Re: RCU qsmask !=0 warnings on large-SMP...

[Steffen Persvold <sp@numascale.com>]

On 1/26/2012 20:26, Paul E. McKenney wrote:
> On Thu, Jan 26, 2012 at 04:04:37PM +0100, Steffen Persvold wrote:
>> On 1/26/2012 02:58, Paul E. McKenney wrote:
>>> On Wed, Jan 25, 2012 at 11:48:58PM +0100, Steffen Persvold wrote:
>> []
>>>
>>> This looks like it will produce useful information, but I am not seeing
>>> output from it below.
>>>
>>> 							Thanx, Paul
>>>
>>>> This run it was CPU24 that triggered the issue :
>>>>
>>
>> This line is the printout for the root level :
>>
>>>> [  231.572688] CPU 24, treason uncloaked, rsp @ ffffffff81a1cd80 (rcu_sched), rnp @ ffffffff81a1cd80(r) qsmask=0x1f, c=5132 g=5132 nc=5132 ng=5133 sc=5132 sg=5133 mc=5132 mg=5133
>
> OK, so the rcu_state structure (sc and sg) believes that grace period
> 5133 has started but not completed, as expected.  Strangely enough, so
> does the root rcu_node structure (nc and ng) and the CPU's leaf rcu_node
> structure (mc and mg).
>
> The per-CPU rcu_data structure (c and g) does not yet know about the
> new 5133 grace period, as expected.
>
> So this is the code in kernel/rcutree.c:rcu_start_gp() that does the
> initialization:
>
> 	rcu_for_each_node_breadth_first(rsp, rnp) {
> 		raw_spin_lock(&rnp->lock);	/* irqs already disabled. */
> 		rcu_preempt_check_blocked_tasks(rnp);
> 		rnp->qsmask = rnp->qsmaskinit;
> 		rnp->gpnum = rsp->gpnum;
> 		rnp->completed = rsp->completed;
> 		if (rnp == rdp->mynode)
> 			rcu_start_gp_per_cpu(rsp, rnp, rdp);
> 		rcu_preempt_boost_start_gp(rnp);
> 		trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
> 					    rnp->level, rnp->grplo,
> 					    rnp->grphi, rnp->qsmask);
> 		raw_spin_unlock(&rnp->lock);	/* irqs remain disabled. */
> 	}
>
> I am assuming that your debug prints are still invoked right after
> the raw_spin_lock() above.  If so, I would expect nc==ng and mc==mg.
> Even if your debug prints followed the assignments to rnp->gpnum and
> rnp->completed, I would expect mc==mg for the root and internal rcu_node
> structures.  But you say below that you get the same values throughout,
> and in that case, I would expect the leaf rcu_node structure to show
> something different than the root and internal structures.
>
> The code really does hold the root rcu_node lock at all calls to
> rcu_gp_start(), so I don't see how we could be getting two CPUs in that
> code at the same time, which would be one way that the rcu_node and
> rcu_data structures might get advance notice of the new grace period,
> but in that case, you would have more than one bit set in ->qsmask.
>
> So, any luck with the trace events for rcu_grace_period and
> rcu_grace_period_init?
>

I've successfully enabled them and it seems to work, however once the 
issue is triggered any attempt to access /sys/kernel/debug/tracing/trace 
just hangs :/

Cheers,
-- 
Steffen Persvold, Chief Architect NumaChip
Numascale AS - www.numascale.com
Tel: +47 92 49 25 54 Skype: spersvold