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SubjectRe: [PATCH] An RCU for SMP with a single CPU garbage collector
On Mon, Mar 07, 2011 at 03:31:06PM -0500, Joe Korty wrote:
> Hi Paul & Fredrick & other interested parties.
>
> We would like for Linux to eventually support user
> dedicated cpus. That is, cpus that are completely free of
> periodic system duties, leaving 100% of their capacity
> available to service user applications. This will
> eventually be important for those realtime applications
> requiring full use of dedicated cpus.
>
> An important milestone to that goal would be to have
> an offical, supported RCU implementation which did not
> make each and every CPU periodically participate in RCU
> garbage collection.
>
> The attached RCU implementation does precisely that.
> Like TinyRCU it is both tiny and very simple, but unlike
> TinyRCU it supports SMP, and unlike the other SMP RCUs,
> it does its garbage collection from a single CPU.
>
> For performance, each cpu is given its own 'current' and
> 'previous' callback queue, and all interactions between
> these queues and the global garbage collector proceed in
> a lock-free manner.
>
> This patch is a quick port to 2.6.38-rc7 from a 2.6.36.4
> implementation developed over the last two weeks. The
> earlier version was tested over the weekend under i386 and
> x86_64, this version has only been spot tested on x86_64.

Hello, Joe,

My biggest question is "what does JRCU do that Frederic's patchset
does not do?" I am not seeing it at the moment. Given that Frederic's
patchset integrates into RCU, thus providing the full RCU API, I really
need a good answer to consider JRCU.

In the meantime, some questions and comments below.

For one, what sort of validation have you done?

Thanx, Paul

> Signed-off-by: Joe Korty <joe.korty@ccur.com>
>
> Index: b/kernel/jrcu.c
> ===================================================================
> --- /dev/null
> +++ b/kernel/jrcu.c
> @@ -0,0 +1,604 @@
> +/*
> + * Joe's tiny single-cpu RCU, for small SMP systems.
> + *
> + * Running RCU end-of-batch operations from a single cpu relieves the
> + * other CPUs from this periodic responsibility. This will eventually
> + * be important for those realtime applications requiring full use of
> + * dedicated cpus. JRCU is also a lockless implementation, currently,
> + * although some anticipated features will eventually require a per
> + * cpu rcu_lock along some minimal-contention paths.
> + *
> + * Author: Joe Korty <joe.korty@ccur.com>
> + *
> + * Acknowledgements: Paul E. McKenney's 'TinyRCU for uniprocessors' inspired
> + * the thought that there could could be something similiarly simple for SMP.
> + * The rcu_list chain operators are from Jim Houston's Alternative RCU.
> + *
> + * Copyright Concurrent Computer Corporation, 2011
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License as published by the
> + * Free Software Foundation; either version 2 of the License, or (at your
> + * option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
> + * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
> + * for more details.
> + *
> + * You should have received a copy of the GNU General Public License along
> + * with this program; if not, write to the Free Software Foundation, Inc.,
> + * 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + */
> +
> +/*
> + * This RCU maintains three callback lists: the current batch (per cpu),
> + * the previous batch (also per cpu), and the pending list (global).
> + */
> +
> +#include <linux/bug.h>
> +#include <linux/smp.h>
> +#include <linux/sched.h>
> +#include <linux/types.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/percpu.h>
> +#include <linux/stddef.h>
> +#include <linux/preempt.h>
> +#include <linux/compiler.h>
> +#include <linux/irqflags.h>
> +#include <linux/rcupdate.h>
> +
> +#include <asm/system.h>
> +
> +/*
> + * Define an rcu list type and operators. An rcu list has only ->next
> + * pointers for the chain nodes; the list head however is special and
> + * has pointers to both the first and last nodes of the chain. Tweaked
> + * so that null head, tail pointers can be used to signify an empty list.
> + */
> +struct rcu_list {
> + struct rcu_head *head;
> + struct rcu_head **tail;
> + int count; /* stats-n-debug */
> +};
> +
> +static inline void rcu_list_init(struct rcu_list *l)
> +{
> + l->head = NULL;
> + l->tail = NULL;
> + l->count = 0;
> +}
> +
> +/*
> + * Add an element to the tail of an rcu list
> + */
> +static inline void rcu_list_add(struct rcu_list *l, struct rcu_head *h)
> +{
> + if (unlikely(l->tail == NULL))
> + l->tail = &l->head;
> + *l->tail = h;
> + l->tail = &h->next;
> + l->count++;
> + h->next = NULL;
> +}

This has interrupts disabled? Or is there some other form of
mutual exclusion? (The only caller does have interrupts disabled,
and calls this only on per-CPU data, so should be OK.)

