Re: [PATCH RFC] CPU hotplug support for preemptible RCU

From: Paul E. McKenney
Date: Thu Aug 16 2007 - 21:26:50 EST

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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.

Thoughts?

Thanx, Paul

> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>
> 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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Next message: Herbert Xu: "Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures"
Previous message: Rusty Russell: "[PATCH] Only initialize hvc_console if needed, cleanup Kconfig help"
Next in thread: Paul E. McKenney: "Re: [PATCH RFC] CPU hotplug support for preemptible RCU"
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