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Date: Tue, 3 May 2011 23:41:45 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Vaibhav Nagarnaik <vnagarnaik@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Michael Rubin <mrubin@google.com>, David Sharp <dhsharp@google.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] tracing: Don't call wakeup() when committing the
 event
Message-ID: <20110503214142.GC2678@nowhere>
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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.