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


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