Re: tracepoints for kernel/mutex.c

[Mathieu Desnoyers]

* Peter Zijlstra (peterz@xxxxxxxxxxxxx) wrote:
> On Fri, 2008-10-17 at 10:48 -0400, Jason Baron wrote:
> > On Thu, Oct 16, 2008 at 11:34:38PM +0200, Peter Zijlstra wrote:
> > > On Thu, 2008-10-16 at 17:04 -0400, Jason Baron wrote:
> > > 
> > > > Below are 3 tracepoints I've been playing with in kernel/mutex.c using
> > > > a SystemTap script. The idea is to detect and determine the cause of
> > > > lock contention. Currently I get the following output:
> > > > 
> > > > <contended mutex nam> <process name and pid of the contention> <time of
> > > > contention> <pid that woke me up(caused the contention?)>
> > > 
> > > > I think this approach has a number of advantages. It has low
> > > > overhead in the off case, since its based on tracepoints. It is
> > > > minimally invasive in the code path (3 tracepoints). It also allows me
> > > > to explore data structures and parts of the kernel by simply modifying
> > > > the SystemTap script. I do not need to re-compile the kernel and reboot.
> > > 
> > > *sigh* this is why I hate markers and all related things...
> > > 
> > > _IFF_ you want to place tracepoints, get them in the same place as the
> > > lock-dep/stat hooks, that way you get all the locks, not only mutexes.
> > 
> > makes sense. So we could layer lock-dep/stat on top of tracepoints? That
> > would potentially also make lock-dep/stat more dynamic.
> 
> I'm afraid that won't work. Both lockdep and lockstat rely on added data
> to the lock structure. But what you can do is expose the hooks as
> tracepoints when lockdep/lockstat is configured.
> 
> > > 
> > > This is the same reason I absolutely _hate_ Edwin's rwsem tracer.
> > > 
> > 
> > i'm trying to get some consensus on these types of patches. Do we
> > want to create a new tracer for each thing we want to trace, or add
> > tracepoints?
> 
> The only thing I'd consider is one lock-tracer that exposes all
> lockdep/lockstat hooks. Any half-assed partial solution won't fly.
> 
> > > Folks, lets please start by getting the tracing infrastructure in and
> > > those few high-level trace-points google proposed.
> > > 
> > > Until we get the basics in, I think I'm going to NAK any and all
> > > tracepoint/marker patches.
> > > 
> > 
> > I think that core locking functions are pretty basic...
> 
> For kernel developers, yes. For userspace stuff like latencytop should
> be good enough to notice something is up.
> 
> And kernel developers can recompile their kernel - that's the only way
> you're going to do anything about lock contention anyway.
> 

We also have to consider if a less specific instrumentation can extract
this kind of data. Scheduler instrumentation can do a big part of that
job, namely identifying the wakeup cause. We would not have the
information about it being mutex-related unless we activate lockdep
tracing, potentially with tracepoints in the lockdep handlers. I think
this would fly. But let's keep this for later.

Mathieu


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Mathieu Desnoyers
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