Re: [PATCH 0/2] rcu: Fix series of spurious RCU softirqs

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Nov 25, 2010 at 11:42:34AM +0800, Lai Jiangshan wrote:
> On 11/24/2010 08:31 AM, Frederic Weisbecker wrote:
> > Hi,
> > 
> > I've observed some not so unfrequent series of spurious rcu
> > softirqs, sometimes happening at each ticks for a random
> > while.
> > 
> > These patches aims at fixing them.
> > 
> > Thanks.
> > 
> > Frederic Weisbecker (2):
> >   rcu: Don't chase unnecessary quiescent states after extended grace periods
> >   rcu: Stop checking quiescent states after grace period completion from remote
> > 
> 
> If we ensure rdp->gpnum >= rdp->completed is always true, the problems as
> you described will not be existed. Or maybe I misunderstand you.
> 
> rdp->gpnum >= rdp->completed is a very important guarantee I think.
> (In my RCURING, it is guaranteed.) I'm afraid there are some other
> problems still hidden if it is not guaranteed.
> 
> so I recommend: (code is better than words)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index d5bc439..af4e87a 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -648,6 +648,13 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
>  
>  		/* Remember that we saw this grace-period completion. */
>  		rdp->completed = rnp->completed;
> +
> +		/* Ensure ->gpnum >= ->completed after NO_HZ */
> +		if (unlikely(rnp->completed - rdp->gpnum > 0
> +				|| rdp->gpnum - rnp->gpnum > 0)) {
> +			rdp->gpnum = rnp->completed;
> +			rdp->qs_pending = 0;


That's an alternative to my first patch yeah. And if rdp->gpnum >= rdp->completed
must be a guarantee outside the rnp lock, then it's certainly better because
the lock is relaxed between rcu_process_gp_end() and note_new_gpnum(), and
both values are async in this lockless frame.

But perhaps this shouldn't touch rdp->qs_pending:

"if (rnp->completed > rdp->gpnum || rdp->gpnum > rnp->gpnum)" is not
a guarantee that we don't need to find quiescent states.

but rnp->completed == rnp->gpnum would provide that guarantee.
That said, note_new_gp_new() would fix the value of rdp->qs_pending.

Thanks.