> +/*
> + * Append the contents of one rcu list to another. The 'from' list is left
> + * corrupted on exit; the caller must re-initialize it before it can be used
> + * again.
> + */
> +static inline void rcu_list_join(struct rcu_list *to, struct rcu_list *from)
> +{
> + if (from->head) {
> + if (unlikely(to->tail == NULL)) {
> + to->tail = &to->head;
> + to->count = 0;
> + }
> + *to->tail = from->head;
> + to->tail = from->tail;
> + to->count += from->count;
> + }
> +}
> +
> +
> +#define RCU_HZ 20 /* max rate at which batches are retired */
> +
> +struct rcu_data {
> + u8 wait; /* goes false when this cpu consents to
> + * the retirement of the current batch */
> + u8 which; /* selects the current callback list */
> + struct rcu_list cblist[2]; /* current & previous callback lists */
> +} ____cacheline_aligned_in_smp;
> +
> +static struct rcu_data rcu_data[NR_CPUS];

Why not DEFINE_PER_CPU(struct rcu_data, rcu_data)?

> +
> +/* debug & statistics stuff */
> +static struct rcu_stats {
> + unsigned nbatches; /* #end-of-batches (eobs) seen */
> + atomic_t nbarriers; /* #rcu barriers processed */
> + u64 ninvoked; /* #invoked (ie, finished) callbacks */
> + atomic_t nleft; /* #callbacks left (ie, not yet invoked) */
> + unsigned nforced; /* #forced eobs (should be zero) */
> +} rcu_stats;
> +
> +int rcu_scheduler_active __read_mostly;
> +int rcu_nmi_seen __read_mostly;
> +static u64 rcu_timestamp;
> +
> +/*
> + * Return our CPU id or zero if we are too early in the boot process to
> + * know what that is. For RCU to work correctly, a cpu named '0' must
> + * eventually be present (but need not ever be online).
> + */
> +static inline int rcu_cpu(void)
> +{
> + return current_thread_info()->cpu;

OK, I'll bite... Why not smp_processor_id()?

And what to do about the architectures that put the CPU number somewhere
else?

> +}
> +
> +/*
> + * Invoke whenever the calling CPU consents to end-of-batch. All CPUs
> + * must so consent before the batch is truly ended.
> + */
> +static inline void rcu_eob(int cpu)
> +{
> + struct rcu_data *rd = &rcu_data[cpu];
> + if (unlikely(rd->wait)) {
> + rd->wait = 0;
> +#ifdef CONFIG_RCU_PARANOID
> + /* not needed, we can tolerate some fuzziness on exactly
> + * when other CPUs see the above write insn. */
> + smp_wmb();
> +#endif
> + }
> +}
> +
> +void rcu_note_context_switch(int cpu)
> +{
> + rcu_eob(cpu);
> +}
> +
> +void __rcu_preempt_sub(void)
> +{
> + rcu_eob(rcu_cpu());
> +}
> +EXPORT_SYMBOL(__rcu_preempt_sub);
> +
> +void rcu_barrier(void)
> +{
> + struct rcu_synchronize rcu;
> +
> + if (!rcu_scheduler_active)
> + return;
> +
> + init_completion(&rcu.completion);
> + call_rcu(&rcu.head, wakeme_after_rcu);
> + wait_for_completion(&rcu.completion);
> + atomic_inc(&rcu_stats.nbarriers);
> +
> +}
> +EXPORT_SYMBOL_GPL(rcu_barrier);

The rcu_barrier() function must wait on all RCU callbacks, regardless of
which CPU they are queued on. This is important when unloading modules
that use call_rcu(). In contrast, the above looks to me like it waits
only on the current CPU's callbacks.

