Re: Infrastructure for tracking driver performance events

[Josh Stone]

On 06/24/2009 10:29 AM, Ben Gamari wrote:
[...]
> I recently wrote a very simple patch to add accounting for these types
> of stalls to the i915 driver[1], exposing a list of wait-event counts to
> userspace through debugfs. While this is useful for giving a general
> overview of the drivers' performance, it does very little to expose
> individual bottlenecks in the driver or userland components. It has been
> suggested[2] that this wait-event tracking functionality would be far more
> useful if we could provide stack backtraces all the way into user space
> for each wait event.
[...]
> Another option seems to be systemtap. It has already been documented[3]
> that this option could provide both user-mode and kernel-mode
> backtraces. The driver could provide a kernel marker at every potential
> wait point (or a single marker in a function called at each wait point,
> for that matter) which would be picked up by systemtap and processed in
> usermode, calling ptrace to acquire a usermode backtrace. This approach
> seems slightly cleaner as it doesn't require the tracing on the entire
> machine to catch what should be reasonably rare events (hopefully).
> 
> Unfortunately, the systemtap approach described in [3] requires that
> each process have an associated "driver" process to get a usermode
> backtrace. It would be nice to avoid this requirement as there are
> generally far more gpu clients than just the X server (i.e. direct
> rendering clients) and tracking them all could get tricky.
[...]
> [3] http://sourceware.org/ml/systemtap/2006-q4/msg00198.html

I have to say, I'm a bit surprised to see my hacky suggestion
resurrected from the archives.  :)  I would guess that that approach
would add way too much overhead to be of use in diagnosing stalls though.

However, I think we can do a lot better with systemtap these days.
We're growing the ability to do userspace backtraces[1] directly within
your systemtap script, which should be much less intrusive.

Please take a look at ubacktrace() and family in recent systemtap and
let us know how you think it could improve.

Thanks,

Josh

[1] http://sourceware.org/ml/systemtap/2009-q2/msg00364.html
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