Re: [PATCH] rcutorture: add random preemption

Paul E. McKenney
2010-06-21T16:43:17Z

On Mon, Jun 21, 2010 at 04:57:42PM +0800, Lai Jiangshan wrote:
> Add random preemption to help we to torture the preemptable rcu.
>
> srcu_read_delay() also calls rcu_read_delay() for shorter delays.

I do like the change to srcu_read_delay(), good to fall back to the
normal rcu_read_delay() behavior when a long delay is not selected.
The change to rcu_read_delay() looks promising as well, but please see
below for some comments on the other change.

And the big question: did you find any failures when testing with this
change? ;-)

Thanx, Paul

> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> ---
> diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
> index 2e2726d..7c81d07 100644
> --- a/kernel/rcutorture.c
> +++ b/kernel/rcutorture.c
> @@ -303,6 +303,10 @@ static void rcu_read_delay(struct rcu_random_state *rrsp)
> mdelay(longdelay_ms);
> if (!(rcu_random(rrsp) % (nrealreaders * 2 * shortdelay_us)))
> udelay(shortdelay_us);
> +#ifdef CONFIG_PREEMPT
> + if (!preempt_count() && !(rcu_random(rrsp) % (nrealreaders * 20000)))
> + preempt_schedule();
> +#endif

This one scared me for a bit -- then I realized that preempt_schedule()
won't actually schedule if preemption is in any way disabled. So the
above really is OK, because Classic RCU and RCU-bh disable preemption.

So, should we have a comment to this effect, or is my hypersensitivity to
RCU semantics unique to me?

> }
>
> static void rcu_torture_read_unlock(int idx) __releases(RCU)
> @@ -536,6 +540,8 @@ static void srcu_read_delay(struct rcu_random_state *rrsp)
> delay = rcu_random(rrsp) % (nrealreaders * 2 * longdelay * uspertick);
> if (!delay)
> schedule_timeout_interruptible(longdelay);
> + else
> + rcu_read_delay(rrsp);
> }
>
> static void srcu_torture_read_unlock(int idx) __releases(&srcu_ctl)
> --
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