Re: [RFC PATCH] tracing: Don't call wakeup() when committing the event

[Frederic Weisbecker <fweisbec@gmail.com>]

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 involves two wake_up() calls but then it scales and you keep
the awakening.