Messages in this thread | | | Date | Wed, 4 May 2011 00:09:51 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFC PATCH] tracing: Don't call wakeup() when committing the event |
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On Tue, May 03, 2011 at 02:56:11PM -0700, Vaibhav Nagarnaik wrote: > 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?
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.
Hm?
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