linux-kernel - Re: [PATCH RFC] rcuperf: make timeout HZ independent

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Date:   Mon, 22 Aug 2016 04:49:43 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Nicholas Mc Guire <hofrat@...dl.org>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcuperf: make timeout HZ independent

On Mon, Aug 22, 2016 at 01:21:05PM +0200, Nicholas Mc Guire wrote:
> Make the probability of ftrace dump not interfering with other writers 
> grace period, HZ independent.
> 
> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>

What we would -really- like is to vary the time based on the clock rate
of the CPUs (could use bogomips, I suppose) and the memory latency of
the system, so that systems with smaller memory latency and faster CPUs
would use shorter timeouts.  However, slower CPUs tend to use smaller
HZ values, so varying based on HZ is not entirely insane.

But what would you suggest?

Good question on schedule_timeout_interruptible()...  I usually do that
if there is some reason to be sensitive to an early wakeup, for example,
to allow shutdown to proceed quickly, but that doesn't make much sense
here, given that ftrace_dump() is likely to take a very long time.

							Thanx, Paul

> ---
> 
> Problem found by coccinelle script
> 
> Passing in jiffies as value allows for this "fixed" delay varying by 
> one order of magnitude. As it is intended to reduce the probability of 
> interference this probability should not be dependent on the systems 
> HZ setting. Its probably more cosmetic but I guess this is the cleaner
> way for fixed delays.
> 
> Q: Could not really figure out why the _interruptible_ version is used
>    here - I would assume that schedule_timeout() would be what is needed
>    here, as this should simply be a fixed delay.
> 
> Patch was compile tested with: x86_64_defconfig + CONFIG_RCU_PERF_TEST=m
> 
> Patch is against 4.8.0-rc2 (localversion-next is -next-20160822)
> 
>  kernel/rcu/rcuperf.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> index 123ccbd..4cd8655 100644
> --- a/kernel/rcu/rcuperf.c
> +++ b/kernel/rcu/rcuperf.c
> @@ -404,7 +404,8 @@ rcu_perf_writer(void *arg)
>  				 perf_type, PERF_FLAG, me, MIN_MEAS);
>  			if (atomic_inc_return(&n_rcu_perf_writer_finished) >=
>  			    nrealwriters) {
> -				schedule_timeout_interruptible(10);
> +				schedule_timeout_interruptible(
> +							msecs_to_jiffies(10));
>  				rcu_ftrace_dump(DUMP_ALL);
>  				PERFOUT_STRING("Test complete");
>  				t_rcu_perf_writer_finished = t;
> -- 
> 2.1.4
>