Re: rt20 scheduling latency testcase and failure data

From: Ingo Molnar
Date: Thu May 18 2006 - 04:56:08 EST

* Sébastien Dugué <sebastien.dugue@xxxxxxxx> 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.

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