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Date: Mon, 30 Jan 2012 08:15:29 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Steffen Persvold <sp@...ascale.com> Cc: Daniel J Blueman <daniel@...ascale-asia.com>, Dipankar Sarma <dipankar@...ibm.com>, linux-kernel@...r.kernel.org, x86@...nel.org Subject: Re: RCU qsmask !=0 warnings on large-SMP... On Sat, Jan 28, 2012 at 10:09:21PM -0800, Paul E. McKenney wrote: > On Fri, Jan 27, 2012 at 12:09:25PM +0100, Steffen Persvold wrote: > > 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 :/ > > Hmmm... I wonder if it waits for a grace period? > > If it cannot be made to work, I can probably put together some > alternative diagnostics, but it will take me a day or three. Actually, another thing to try is "torture_type=rcu_bh" on the modprobe line for rcutorture. Also, it would be good to get a stack dump of the hung process -- it might be hung for some other reason. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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