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

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Date:	Tue, 3 May 2011 14:56:11 -0700
From:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
To:	Frederic Weisbecker <fweisbec@...il.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 3, 2011 at 2:41 PM, Frederic Weisbecker <fweisbec@...il.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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