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

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

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.
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