So, what am I missing?

> +
> +void rcu_force_quiescent_state(void)
> +{
> +}
> +EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
> +
> +
> +/*
> + * Insert an RCU callback onto the calling CPUs list of 'current batch'
> + * callbacks. Lockless version, can be invoked anywhere except under NMI.
> + */
> +void call_rcu(struct rcu_head *cb, void (*func)(struct rcu_head *rcu))
> +{
> + unsigned long flags;
> + struct rcu_data *rd;
> + struct rcu_list *cblist;
> + int which;
> +
> + cb->func = func;
> + cb->next = NULL;
> +
> + raw_local_irq_save(flags);
> + smp_rmb();
> +
> + rd = &rcu_data[rcu_cpu()];

Why not a per-CPU variable rather than an array?

> + which = ACCESS_ONCE(rd->which) & 1;
> + cblist = &rd->cblist[which];
> +
> + /* The following is not NMI-safe, therefore call_rcu()
> + * cannot be invoked under NMI. */
> + rcu_list_add(cblist, cb);
> + smp_wmb();
> + raw_local_irq_restore(flags);
> + atomic_inc(&rcu_stats.nleft);
> +}
> +EXPORT_SYMBOL_GPL(call_rcu);
> +
> +/*
> + * For a given cpu, push the previous batch of callbacks onto a (global)
> + * pending list, then make the current batch the previous. A new, empty
> + * current batch exists after this operation.
> + *
> + * Locklessly tolerates changes being made by call_rcu() to the current
> + * batch, locklessly tolerates the current batch becoming the previous
> + * batch, and locklessly tolerates a new, empty current batch becoming
> + * available. Requires that the previous batch be quiescent by the time
> + * rcu_end_batch is invoked.
> + */
> +static void rcu_end_batch(struct rcu_data *rd, struct rcu_list *pending)
> +{
> + int prev;
> + struct rcu_list *plist; /* some cpus' previous list */
> +
> + prev = (ACCESS_ONCE(rd->which) & 1) ^ 1;
> + plist = &rd->cblist[prev];
> +
> + /* Chain previous batch of callbacks, if any, to the pending list */
> + if (plist->head) {
> + rcu_list_join(pending, plist);
> + rcu_list_init(plist);
> + smp_wmb();
> + }
> + /*
> + * Swap current and previous lists. Other cpus must not see this
> + * out-of-order w.r.t. the just-completed plist init, hence the above
> + * smp_wmb().
> + */
> + rd->which++;

You do seem to have interrupts disabled when sampling ->which, but
this is not safe for cross-CPU accesses to ->which, right? The other
CPU might queue onto the wrong element. This would mean that you
would not be guaranteed a full 50ms delay from quiescent state to
corresponding RCU callback invocation.

Or am I missing something subtle here?

