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Date: Fri, 17 Oct 2008 21:05:13 +0530
From: Gautham R Shenoy <ego@...ibm.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
mingo@...e.hu, akpm@...ux-foundation.org, manfred@...orfullife.com,
dipankar@...ibm.com, josht@...ux.vnet.ibm.com, schamp@....com,
niv@...ibm.com, dvhltc@...ibm.com, laijs@...fujitsu.com,
rostedt@...dmis.org, peterz@...radead.org, penberg@...helsinki.fi,
andi@...stfloor.org, tglx@...utronix.de
Subject: Re: [PATCH, RFC] v7 scalable classic RCU implementation
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.
> > +};
> > +
> > +/* 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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