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    SubjectRe: [PATCH RFC] CPU hotplug support for preemptible RCU
    On Thu, Aug 16, 2007 at 06:26:18PM -0700, Paul E. McKenney wrote:
    > On Tue, Aug 14, 2007 at 04:41:16PM -0700, Paul E. McKenney wrote:
    > > Hello!
    > >
    > > Work in progress, not for inclusion.
    > >
    > > The attached patch passes multiple hours of rcutorture while hotplugging
    > > CPUs every ten seconds on 64-bit PPC and x86_64. It fails miserably on
    > > 32-bit i386 after a few hotplugs, but then again, so does stock 2.6.22
    > > even without running rcutorture simultaneously.
    > >
    > > Is there some extra patch or hardware dependency for CPU hotplug on
    > > 32-bit i386?
    >
    > Progress report: turns out to be at least partly a hardware dependency.
    > I have found a couple of x86 machines that survive a goodly number of
    > CPU-hotplug operations on stock 2.6.22. They also have some other
    > problems, in one case, only booting at odd intervals regardless of
    > what kernel you are using. Though when it does boot, it does seems to
    > run reliably. :-/
    >
    > In contrast, doing even a small number of CPU hotplug operations on the
    > other machines eventually causes themselves to NMI themselves to death
    > regardless of what kernel I boot on them. One of these machines ends up
    > writing LOCK_PREFIX and nop into kernel text on CPU hotplug operations.
    > While other CPUs are running. I cannot believe that this is a good
    > thing, but suppressing this with the smp-alt-boot (see the bootonly()
    > function) doesn't seem to improve matters. Even so, seems to me that
    > smp-alt-boot should be enabled by default. Unless there are objections,
    > I will send a patch to do this.

    And I finally located a couple of i386 machines that tolerate CPU hotplug
    operations. They both passed an overnight CPU-hotplug-plus-rcutorture
    run. ;-)

    Thanx, Paul

    > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > ---
    > >
    > > include/linux/rcuclassic.h | 2
    > > include/linux/rcupreempt.h | 2
    > > kernel/rcuclassic.c | 8 +++
    > > kernel/rcupreempt.c | 93 +++++++++++++++++++++++++++++++++++++++++++--
    > > 4 files changed, 100 insertions(+), 5 deletions(-)
    > >
    > > diff -urpNa -X dontdiff linux-2.6.22-d-schedclassic/include/linux/rcuclassic.h linux-2.6.22-e-hotplugcpu/include/linux/rcuclassic.h
    > > --- linux-2.6.22-d-schedclassic/include/linux/rcuclassic.h 2007-08-06 10:16:16.000000000 -0700
    > > +++ linux-2.6.22-e-hotplugcpu/include/linux/rcuclassic.h 2007-08-07 18:15:08.000000000 -0700
    > > @@ -83,6 +83,8 @@ static inline void rcu_bh_qsctr_inc(int
    > > #define rcu_check_callbacks_rt(cpu, user)
    > > #define rcu_init_rt()
    > > #define rcu_needs_cpu_rt(cpu) 0
    > > +#define rcu_offline_cpu_rt(cpu)
    > > +#define rcu_online_cpu_rt(cpu)
    > > #define rcu_pending_rt(cpu) 0
    > > #define rcu_process_callbacks_rt(unused)
    > >
    > > diff -urpNa -X dontdiff linux-2.6.22-d-schedclassic/include/linux/rcupreempt.h linux-2.6.22-e-hotplugcpu/include/linux/rcupreempt.h
    > > --- linux-2.6.22-d-schedclassic/include/linux/rcupreempt.h 2007-08-06 14:56:00.000000000 -0700
    > > +++ linux-2.6.22-e-hotplugcpu/include/linux/rcupreempt.h 2007-08-07 18:15:10.000000000 -0700
    > > @@ -59,6 +59,8 @@ extern void rcu_advance_callbacks_rt(int
    > > extern void rcu_check_callbacks_rt(int cpu, int user);
    > > extern void rcu_init_rt(void);
    > > extern int rcu_needs_cpu_rt(int cpu);
