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    SubjectRe: tracepoints for kernel/mutex.c
    * Peter Zijlstra (peterz@infradead.org) 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


    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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