> +}
> +
> +/*
> + * Invoke all callbacks on the passed-in list.
> + */
> +static void rcu_invoke_callbacks(struct rcu_list *pending)
> +{
> + struct rcu_head *curr, *next;
> +
> + for (curr = pending->head; curr;) {
> + next = curr->next;
> + curr->func(curr);
> + curr = next;
> + rcu_stats.ninvoked++;
> + atomic_dec(&rcu_stats.nleft);
> + }
> +}
> +
> +/*
> + * Check if the conditions for ending the current batch are true. If
> + * so then end it.
> + *
> + * Must be invoked periodically, and the periodic invocations must be
> + * far enough apart in time for the previous batch to become quiescent.
> + * This is a few tens of microseconds unless NMIs are involved; an NMI
> + * stretches out the requirement by the duration of the NMI.
> + *
> + * "Quiescent" means the owning cpu is no longer appending callbacks
> + * and has completed execution of a trailing write-memory-barrier insn.
> + */
> +static void __rcu_delimit_batches(struct rcu_list *pending)
> +{
> + struct rcu_data *rd;
> + int cpu, eob;
> + u64 rcu_now;
> +
> + /* If an NMI occured then the previous batch may not yet be
> + * quiescent. Let's wait till it is.
> + */
> + if (rcu_nmi_seen) {
> + rcu_nmi_seen = 0;
> + return;
> + }
> +
> + if (!rcu_scheduler_active)
> + return;
> +
> + /*
> + * Find out if the current batch has ended
> + * (end-of-batch).
> + */
> + eob = 1;
> + for_each_online_cpu(cpu) {
> + rd = &rcu_data[cpu];
> + if (rd->wait) {
> + eob = 0;
> + break;
> + }
> + }
> +
> + /*
> + * Force end-of-batch if too much time (n seconds) has
> + * gone by. The forcing method is slightly questionable,
> + * hence the WARN_ON.
> + */
> + rcu_now = sched_clock();
> + if (!eob && !rcu_timestamp
> + && ((rcu_now - rcu_timestamp) > 3LL * NSEC_PER_SEC)) {
> + rcu_stats.nforced++;
> + WARN_ON_ONCE(1);
> + eob = 1;
> + }
> +
> + /*
> + * Just return if the current batch has not yet
> + * ended. Also, keep track of just how long it
> + * has been since we've actually seen end-of-batch.
> + */
> +
> + if (!eob)
> + return;
> +
> + rcu_timestamp = rcu_now;
> +
> + /*
> + * End the current RCU batch and start a new one.
> + */
> + for_each_present_cpu(cpu) {
> + rd = &rcu_data[cpu];

And here we get the cross-CPU accesses that I was worried about above.