    > > +extern void rcu_offline_cpu_rt(int cpu);
    > > +extern void rcu_online_cpu_rt(int cpu);
    > > extern int rcu_pending_rt(int cpu);
    > > struct softirq_action;
    > > extern void rcu_process_callbacks_rt(struct softirq_action *unused);
    > > diff -urpNa -X dontdiff linux-2.6.22-d-schedclassic/kernel/rcuclassic.c linux-2.6.22-e-hotplugcpu/kernel/rcuclassic.c
    > > --- linux-2.6.22-d-schedclassic/kernel/rcuclassic.c 2007-08-06 14:07:26.000000000 -0700
    > > +++ linux-2.6.22-e-hotplugcpu/kernel/rcuclassic.c 2007-08-11 08:25:55.000000000 -0700
    > > @@ -404,14 +404,19 @@ static void __rcu_offline_cpu(struct rcu
    > > static void rcu_offline_cpu(int cpu)
    > > {
    > > struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
    > > +#ifdef CONFIG_CLASSIC_RCU
    > > struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);
    > > +#endif /* #ifdef CONFIG_CLASSIC_RCU */
    > >
    > > __rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
    > > &per_cpu(rcu_data, cpu));
    > > +#ifdef CONFIG_CLASSIC_RCU
    > > __rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
    > > &per_cpu(rcu_bh_data, cpu));
    > > - put_cpu_var(rcu_data);
    > > put_cpu_var(rcu_bh_data);
    > > +#endif /* #ifdef CONFIG_CLASSIC_RCU */
    > > + put_cpu_var(rcu_data);
    > > + rcu_offline_cpu_rt(cpu);
    > > }
    > >
    > > #else
    > > @@ -561,6 +566,7 @@ static void __devinit rcu_online_cpu(int
    > > rdp->passed_quiesc = &per_cpu(rcu_data_passed_quiesc, cpu);
    > > rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
    > > bh_rdp->passed_quiesc = &per_cpu(rcu_data_bh_passed_quiesc, cpu);
    > > + rcu_online_cpu_rt(cpu);
    > > }
    > >
    > > static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
    > > diff -urpNa -X dontdiff linux-2.6.22-d-schedclassic/kernel/rcupreempt.c linux-2.6.22-e-hotplugcpu/kernel/rcupreempt.c
    > > --- linux-2.6.22-d-schedclassic/kernel/rcupreempt.c 2007-08-06 14:58:07.000000000 -0700
    > > +++ linux-2.6.22-e-hotplugcpu/kernel/rcupreempt.c 2007-08-11 04:02:10.000000000 -0700
    > > @@ -125,6 +125,8 @@ enum rcu_mb_flag_values {
    > > };
    > > static DEFINE_PER_CPU(enum rcu_mb_flag_values, rcu_mb_flag) = rcu_mb_done;
    > >
    > > +static cpumask_t rcu_cpu_online_map = CPU_MASK_NONE;
    > > +
    > > /*
    > > * Macro that prevents the compiler from reordering accesses, but does
    > > * absolutely -nothing- to prevent CPUs from reordering. This is used
    > > @@ -400,7 +402,7 @@ rcu_try_flip_idle(void)
    > >
    > > /* Now ask each CPU for acknowledgement of the flip. */
    > >
    > > - for_each_possible_cpu(cpu)
    > > + for_each_cpu_mask(cpu, rcu_cpu_online_map)
    > > per_cpu(rcu_flip_flag, cpu) = rcu_flipped;
    > >
    > > return 1;
    > > @@ -416,7 +418,7 @@ rcu_try_flip_waitack(void)
    > > int cpu;
    > >
    > > RCU_TRACE_ME(rcupreempt_trace_try_flip_a1);
    > > - for_each_possible_cpu(cpu)
    > > + for_each_cpu_mask(cpu, rcu_cpu_online_map)
    > > if (per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {
    > > RCU_TRACE_ME(rcupreempt_trace_try_flip_ae1);
    > > return 0;
    > > @@ -460,7 +462,7 @@ rcu_try_flip_waitzero(void)
    > >
    > > /* Call for a memory barrier from each CPU. */
    > >
    > > - for_each_possible_cpu(cpu)
    > > + for_each_cpu_mask(cpu, rcu_cpu_online_map)
    > > per_cpu(rcu_mb_flag, cpu) = rcu_mb_needed;
    > >
    > > RCU_TRACE_ME(rcupreempt_trace_try_flip_z2);
    > > @@ -478,7 +480,7 @@ rcu_try_flip_waitmb(void)
    > > int cpu;
    > >
    > > RCU_TRACE_ME(rcupreempt_trace_try_flip_m1);
