linux-kernel - Re: [PATCH 4/7] ring-buffer: change test to be more latency friendly

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Date:	Thu, 7 May 2009 10:31:47 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 4/7] ring-buffer: change test to be more latency
	friendly


* Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <srostedt@...hat.com>
> 
> The ring buffer benchmark/test runs a producer for 10 seconds. 
> This is done with preemption and interrupts enabled. But if the 
> kernel is not compiled with CONFIG_PREEMPT, it basically stops 
> everything but interrupts for 10 seconds.
> 
> Although this is just a test and is not for production, this attribute
> can be quite annoying. It can also spawn badness elsewhere.

Yep, this probably explains that lockdep splat i got in a networking 
driver. Some functionality (a workqueue iirc) of the driver got 
starved and a time-out timer triggered - where lockdep caught 
locking badness.

> This patch solves the issues by calling "cond_resched" when the 
> system is not compiled with CONFIG_PREEMPT. It also keeps track of 
> the time spent to call cond_resched such that it does not go 
> against the time calculations. That is, if the task schedules 
> away, the time scheduled out is removed from the test data. Note, 
> this only works for non PREEMPT because we do not know when the 
> task is scheduled out if we have PREEMPT enabled.
> 
> [ Impact: prevent test from stopping the world for 10 seconds ]
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/trace/ring_buffer_benchmark.c |   31 +++++++++++++++++++++++++++++++
>  1 files changed, 31 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer_benchmark.c b/kernel/trace/ring_buffer_benchmark.c
> index dcd75e9..a26fc67 100644
> --- a/kernel/trace/ring_buffer_benchmark.c
> +++ b/kernel/trace/ring_buffer_benchmark.c
> @@ -185,6 +185,35 @@ static void ring_buffer_consumer(void)
>  	complete(&read_done);
>  }
>  
> +/*
> + * If we are a non preempt kernel, the 10 second run will
> + * stop everything while it runs. Instead, we will call cond_resched
> + * and also add any time that was lost by a rescedule.
> + */
> +#ifdef CONFIG_PREEMPT
> +static void sched_if_needed(struct timeval *start_tv, struct timeval *end_tv)
> +{
> +}
> +#else
> +static void sched_if_needed(struct timeval *start_tv, struct timeval *end_tv)
> +{
> +	struct timeval tv;
> +
> +	cond_resched();
> +	do_gettimeofday(&tv);
> +	if (tv.tv_usec < end_tv->tv_usec) {
> +		tv.tv_usec += 1000000;
> +		tv.tv_sec--;
> +	}
> +	start_tv->tv_sec += tv.tv_sec - end_tv->tv_sec;
> +	start_tv->tv_usec += tv.tv_usec - end_tv->tv_usec;
> +	if (start_tv->tv_usec > 1000000) {
> +		start_tv->tv_usec -= 1000000;
> +		start_tv->tv_sec++;
> +	}
> +}
> +#endif

This is _way_ too ugly. Why not just add a cond_resched() to the 
inner loop and be done with it? cond_resched() is conditional 
already, so it will only schedule 'if needed'.

If the test's timing gets skewed, what's the big deal? If its being 
preempted there will be impact _anyway_. (due to cache footprint 
elimination, etc.) People obviously should only rely on the numbers 
if the system is idle.

	Ingo
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