> + rcu_end_batch(rd, pending);
> + if (cpu_online(cpu)) /* wins race with offlining every time */
> + rd->wait = preempt_count_cpu(cpu) > idle_cpu(cpu);
> + else
> + rd->wait = 0;
> + }
> + rcu_stats.nbatches++;
> +}
> +
> +static void rcu_delimit_batches(void)
> +{
> + unsigned long flags;
> + struct rcu_list pending;
> +
> + rcu_list_init(&pending);
> +
> + raw_local_irq_save(flags);
> + smp_rmb();
> + __rcu_delimit_batches(&pending);
> + smp_wmb();
> + raw_local_irq_restore(flags);
> +
> + if (pending.head)
> + rcu_invoke_callbacks(&pending);
> +}
> +
> +/* ------------------ interrupt driver section ------------------ */
> +
> +/*
> + * We drive RCU from a periodic interrupt during most of boot. Once boot
> + * is complete we (optionally) transition to a daemon.
> + */
> +
> +#include <linux/time.h>
> +#include <linux/delay.h>
> +#include <linux/hrtimer.h>
> +#include <linux/interrupt.h>
> +
> +#define RCU_PERIOD_NS (NSEC_PER_SEC / RCU_HZ)
> +#define RCU_PERIOD_DELTA_NS (((NSEC_PER_SEC / HZ) * 3) / 2)
> +
> +#define RCU_PERIOD_MIN_NS RCU_PERIOD_NS
> +#define RCU_PERIOD_MAX_NS (RCU_PERIOD_NS + RCU_PERIOD_DELTA_NS)
> +
> +static struct hrtimer rcu_timer;
> +
> +static void rcu_softirq_func(struct softirq_action *h)
> +{
> + rcu_delimit_batches();
> +}
> +
> +static enum hrtimer_restart rcu_timer_func(struct hrtimer *t)
> +{
> + ktime_t next;
> +
> + raise_softirq(RCU_SOFTIRQ);
> +
> + next = ktime_add_ns(ktime_get(), RCU_PERIOD_NS);
> + hrtimer_set_expires_range_ns(&rcu_timer, next, RCU_PERIOD_DELTA_NS);
> + return HRTIMER_RESTART;
> +}
> +
> +static void rcu_timer_restart(void)
> +{
> + pr_info("JRCU: starting timer. rate is %d Hz\n", RCU_HZ);
> + hrtimer_forward_now(&rcu_timer, ns_to_ktime(RCU_PERIOD_NS));
> + hrtimer_start_expires(&rcu_timer, HRTIMER_MODE_ABS);
> +}
> +
> +static __init int rcu_timer_start(void)
> +{
> + open_softirq(RCU_SOFTIRQ, rcu_softirq_func);
> +
> + hrtimer_init(&rcu_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
> + rcu_timer.function = rcu_timer_func;
> + rcu_timer_restart();
> +
> + return 0;
> +}
> +
> +#ifdef CONFIG_JRCU_DAEMON
> +static void rcu_timer_stop(void)
> +{
> + int stat;
> +
> + stat = hrtimer_cancel(&rcu_timer);
> + if (stat)
> + pr_info("JRCU: timer canceled.\n");
> +}
> +#endif
> +
> +/*
> + * Transition from a simple to a full featured, interrupt driven RCU.
> + *
> + * This is to protect us against RCU being used very very early in the boot
> + * process, where ideas like 'tasks' and 'cpus' and 'timers' and such are
> + * not yet fully formed. During this very early time, we use a simple,
> + * not-fully-functional braindead version of RCU.
> + *
> + * Invoked from main() at the earliest point where scheduling and timers
> + * are functional.
> + */
> +void __init rcu_scheduler_starting(void)
> +{
> + int stat;
> +
> + stat = rcu_timer_start();
> + if (stat) {
> + pr_err("JRCU: failed to start. This is fatal.\n");
> + return;
> + }
> +
> + rcu_scheduler_active = 1;
> + smp_wmb();
> +
> + pr_info("JRCU: started\n");
> +}
> +
> +#ifdef CONFIG_JRCU_DAEMON
> +
> +/* ------------------ daemon driver section --------------------- */
> +
> +#define RCU_PERIOD_MIN_US (RCU_PERIOD_MIN_NS / NSEC_PER_USEC)
> +#define RCU_PERIOD_MAX_US (RCU_PERIOD_MAX_NS / NSEC_PER_USEC)
> +
> +/*
> + * Once the system is fully up, we will drive the periodic-polling part
> + * of JRCU from a kernel daemon, jrcud. Until then it is driven by
> + * an interrupt.
> + */
> +#include <linux/err.h>
> +#include <linux/param.h>
> +#include <linux/kthread.h>
> +
> +static int jrcud_func(void *arg)
> +{
> + set_user_nice(current, -19);
> + current->flags |= PF_NOFREEZE;
> +
> + pr_info("JRCU: daemon started. Will operate at ~%d Hz.\n", RCU_HZ);
> + rcu_timer_stop();
> +
> + while (!kthread_should_stop()) {
> + usleep_range(RCU_PERIOD_MIN_US, RCU_PERIOD_MAX_US);
> + rcu_delimit_batches();
> + }
> +
> + pr_info("JRCU: daemon exiting\n");
> + rcu_timer_restart();
> + return 0;
> +}
> +
> +static __init int jrcud_start(void)
> +{
