Re: [PATCH 8/9] stacktrace: implement save_stack_trace_quick()

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Jan 10, 2012 at 10:28:25AM -0800, Tejun Heo wrote:
> Implement save_stack_trace_quick() which only considers the usual
> contexts (ie. thread and irq) and doesn't handle links between
> different contexts - if %current is in irq context, only backtrace in
> the irq stack is considered.

The thing I don't like is the duplication that involves not only on
stack unwinding but also on the safety checks.

What about making struct stacktrace_ops::stack() return a value
that either stops or continue the trace? In your case EOE/EOI would
be the triggering condition.

Filtering stack contexts might in fact be a desirable generic feature
overall.

At least in perf we could be interested in filtering kernel/user contexts.
And in your case in stopping after the first context. I also don't know if
we will be interested in filtering irq/exception/process stacks in the future
but I prefer to ensure we have a flexible enough interface to allow that.

So it may be a good idea to reuse the exisiting code for your needs like
a stack() return value as above. And if the post processing will be done
from userspace (which I really hope) then extend the ftrace/perf interface
to allow your quick filtering, something that can be later extended to
allow more finegrained stacktrace filtering.

> This is subset of dump_trace() done in much simpler way.  It's
> intended to be used in hot paths where the overhead of dump_trace()
> can be too heavy.

Is it? Have you found a measurable impact (outside the fact you record only
one stack).

Thanks.