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    SubjectRe: [PATCH, RFC] v7 scalable classic RCU implementation
    On Fri, Oct 17, 2008 at 09:05:13PM +0530, Gautham R Shenoy wrote:
    > On Fri, Oct 17, 2008 at 02:04:52PM +0530, Gautham R Shenoy wrote:
    > > On Fri, Oct 10, 2008 at 09:09:30AM -0700, Paul E. McKenney wrote:
    > > > Hello!
    > > Hi Paul,
    > >
    > > Looks interesting. Couple of minor nits. Comments interspersed. Search for "=>"
    > Search is too tedius, even for me. Trimming it down.

    The "/" command in "vi" works pretty well for me. ;-)

    These are the same as the ones in your earlier note, correct?

    Thanx, Paul

    > > > +};
    > > > +
    > > > +/* Values for signaled field in struc rcu_data. */
    > ^^^^^^^^^^^^^^^^^^
    > should be struct rcu_state.
    > > > +#define RCU_SAVE_DYNTICK 0 /* Need to scan dyntick state. */
    > > > +#define RCU_FORCE_QS 1 /* Need to force quiescent state. */
    > > > +#ifdef CONFIG_NO_HZ
    > > > +#define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK
    > > > +#else /* #ifdef CONFIG_NO_HZ */
    > > > +#define RCU_SIGNAL_INIT RCU_FORCE_QS
    > > > +#endif /* #else #ifdef CONFIG_NO_HZ */
    > > > +
    > > > +}
    > > > +
    >
    >
    >
    > > > +#ifdef CONFIG_SMP
    > > > +
    > > > +/*
    > > > + * If the specified CPU is offline, tell the caller that it is in
    > > > + * a quiescent state. Otherwise, whack it with a reschedule IPI.
    > > > + * Grace periods can end up waiting on an offline CPU when that
    > > > + * CPU is in the process of coming online -- it will be added to the
    > > > + * rcu_node bitmasks before it actually makes it online.
    >
    > This can also happen when a CPU has just gone offline,
    > but RCU hasn't yet marked it as offline. However, it's impact
    > on delaying the grace period may not be high as in the
    > CPU-online case.
    > >
    > > > + * Because this
    > > > + * race is quite rare, we check for it after detecting that the grace
    > > > + * period has been delayed rather than checking each and every CPU
    > > > + * each and every time we start a new grace period.
    > > > + */
    > > > +static int rcu_implicit_offline_qs(struct rcu_data *rdp)
    > > > +{
    > > > + /*
    > > > + * If the CPU is offline, it is in a quiescent state. We can
    > > > + * trust its state not to change because interrupts are disabled.
    > > > + */
    > > > + if (cpu_is_offline(rdp->cpu)) {
    > > > + rdp->offline_fqs++;
    > > > + return 1;
    > > > + }
    > > > +
    > > > + /* The CPU is online, so send it a reschedule IPI. */
    > > > + if (rdp->cpu != smp_processor_id())
    >
    > This check is safe here since this callpath is invoked
    > from a softirq, and thus the system cannot do a stop_machine()
    > as yet. This implies that the cpu in question cannot go offline
    > until we're done.
    >
    > > > + smp_send_reschedule(rdp->cpu);
    > > > + else
    > > > + set_need_resched();
    > > > + rdp->resched_ipi++;
    > > > + return 0;
    > > > +}
    > > > +
    > > > +#endif /* #ifdef CONFIG_SMP */
    > > > +/*
    >
    > > > + * Record the specified "completed" value, which is later used to validate
    > > > + * dynticks counter manipulations. Specify "rsp->complete - 1" to
    > ^^^^^^^^^^^^^^^^^^^
    > "rsp->completed - 1" ?
    >
    >
    > > > + * unconditionally invalidate any future dynticks manipulations (which is
    > > > + * useful at the beginning of a grace period).
    >
    >
    > > > +
    > > > +static void print_other_cpu_stall(struct rcu_state *rsp)
    > > > +{
    > > > + int cpu;
    > > > + long delta;
    > > > + unsigned long flags;
    > > > + struct rcu_node *rnp = rcu_get_root(rsp);
    > > > + struct rcu_node *rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
