Re: ftrace global trace_pipe_raw

[Claudio <claudio.fontana@gliwa.com>]

Hello,

just a small update below:

On 10/08/2018 09:18 PM, Claudio wrote:
> Hi Steven,
> 
> thank you for your answer,
> 
> On 10/08/2018 06:16 PM, Steven Rostedt wrote:
>> On Mon, 8 Oct 2018 18:07:49 +0200
>> Claudio <claudio.fontana@gliwa.com> wrote:
>>
>>> Hello Steven,
>>>
>>> On 07/24/2018 04:25 PM, Steven Rostedt wrote:
>>>> On Tue, 24 Jul 2018 10:23:16 -0400
>>>> Steven Rostedt <rostedt@goodmis.org> wrote:
>>>>   
>>>>>>
>>>>>> Would work in the direction of adding a global trace_pipe_raw be considered
>>>>>> for inclusion?    
>>>>>
>>>>> The design of the lockless ring buffer requires not to be preempted,
>>>>> and that the data cannot be written to from more than one location. To
>>>>> do so, we make a per CPU buffer, and disable preemption when writing.
>>>>> This means that we have only one writer at a time. It can handle
>>>>> interrupts and NMIs, because they will finish before they return and
>>>>> this doesn't break the algorithm. But having writers from multiple CPUs
>>>>> would require locking or other heaving synchronization operations that
>>>>> will greatly reduce the speed of writing to the buffers (not to mention
>>>>> the cache thrashing).  
>>>>
>>>> And why would you need a single buffer? Note, we are working on making
>>>> libtracecmd.so that will allow applications to read the buffers and the
>>>> library will take care of the interleaving of the raw data. This should
>>>> hopefully be ready in about three months or so.
>>>>
>>>> -- Steve
>>>>   
>>>
>>> Is this something you will showcase in the linux tracing summit?
>>> Is there a repo / branch I should be following?
>>
>> We are preparing the code in tools/lib/traceevent of the Linux kernel
>> to turn that into a library.
>>
>> At the same time, we are looking at making libtracecmd or perhaps we'll
>> call it libftrace? to implement all the trace-cmd code as a library as
>> well. But that's happening in the main trace-cmd repo:
>>
>>  git://git.kernel.org/pub/scm/utils/trace-cmd/trace-cmd.git
>>
> 
> I think that the binary raw event streaming aspect will be part of this last component right?
> 
> libftrace is way better as a name I think.
> 
> 
>>>
>>> The reason why we need to end up with a single stream of events is to be
>>> able to do "online" task state correlation and timing parameters calculations
>>> for all task-related events independent of cores.
>>
>> Well, all the events are timestamped, and you can pick different clocks
>> to use, and a simple merge sort gives all the information you need.
> 
> Yes, we are about to do the merge sorting of the streams ourselves,
> but if the library does it, even better ;-)

I think that with K streams, instead of merge sort it would be optimal to
use min heap sort for the partial, online sorting of the events coming
from the cpu streams.

Do you think this could be part of the library, so it would be able
to provide a constant stream of already sorted events regardless of the cpu,
for full sched task state correlation?

Or it it your view that this functionality should never be part of the
library, and needs to be implemented by every user of the library separately?

For me, it would be I think the main factor to decide whether to use the
library or not.

Thank you,

Claudio

> 
>> Note, having per cpu buffers makes things much more efficient as you
>> don't need to do synchronizing with atomics.>>
>> -- Steve
>>
>>>
>>> Currently we have this on QNX, and we are trying to enable it for Linux as well.
>>>
>>> Thank you,
>>>
>>> Claudio
>>
> 
> Thanks,
> 
> Claudio
>