Messages in this thread | | | Date | Thu, 25 Nov 2010 08:38:59 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 0/2] rcu: Fix series of spurious RCU softirqs |
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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.
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