    > > > + struct rcu_node *rnp_end = &rsp->node[NUM_RCU_NODES];
    > > > +
    > > > + /* Only let one CPU complain about others per time interval. */
    > > > +
    > > > + spin_lock_irqsave(&rnp->lock, flags);
    > > > + delta = jiffies - rsp->jiffies_stall;
    > > > + if (delta < RCU_STALL_RAT_DELAY || rsp->gpnum != rsp->completed) {
    > ----------------> [1]
    > See comment in check_cpu_stall()
    >
    > > > + spin_unlock_irqrestore(&rnp->lock, flags);
    > > > + return;
    > > > + }
    > > > + rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
    > > > + spin_unlock_irqrestore(&rnp->lock, flags);
    > > > +
    > > > + /* OK, time to rat on our buddy... */
    > > > +
    > > > + printk(KERN_ERR "RCU detected CPU stalls:");
    > > > + for (; rnp_cur < rnp_end; rnp_cur++) {
    > > > + if (rnp_cur->qsmask == 0)
    > > > + continue;
    > > > + for (cpu = 0; cpu <= rnp_cur->grphi - rnp_cur->grplo; cpu++)
    > > > + if (rnp_cur->qsmask & (1UL << cpu))
    > > > + printk(" %d", rnp_cur->grplo + cpu);
    > > > + }
    > > > + printk(" (detected by %d, t=%ld jiffies)\n",
    > > > + smp_processor_id(), (long)(jiffies - rsp->gp_start));
    > > > + force_quiescent_state(rsp, 0); /* Kick them all. */
    > > > +}
    > > > +
    > > > +static void print_cpu_stall(struct rcu_state *rsp)
    > > > +{
    > > > + unsigned long flags;
    > > > + struct rcu_node *rnp = rcu_get_root(rsp);
    > > > +
    > > > + printk(KERN_ERR "RCU detected CPU %d stall (t=%lu jiffies)\n",
    > > > + smp_processor_id(), jiffies - rsp->gp_start);
    > > > + dump_stack();
    > > > + spin_lock_irqsave(&rnp->lock, flags);
    > > > + if ((long)(jiffies - rsp->jiffies_stall) >= 0)
    > > > + rsp->jiffies_stall =
    > > > + jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
    > > > + spin_unlock_irqrestore(&rnp->lock, flags);
    > > > + set_need_resched(); /* kick ourselves to get things going. */
    > > > +}
    > > > +
    > > > +static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
    > > > +{
    > > > + long delta;
    > > > + struct rcu_node *rnp;
    > > > +
    > > > + delta = jiffies - rsp->jiffies_stall;
    > > > + rnp = rdp->mynode;
    > > > + if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {
    > > > +
    > > > + /* We haven't checked in, so go dump stack. */
    > > > + print_cpu_stall(rsp);
    > > > +
    > > > + } else if (rsp->gpnum != rsp->completed &&
    > > > + delta >= RCU_STALL_RAT_DELAY) {
    > >
    > If this condition is true, then,
    > rsp->gpnum != rsp->completed. Hence, we will always enter
    > the if() condition in print_other_cpu_stall() at
    > [1] (See above), and return without ratting our buddy.
    >
    > That defeats the purpose of the stall check or I am
    > missing the obvious, which is quite possible :-)
    > > > +
    > > > + /* They had two time units to dump stack, so complain. */
    > > > + print_other_cpu_stall(rsp);
    > > > + }
    > > > +}
    > > > +
    > > > +#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
    > > > +
    > > > +static void record_gp_stall_check_time(struct rcu_state *rsp)
    > > > +{
    > > > +}
    >
    >
    > > > +
    > > > +static void __cpuinit rcu_online_cpu(int cpu)
    > > > +{
    > > > +#ifdef CONFIG_NO_HZ
    > > > + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
    > > > +
    > > > + rdtp->dynticks_nesting = 1;
    > > > + rdtp->dynticks |= 1; /* need consecutive #s even for hotplug. */
    > > > + rdtp->dynticks_nmi = (rdtp->dynticks + 1) & ~0x1;
    > rdtp->dynticks is odd. Hence rdtp->dynticks + 1 should be even.
    > Why is the additional & ~0x1 ?
    >
    >
    > >
    > > > +#endif /* #ifdef CONFIG_NO_HZ */
    > > > + rcu_init_percpu_data(cpu, &rcu_state);
    > --
    > Thanks and Regards
    > gautham


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