Re: [PATCH 1/5] trace: do not disable wake up tracer on output of trace

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <srostedt@redhat.com>
> 
> Impact: fix to erased trace output
> 
> To try not to have the outputing of a trace interfere with the wakeup 
> tracer, it would disable tracing while the output was printing. But if a 
> trace had started when it was disabled, it can show a partial trace. To 
> try to solve this, on closing of the tracer, it would clear the trace 
> buffer.
> 
> The latency tracers (wakeup and irqsoff) have two buffers. One for 
> recording and one for holding the max trace that is printed. The 
> clearing of the trace above should only affect the recording buffer. But 
> for some reason it would move the erased trace to the print buffer. 
> Probably due to a race with the closing of the trace and the saving ofhe 
> max race.

hm, that race needs to be fixed then.

> The above is all pretty useless, and if the user does not want the 
> printing of the trace to be traced itself, then the user can manual 
> disable tracing. This patch removes all the code that tries to keep the 
> output of the tracer from modifying the trace.

printing of the trace should not be traced. I cannot imagine any usage 
mode where that would be interesting - and i can imagine a ton of cases 
where users would be confused/distracted by the tracer in essence zapping 
their measurement by replacing it with some uninteresting 'the tracer 
itself has wakeup delays' data.

auto-disabling latency tracing while the trace is being output is 
essential. Measurement should never impact the workload that is being 
measured.

We should fix that race instead.

	Ingo