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    SubjectRe: [PATCH, RFC] v4 scalable classic RCU implementation
    Hi Paul,

    Paul E. McKenney wrote:
    > +/*
    > + * Scan the leaf rcu_node structures, processing dyntick state for any that
    > + * have not yet encountered a quiescent state, using the function specified.
    > + * Returns 1 if the current grace period ends while scanning (possibly
    > + * because we made it end).
    > + */
    > +static int rcu_process_dyntick(struct rcu_state *rsp, long lastcomp,
    > + int (*f)(struct rcu_data *))
    > +{
    > + unsigned long bit;
    > + int cpu;
    > + unsigned long flags;
    > + unsigned long mask;
    > + struct rcu_node *rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
    > + struct rcu_node *rnp_end = &rsp->node[NUM_RCU_NODES];
    > +
    > + for (; rnp_cur < rnp_end; rnp_cur++) {
    > + mask = 0;
    > + spin_lock_irqsave(&rnp_cur->lock, flags);
    > + if (rsp->completed != lastcomp) {
    > + spin_unlock_irqrestore(&rnp_cur->lock, flags);
    > + return 1;
    > + }
    > + if (rnp_cur->qsmask == 0) {
    > + spin_unlock_irqrestore(&rnp_cur->lock, flags);
    > + continue;
    > + }
    > + cpu = rnp_cur->grplo;
    > + bit = 1;
    > + mask = 0;
    > + for (; cpu <= rnp_cur->grphi; cpu++, bit <<= 1) {
    > + if ((rnp_cur->qsmask & bit) != 0 && f(rsp->rda[cpu]))
    > + mask |= bit;
    > + }
    >
    I'm still comparing my implementation with your code:
    - f is called once for each cpu in the system, correct?
    - if at least one cpu is in nohz mode, this loop will be needed for
    every grace period.

    That means an O(NR_CPUS) loop with disabled local interrupts :-(
    Is that correct?

    Unfortunately, my solution is even worse:
    My rcu_irq_exit() acquires a global spinlock when called on a nohz cpus.
    A few cpus in cpu_idle, nohz, executing 50k network interrupts/sec would
    cacheline-trash that spinlock.
    I'm considering counting interrupts: if a nohz cpu executes more than a
    few interrupts/tick, then add a timer that check rcu_pending().

    Perhaps even wouldn't be enough: I remember that the initial unhandled
    irq detection code broke miserably on large SGI systems:
    An atomic_inc(&global_var) in the local timer interrupt (i.e.:
    NR_CPUS*HZ calls/sec) caused so severe trashing that the system wouldn't
    boot. IIRC that was with 512 cpus.


    --
    Manfred


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