> + struct task_struct *p;
> +
> + p = kthread_run(jrcud_func, NULL, "jrcud");
> + if (IS_ERR(p)) {
> + pr_warn("JRCU: daemon not started\n");
> + return -ENODEV;
> + }
> + return 0;
> +}
> +late_initcall(jrcud_start);
> +
> +#endif /* CONFIG_JRCU_DAEMON */
> +
> +/* ------------------ debug and statistics section -------------- */
> +
> +#ifdef CONFIG_RCU_TRACE
> +
> +#include <linux/debugfs.h>
> +#include <linux/seq_file.h>
> +
> +static int rcu_debugfs_show(struct seq_file *m, void *unused)
> +{
> + int cpu, q, s[2], msecs;
> +
> + raw_local_irq_disable();
> + msecs = div_s64(sched_clock() - rcu_timestamp, NSEC_PER_MSEC);
> + raw_local_irq_enable();
> +
> + seq_printf(m, "%14u: #batches seen\n",
> + rcu_stats.nbatches);
> + seq_printf(m, "%14u: #barriers seen\n",
> + atomic_read(&rcu_stats.nbarriers));
> + seq_printf(m, "%14llu: #callbacks invoked\n",
> + rcu_stats.ninvoked);
> + seq_printf(m, "%14u: #callbacks left to invoke\n",
> + atomic_read(&rcu_stats.nleft));
> + seq_printf(m, "%14u: #msecs since last end-of-batch\n",
> + msecs);
> + seq_printf(m, "%14u: #passes forced (0 is best)\n",
> + rcu_stats.nforced);
> + seq_printf(m, "\n");
> +
> + for_each_online_cpu(cpu)
> + seq_printf(m, "%4d ", cpu);
> + seq_printf(m, " CPU\n");
> +
> + s[1] = s[0] = 0;
> + for_each_online_cpu(cpu) {
> + struct rcu_data *rd = &rcu_data[cpu];
> + int w = ACCESS_ONCE(rd->which) & 1;
> + seq_printf(m, "%c%c%c%d ",
> + '-',
> + idle_cpu(cpu) ? 'I' : '-',
> + rd->wait ? 'W' : '-',
> + w);
> + s[w]++;
> + }
> + seq_printf(m, " FLAGS\n");
> +
> + for (q = 0; q < 2; q++) {
> + for_each_online_cpu(cpu) {
> + struct rcu_data *rd = &rcu_data[cpu];
> + struct rcu_list *l = &rd->cblist[q];
> + seq_printf(m, "%4d ", l->count);
> + }
> + seq_printf(m, " Q%d%c\n", q, " *"[s[q] > s[q^1]]);
> + }
> + seq_printf(m, "\nFLAGS:\n");
> + seq_printf(m, " I - cpu idle, 0|1 - Q0 or Q1 is current Q, other is previous Q,\n");
> + seq_printf(m, " W - cpu does not permit current batch to end (waiting),\n");
> + seq_printf(m, " * - marks the Q that is current for most CPUs.\n");
> +
> + return 0;
> +}
> +
> +static int rcu_debugfs_open(struct inode *inode, struct file *file)
> +{
> + return single_open(file, rcu_debugfs_show, NULL);
> +}
> +
> +static const struct file_operations rcu_debugfs_fops = {
> + .owner = THIS_MODULE,
> + .open = rcu_debugfs_open,
> + .read = seq_read,
> + .llseek = seq_lseek,
> + .release = single_release,
> +};
> +
> +static struct dentry *rcudir;
> +
> +static int __init rcu_debugfs_init(void)
> +{
> + struct dentry *retval;
> +
> + rcudir = debugfs_create_dir("rcu", NULL);
> + if (!rcudir)
> + goto error;
> +
> + retval = debugfs_create_file("rcudata", 0444, rcudir,
> + NULL, &rcu_debugfs_fops);
> + if (!retval)
> + goto error;
> +
> + pr_info("JRCU: Created debugfs files\n");
> + return 0;
> +
> +error:
> + debugfs_remove_recursive(rcudir);
> + pr_warn("JRCU: Could not create debugfs files\n");
> + return -ENOSYS;
> +}
> +late_initcall(rcu_debugfs_init);
> +#endif /* CONFIG_RCU_TRACE */
> Index: b/include/linux/jrcu.h
> ===================================================================
> --- /dev/null
> +++ b/include/linux/jrcu.h
> @@ -0,0 +1,75 @@
> +/*
> + * JRCU - A tiny single-cpu RCU for small SMP systems.
> + *
> + * Author: Joe Korty <joe.korty@ccur.com>
> + * Copyright Concurrent Computer Corporation, 2011
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU General Public License as published by the
> + * Free Software Foundation; either version 2 of the License, or (at your
> + * option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful, but
> + * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
> + * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
> + * for more details.
> + *
> + * You should have received a copy of the GNU General Public License along
> + * with this program; if not, write to the Free Software Foundation, Inc.,
> + * 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + */
> +#ifndef __LINUX_JRCU_H
> +#define __LINUX_JRCU_H
> +