    > > - for_each_possible_cpu(cpu)
    > > + for_each_cpu_mask(cpu, rcu_cpu_online_map)
    > > if (per_cpu(rcu_mb_flag, cpu) != rcu_mb_done) {
    > > RCU_TRACE_ME(rcupreempt_trace_try_flip_me1);
    > > return 0;
    > > @@ -583,6 +585,89 @@ void rcu_advance_callbacks_rt(int cpu, i
    > > spin_unlock_irqrestore(&rdp->lock, oldirq);
    > > }
    > >
    > > +#ifdef CONFIG_HOTPLUG_CPU
    > > +
    > > +#define rcu_offline_cpu_rt_enqueue(srclist, srctail, dstlist, dsttail) do { \
    > > + *dsttail = srclist; \
    > > + if (srclist != NULL) { \
    > > + dsttail = srctail; \
    > > + srclist = NULL; \
    > > + srctail = &srclist;\
    > > + } \
    > > + } while (0)
    > > +
    > > +
    > > +void rcu_offline_cpu_rt(int cpu)
    > > +{
    > > + int i;
    > > + struct rcu_head *list = NULL;
    > > + unsigned long oldirq;
    > > + struct rcu_data *rdp = RCU_DATA_CPU(cpu);
    > > + struct rcu_head **tail = &list;
    > > +
    > > + /* Remove all callbacks from the newly dead CPU, retaining order. */
    > > +
    > > + spin_lock_irqsave(&rdp->lock, oldirq);
    > > + rcu_offline_cpu_rt_enqueue(rdp->donelist, rdp->donetail, list, tail);
    > > + for (i = GP_STAGES - 1; i >= 0; i--)
    > > + rcu_offline_cpu_rt_enqueue(rdp->waitlist[i], rdp->waittail[i],
    > > + list, tail);
    > > + rcu_offline_cpu_rt_enqueue(rdp->nextlist, rdp->nexttail, list, tail);
    > > + spin_unlock_irqrestore(&rdp->lock, oldirq);
    > > + rdp->waitlistcount = 0;
    > > +
    > > + /* Disengage the newly dead CPU from grace-period computation. */
    > > +
    > > + spin_lock_irqsave(&rcu_ctrlblk.fliplock, oldirq);
    > > + rcu_check_mb(cpu);
    > > + if (per_cpu(rcu_flip_flag, cpu) == rcu_flipped) {
    > > + smp_mb(); /* Subsequent counter accesses must see new value */
    > > + per_cpu(rcu_flip_flag, cpu) = rcu_flip_seen;
    > > + smp_mb(); /* Subsequent RCU read-side critical sections */
    > > + /* seen -after- acknowledgement. */
    > > + }
    > > + cpu_clear(cpu, rcu_cpu_online_map);
    > > + spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, oldirq);
    > > +
    > > + /*
    > > + * Place the removed callbacks on the current CPU's queue.
    > > + * Make them all start a new grace period: simple approach,
    > > + * in theory could starve a given set of callbacks, but
    > > + * you would need to be doing some serious CPU hotplugging
    > > + * to make this happen. If this becomes a problem, adding
    > > + * a synchronize_rcu() to the hotplug path would be a simple
    > > + * fix.
    > > + */
    > > +
    > > + rdp = RCU_DATA_ME();
    > > + spin_lock_irqsave(&rdp->lock, oldirq);
    > > + *rdp->nexttail = list;
    > > + if (list)
    > > + rdp->nexttail = tail;
    > > + spin_unlock_irqrestore(&rdp->lock, oldirq);
    > > +}
    > > +
    > > +void __devinit rcu_online_cpu_rt(int cpu)
    > > +{
    > > + unsigned long oldirq;
    > > +
    > > + spin_lock_irqsave(&rcu_ctrlblk.fliplock, oldirq);
    > > + cpu_set(cpu, rcu_cpu_online_map);
    > > + spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, oldirq);
    > > +}
    > > +
    > > +#else /* #ifdef CONFIG_HOTPLUG_CPU */
    > > +
    > > +void rcu_offline_cpu(int cpu)
    > > +{
    > > +}
    > > +
    > > +void __devinit rcu_online_cpu_rt(int cpu)
    > > +{
    > > +}
    > > +
    > > +#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */
    > > +
    > > void rcu_process_callbacks_rt(struct softirq_action *unused)
    > > {
    > > unsigned long flags;
    -
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