Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 25 Nov 2010 10:27:08 +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 04:35:30PM +0800, Lai Jiangshan wrote: > On 11/25/2010 03:38 PM, Frederic Weisbecker wrote: > > 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. > > Since rdp->gpnum >= rdp->completed is guaranteed. > your second patch is not needed, the problem is also fixed. > > if rnp->gpnum == rnp->completed, rcu_report_qs_rdp() will not be called. > it is because rdp->qs_pending == 0 when rnp->gpnum == rnp->completed.
Aaah...
> > 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 rdp->gpnum == rnp->completed, it means we don't need a qs for rdp->gpnum, > it is completed. so we must set rdp->qs_pending = 0; > > when we really need a qs, rdp->qs_pending will be fixed in note_new_gp_new().
Ok that makes all sense now!
I'm just not sure about your check above.
(rdp->gpnum - rnp->gpnum > 0) can never happen, right?
Also perhaps we should set rdp->qs_pending = 0 only if rnp->completed == rdp->completed?
Which in the end would be:
/* Remember that we saw this grace-period completion. */ rdp->completed = rnp->completed;
+ if (rdp->gpnum < rdp->completed) + rdp->gpnum = rdp->completed; + + if (rdp->gpnum == rdp->completed) + rdp->qs_pending = 0;
And then if there is a new grace period to handle, this will be done through note_new_pgnum().
Hm?
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