Re: [PATCH v6 45/45] trace-cmd: Update trace.dat man page

[Tzvetomir Stoyanov <tz.stoyanov@gmail.com>]

On Tue, Jun 22, 2021 at 3:37 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Mon, 14 Jun 2021 10:50:29 +0300
> "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com> wrote:
>
> > Updated the trace.dat man page with the changes related to file
> > version 7 and compression.
> >
> > Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
> > ---
> >  Documentation/trace-cmd/trace-cmd.dat.5.txt | 56 ++++++++++++++++++---
> >  1 file changed, 50 insertions(+), 6 deletions(-)
> >
> > diff --git a/Documentation/trace-cmd/trace-cmd.dat.5.txt b/Documentation/trace-cmd/trace-cmd.dat.5.txt
> > index 8d285353..e80d460e 100644
> > --- a/Documentation/trace-cmd/trace-cmd.dat.5.txt
> > +++ b/Documentation/trace-cmd/trace-cmd.dat.5.txt
> > @@ -52,6 +52,23 @@ INITIAL FORMAT
> >    The next 4 bytes are a 32-bit word that defines what the traced
> >    host machine page size was.
> >
> > +  If the file version is 7 or greater, the compression header is
> > +  written next:
> > +     "name version\0"
>
> I wonder if we should make it: "name\0version\0"
>
> Also, I think "none" is acceptable, where none of the sections are
> compressed. If we add something special for a version 7 but don't want
> to compress, we need to support that.
>
> > +  where "name" and "version" are strings, name and version of the
> > +  compression algorithm used to compress the trace file.
> > +
> > +COMPRESSION FORMAT OF THE HEADER SECTIONS
> > +-----------------------------------------
> > +  If the file version is 7 or greater, some header sections are compressed
> > +  with the compression algorithm, specified in the compression header.
> > +  The format of these compressed sections is:
> > +     <4 bytes> unsigned int, size of compressed data in the next block.
> > +     <4 bytes> unsigned int, size of uncompressed data.
> > +     <data> binary compressed data, with the specified size.
> > +  These sections must be uncompressed on reading. The described format of
> > +  the sections refers to the uncomperssed data.
>
> I think each section should have a flag that states that it is
> compressed or not. That way we could have options that determine "what"
> gets compressed, and not have it be all or none.

I was thinking the same, but could not find a use case. That means to
give control to the user to decide what parts should be compressed.
This will complicate the implementation, new trace-cmd parameters
should be added. As I couldn't thought of a use case, decided to go
with the simpler approach. May be it makes sense only for the trace
data, but the metadata should be always compressed if possible.

>
> > +
> >  HEADER INFO FORMAT
> >  ------------------
> >
> > @@ -93,7 +110,8 @@ FTRACE EVENT FORMATS
> >
> >    Directly after the header information comes the information about
> >    the Ftrace specific events. These are the events used by the Ftrace plugins
> > -  and are not enabled by the event tracing.
> > +  and are not enabled by the event tracing. If the file version is 7 or
> > +  greater, this section is compressed.
>
> Perhaps add a single byte ahead of each section, where "0" is not
> compressed, and "1" is compressed?
>
> >
> >    The next 4 bytes contain a 32-bit word of the number of Ftrace event
> >    format files that are stored in the file.
> > @@ -110,7 +128,8 @@ EVENT FORMATS
> >  -------------
> >
> >    Directly after the Ftrace formats comes the information about
> > -  the event layout.
> > +  the event layout. If the file version is 7 or greater, this section
> > +  is compressed.
> >
> >    The next 4 bytes are a 32-bit word containing the number of
> >    event systems that are stored in the file. These are the
> > @@ -137,7 +156,8 @@ KALLSYMS INFORMATION
> >  --------------------
> >
> >    Directly after the event formats comes the information of the mapping
> > -  of function addresses to the function names.
> > +  of function addresses to the function names. If the file version is 7
> > +  or greater, this section is compressed.
> >
> >    The next 4 bytes are a 32-bit word containing the size of the
> >    data holding the function mappings.
> > @@ -154,6 +174,7 @@ TRACE_PRINTK INFORMATION
> >    store the format string outside the ring buffer.
> >    This information can be found in:
> >    debugfs/tracing/printk_formats
> > +  If the file version is 7 or greater, this section is compressed.
> >
> >    The next 4 bytes are a 32-bit word containing the size of the
> >    data holding the printk formats.
> > @@ -166,7 +187,8 @@ PROCESS INFORMATION
> >  -------------------
> >
> >    Directly after the trace_printk formats comes the information mapping
> > -  a PID to a process name.
> > +  a PID to a process name. If the file version is 7 or greater, this
> > +  section is compressed.
> >
> >    The next 8 bytes contain a 64-bit word that holds the size of the
> >    data mapping the PID to a process name.
> > @@ -193,10 +215,11 @@ REST OF TRACE-CMD HEADER
> >
> >      "flyrecord\0"
> >
> > -  If it is "options  \0" then:
> > +  If it is "options  \0" then follows a section with trace options.
> > +  If the file version is 7 or greater, this section is compressed.
> >
> >    The next 2 bytes are a 16-bit word defining the current option.
> > -  If the the value is zero then there are no more options.
> > +  If the value is zero then there are no more options.
> >
> >    Otherwise, the next 4 bytes contain a 32-bit word containing the
> >    option size. If the reader does not know how to handle the option
> > @@ -206,6 +229,25 @@ REST OF TRACE-CMD HEADER
> >    The next option will be directly after the previous option, and
> >    the options ends with a zero in the option type field.
> >
> > +COMPRESSION FORMAT OF THE TRACE DATA
> > +------------------------------------
> > +
> > +  If the file version is 7 or greater, the tarce data is compressed
>
> Typo "trace data"
>
> And this is where we definitely need to make it optional. We currently
> do not have a safe way to read this file. The "uncompress to /tmp" is
> not a reliable way to do this. And again, people can likely want to
> have the header compressed but not the data, due to speed in reading.
>
> -- Steve
>
> > +  with the compression algorithm, specified in the compression header.
> > +  The data is compressed in chunks. The size of one compression chunk
> > +  is defined when the file is written. The format of compressed trace
> > +  data is:
> > +     <4 bytes> unsigned int, count of chunks.
> > +     Follows the compressed chunks of givent count. For each chunk:
> > +        <4 bytes> unsigned int, size of compressed data in this chunk.
> > +        <4 bytes> unsigned int, size of uncompressed data.
> > +        <data> binary compressed data, with the specified size.
> > +  These chunks must be uncompressed on reading. The described format of
> > +  trace data refers to the uncomperssed data.
> > +
> > +TRACE DATA
> > +----------
> > +
> >    The next 10 bytes after the options are one of the following:
> >
> >    "latency  \0"
> > @@ -217,6 +259,7 @@ REST OF TRACE-CMD HEADER
> >    If the value is "latency  \0", then the rest of the file is
> >    simply ASCII text that was taken from the target's:
> >    debugfs/tracing/trace
> > +  If the file version is 7 or greater, the latency data is compressed.
> >
> >    If the value is "flyrecord\0", the following is present:
> >
> > @@ -232,6 +275,7 @@ REST OF TRACE-CMD HEADER
> >  CPU DATA
> >  --------
> >
> > +  If the file version is 7 or greater, the CPU data is compressed.
> >    The CPU data is located in the part of the file that is specified
> >    in the end of the header. Padding is placed between the header and
> >    the CPU data, placing the CPU data at a page aligned (target page) position
>


-- 
Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center