Messages in this thread | | | Date | Thu, 25 Nov 2010 16:35:30 +0800 | From | Lai Jiangshan <> | Subject | Re: [PATCH 0/2] rcu: Fix series of spurious RCU softirqs |
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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.
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().
> > "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. > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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