Re: [PATCH v4 2/2] trace: Example of non-tracing recorder use

[Markus Armbruster]

Stefan Hajnoczi <stefanha@redhat.com> writes:

> On Thu, Jul 23, 2020 at 03:29:03PM +0200, Christophe de Dinechin wrote:
>> This patch is a simple example showing how the recorder can be used to
>> have one "topic" covering multiple entries. Here, the topic is "lock",
>> the idea being to have the latest lock changes for instance in case of
>> a crash or hang.
>> 
>> Here are a few use cases:
>> 
>> * Tracing  lock updates:
>>     RECORDER_TRACES=lock qemu
>> * Showing lock changes prior to a hang
>>     RECORDER_TRACES=lock qemu &
>>     # Wait until hang
>>     killall -USR2 qemu  # This will trigger a dump
>> * Graphic visualization of lock states:
>>     RECORDER_TRACES="lock=state,id" qemu &
>>     recorder_scope state
>>     # Hit the 't' key to toggle timing display
>>     # Hit the 'c' key to dump the screen data as CSV
>>     cat recorder_scope_data-1.csv
>
> Dan raised a good point regarding integrating recorder functionality
> behind the tracetool interface.
>
> On the other hand, I would like to see where this goes so that we have
> enough experience to design the tracetool interface, if necessary.
>
> Therefore I am for merging this as-is and taking action when it's clear
> that duplication is taking place.

Sounds like we should not yet commit to a stable external interface.

The monitor command is HMP only.  Not a stable interface.  A QMP command
would have to be marked experimental with the customary x- prefix.

The environment variable is an external interface of the recorder
library.  Attempting to police such interfaces of libraries we use seems
futile.