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    SubjectRe: [RFC PATCH] tracing: Don't call wakeup() when committing the event
    On Tue, May 03, 2011 at 02:56:11PM -0700, Vaibhav Nagarnaik wrote:
    > On Tue, May 3, 2011 at 2:41 PM, Frederic Weisbecker <> wrote:
    > > On Tue, May 03, 2011 at 02:03:36PM -0700, Vaibhav Nagarnaik wrote:
    > >> In using syscall tracing by concurrent processes, the wakeup() that is
    > >> called in the event commit function causes contention on the spin lock
    > >> of the waitqueue. I enabled sys_enter_getuid and sys_exit_getuid
    > >> tracepoints, and by running getuid_microbench from autotest in parallel
    > >> I found that the contention causes exponential latency increase in the
    > >> tracing path.
    > >>
    > >> The autotest binary getuid_microbench calls getuid() in a tight loop for
    > >> the given number of iterations and measures the average time required to
    > >> complete a single invocation of syscall.
    > >>
    > >> The patch here points to the problem and provides a naive solution to
    > >> start the discussion. It is not intended to be a definitive solution.
    > >
    > > Right, so another solution could be to have per cpu waitqueues for
    > > the per_cpu trace_pipe/trace_pipe_raw files, and one big for the main
    > > trace_pipe file.
    > That could be another way. But if there is still *one* common waitqueue for
    > the main trace file, we are still going to get contention on waking up that
    > common waitqueue.
    > Unless I am missing something, can you explain why there won't be contention
    > in your suggested solution?

    Ah right, I missed that.

    I wonder if we should have a lite version of wake_up() that checks
    if the list of waiters is empty before locking the queue.
    After all we don't care much about tight races for tracing.


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