Messages in this thread | | | From | Vaibhav Nagarnaik <> | Date | Tue, 3 May 2011 14:56:11 -0700 | Subject | Re: [RFC PATCH] tracing: Don't call wakeup() when committing the event |
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On Tue, May 3, 2011 at 2:41 PM, Frederic Weisbecker <fweisbec@gmail.com> 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?
> > That involves two wake_up() calls but then it scales and you keep > the awakening. >
Vaibhav Nagarnaik
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