linux-kernel - Re: [PATCH 8/9] stacktrace: implement save_stack_trace

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Message-ID: <20120111162642.GC7991@somewhere.redhat.com>
Date:	Wed, 11 Jan 2012 17:26:44 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, mingo@...hat.com, rostedt@...dmis.org,
	teravest@...gle.com, slavapestov@...gle.com, ctalbott@...gle.com,
	dhsharp@...gle.com, linux-kernel@...r.kernel.org,
	winget@...gle.com, namhyung@...il.com,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 8/9] stacktrace: implement save_stack_trace_quick()

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.
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