This is getting better, but I still have some nits.
>> >
>>
>> Thanks.
>>
>> >>
>> >> Signed-off-by: Chunyan Zhang <zhang.chunyan@xxxxxxxxxx>
>> >> ---
>> >>  include/linux/trace.h |  28 +++++++++++
>> >>  kernel/trace/trace.c  | 132 +++++++++++++++++++++++++++++++++++++++++++++++++-
>> >>  2 files changed, 159 insertions(+), 1 deletion(-)
>> >>  create mode 100644 include/linux/trace.h
>> >>
>> >> diff --git a/include/linux/trace.h b/include/linux/trace.h
>> >> new file mode 100644
>> >> index 0000000..eb1c5b8
>> >> --- /dev/null
>> >> +++ b/include/linux/trace.h
>> >> @@ -0,0 +1,28 @@
>> >> +#ifndef _LINUX_TRACE_H
>> >> +#define _LINUX_TRACE_H
>> >> +
>> >> +#ifdef CONFIG_TRACING
>> >> +/*
>> >> + * The trace export - an export of Ftrace output. The trace_export
>> >> + * can process traces and export them to a registered destination as
>> >> + * an addition to the current only output of Ftrace - i.e. ring buffer.
>> >> + *
>> >> + * If you want traces to be sent to some other place rather than ring
>> >> + * buffer only, just need to register a new trace_export and implement
>> >> + * its own .write() function for writing traces to the storage.
>> >> + *
>> >> + * next              - pointer to the next trace_export
>> >> + * write     - copy traces which have been delt with ->commit() to
>> >> + *             the destination
>> >> + */
>> >> +struct trace_export {
>> >> +     struct trace_export __rcu       *next;
>> >> +     void (*write)(const char *, unsigned int);
>> >
>> > Why const char*? Why not const void *? This will never be a string.
>> >
>>
>> Will revise this.
>>
>> >
>> >> +};
>> >> +
>> >> +int register_ftrace_export(struct trace_export *export);
>> >> +int unregister_ftrace_export(struct trace_export *export);
>> >> +
>> >> +#endif       /* CONFIG_TRACING */
>> >> +
>> >> +#endif       /* _LINUX_TRACE_H */
>> >> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> >> index 8696ce6..db94ec1 100644
>> >> --- a/kernel/trace/trace.c
>> >> +++ b/kernel/trace/trace.c
>> >> @@ -40,6 +40,7 @@
>> >>  #include <linux/poll.h>
>> >>  #include <linux/nmi.h>
>> >>  #include <linux/fs.h>
>> >> +#include <linux/trace.h>
>> >>  #include <linux/sched/rt.h>
>> >>
>> >>  #include "trace.h"
>> >> @@ -2128,6 +2129,132 @@ void trace_buffer_unlock_commit_regs(struct trace_array *tr,
>> >>       ftrace_trace_userstack(buffer, flags, pc);
>> >>  }
>> >>
>> >> +static void
>> >> +trace_process_export(struct trace_export *export,
>> >> +            struct ring_buffer_event *event)
>> >> +{
>> >> +     struct trace_entry *entry;
>> >> +     unsigned int size = 0;
>> >> +
>> >> +     entry = ring_buffer_event_data(event);
>> >> +
>> >> +     size = ring_buffer_event_length(event);
>> >> +
>> >> +     if (export->write)
>> >> +             export->write((char *)entry, size);
>> >
>> > Is there ever going to be a time where export->write wont be set?
>>
>> There hasn't been since only one trace_export (i.e. stm_ftrace) was
>> added in this patch-set , I just wanted to make sure the write() has
>> been set before registering trace_export like what I added in 2/3 of
>> this series.
>>
>> >
>> > And if there is, this can be racy. As in
>> >
>> >
>> >         CPU 0:                  CPU 1:
>> >         ------                  ------
>> >         if (export->write)
>> >
>> >                                 export->write = NULL;
>>
>> Is there going to be this kind of use case? Why some one needs to
>> change export->write() rather than register a new trace_export?
>
> Then why have a
>
>         if (export->write)
>
>
> Is there every going to be a case where export will not have a write
> function?

There shouldn't be.

I can move this if statement to the register_ftrace_export() to ensure
users won't wrongly use it, that's saying the write() of trace_export
has been set before being registered to 'ftrace_exports_list'.

Thanks,
Chunyan

>
> -- Steve
>
>>
>> I probably haven't understood your point thoroughly, please correct me
>> if my guess was wrong.
>>
>>
>> Thanks for the review,
>> Chunyan
>>
>> >
>> >         export->write(entry, size);
>> >
>> >         BOOM!
>> >
>> >
>> > -- Steve