Re: Re: [PATCH V4 1/5] trace-cmd/listen: Apply the trace-msg protocol for communication between a server and clients

[Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>]

Hi Steven,

Thank you for your review.

(2014/07/23 0:04), Steven Rostedt wrote:
> Sorry for taking so long to reply, I've been hacking on the kernel a
> bit and that takes precedence over user tools :-/
>
>
> On Fri, 11 Jul 2014 00:58:26 +0000
> Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com> wrote:
>
>> Apply trace-msg protocol for communication between a server and clients.
>>
>> Currently, trace-listen(server) and trace-record -N(client) operate as follows:
>>
>>       <server>                 <client>
>>        listen to socket fd
>>                                connect to socket fd
>>        accept the client
>>        send "tracecmd"
>>                 +------------> receive "tracecmd"
>>                                check "tracecmd"
>>                                send cpus
>>        receive cpus <------------+
>>        print "cpus=XXX"
>>                                send pagesize
>>                                  |
>>        receive pagesize <--------+
>>        print "pagesize=XXX"
>>                                send option
>>                                  |
>>        receive option <----------+
>>        understand option
>>        send port_array
>>                 +------------> receive port_array
>>                                understand port_array
>>                                send meta data
>>        receive meta data <-------+
>>        record meta data
>>                          (snip)
>>        read block
>>       --- start sending trace data on child processes ---
>>
>>       --- When client finishes sending trace data ---
>>                                close(socket fd)
>>        read size = 0
>>        close(socket fd)
>>
>> All messages are unstructured character strings, so server(client) using the
>> protocol must parse the unstructured messages. Since it is hard to
>> add complex contents in the protocol, structured binary message trace-msg
>> is introduced as the communication protocol.
>>
>> By applying this patch, server and client operate as follows:
>>
>>       <server>                 <client>
>>        listen to socket fd
>>                                connect to socket fd
>>        accept the client
>>        send "tracecmd"
>>                 +------------> receive "tracecmd"
>>                                check "tracecmd"
>>                                send "V2\0<MAGIC_NUMBER>\00" as the v2 protocol
>
> Lets change this to "-1V2\0<MAGIC_NUMBER>\00"
>
> The -1 will cause an old server to exit as it will not accept a -1 for
> CPU count. Then you can check if the return of the next read is -1, as
> the client would have disconnected.

Sure.

> The reason I ask this, is because once you send a valid CPU count (and
> unfortunately, 0 happens to be valid :-p, the server side creates a
> file. When you close it, that file stays around as zero length.
>
> By sending -1, the old server will error out and never create a file.

Yes, I also thought this should be fixed.
I'll submit fixed patch.

Thank you,
Yoshihiro YUNOMAE

-- 
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@hitachi.com