Re: rt20 scheduling latency testcase and failure data

[Ingo Molnar <mingo@elte.hu>]

* Sébastien Dugué <sebastien.dugue@bull.net> wrote:

> > thanks for tracking this down. FYI, the latency of stopping the trace is 
> > that expensive because we are copying large amounts of trace data 
> > around, to ensure that /proc/latency_trace is always consistent and is 
> > updated atomically, and to make sure that we can update the trace from 
> > interrupt contexts too - without /proc/latency_trace accesses blocking 
> > them. The latency of stopping the trace is hidden from the tracer itself 
> > - but it cannot prevent indirect effects such as your app from missing 
> > periods, if the periods are in the 5msec range.
> > 
> 
>   Thanks for the explanation, will have to look deeper into the code 
> to understand how it works though.

there's another complexity on SMP: if trace_all_cpus is set then the 
per-cpu trace buffers are sorted chronologically as well while the 
copying into the current-max-trace-buffer, to produce easier to read 
latency_trace output.

	Ingo