> +#define __rcu_read_lock() preempt_disable()
> +#define __rcu_read_unlock() preempt_enable()
> +
> +#define __rcu_read_lock_bh() __rcu_read_lock()
> +#define __rcu_read_unlock_bh() __rcu_read_unlock()
> +
> +extern void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu));
> +
> +#define call_rcu_sched call_rcu
> +#define call_rcu_bh call_rcu
> +
> +extern void rcu_barrier(void);
> +
> +#define rcu_barrier_sched rcu_barrier
> +#define rcu_barrier_bh rcu_barrier
> +
> +#define synchronize_rcu rcu_barrier
> +#define synchronize_sched rcu_barrier
> +#define synchronize_sched_expedited rcu_barrier
> +#define synchronize_rcu_bh rcu_barrier
> +#define synchronize_rcu_expedited rcu_barrier
> +#define synchronize_rcu_bh_expedited rcu_barrier
> +
> +#define rcu_init(cpu) do { } while (0)
> +#define rcu_init_sched() do { } while (0)
> +#define exit_rcu() do { } while (0)
> +
> +static inline void __rcu_check_callbacks(int cpu, int user) { }
> +#define rcu_check_callbacks __rcu_check_callbacks
> +
> +#define rcu_needs_cpu(cpu) (0)
> +#define rcu_batches_completed() (0)
> +#define rcu_batches_completed_bh() (0)
> +#define rcu_preempt_depth() (0)
> +
> +extern void rcu_force_quiescent_state(void);
> +
> +#define rcu_sched_force_quiescent_state rcu_force_quiescent_state
> +#define rcu_bh_force_quiescent_state rcu_force_quiescent_state
> +
> +#define rcu_enter_nohz() do { } while (0)
> +#define rcu_exit_nohz() do { } while (0)
> +
> +extern void rcu_note_context_switch(int cpu);
> +
> +#define rcu_sched_qs rcu_note_context_switch
> +#define rcu_bh_qs rcu_note_context_switch
> +
> +extern void rcu_scheduler_starting(void);
> +extern int rcu_scheduler_active __read_mostly;
> +
> +#endif /* __LINUX_JRCU_H */
> Index: b/include/linux/rcupdate.h
> ===================================================================
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -145,6 +145,8 @@ static inline void rcu_exit_nohz(void)
> #include <linux/rcutree.h>
> #elif defined(CONFIG_TINY_RCU) || defined(CONFIG_TINY_PREEMPT_RCU)
> #include <linux/rcutiny.h>
> +#elif defined(CONFIG_JRCU)
> +#include <linux/jrcu.h>
> #else
> #error "Unknown RCU implementation specified to kernel configuration"
> #endif
> Index: b/init/Kconfig
> ===================================================================
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -384,6 +384,22 @@ config TREE_PREEMPT_RCU
> is also required. It also scales down nicely to
> smaller systems.
>
> +config JRCU
> + bool "A tiny single-CPU RCU for small SMP systems"
> + depends on PREEMPT
> + depends on SMP
> + help
> + This option selects a minimal-footprint RCU suitable for small SMP
> + systems -- that is, those with fewer than 16 or perhaps 32, and
> + certainly less than 64 processors.
> +
> + This RCU variant may be a good choice for systems with low latency
> + requirements. It does RCU garbage collection from a single CPU
> + rather than have each CPU do its own. This frees up all but one
> + CPU from interference by this periodic requirement.
> +
> + Most users should say N here.
> +
> config TINY_RCU
> bool "UP-only small-memory-footprint RCU"
> depends on !SMP
> @@ -409,6 +425,17 @@ config PREEMPT_RCU
> This option enables preemptible-RCU code that is common between
> the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations.
>
> +config JRCU_DAEMON
> + bool "Drive JRCU from a daemon"
> + depends on JRCU
> + default Y
> + help
> + Normally JRCU end-of-batch processing is driven from a SoftIRQ
> + 'interrupt' driver. If you consider this to be too invasive,
> + this option can be used to drive JRCU from a kernel daemon.
> +
> + If unsure, say Y here.
> +
> config RCU_TRACE
> bool "Enable tracing for RCU"
> help
> Index: b/kernel/Makefile
> ===================================================================
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -87,6 +87,7 @@ obj-$(CONFIG_TREE_PREEMPT_RCU) += rcutre
> obj-$(CONFIG_TREE_RCU_TRACE) += rcutree_trace.o
> obj-$(CONFIG_TINY_RCU) += rcutiny.o
> obj-$(CONFIG_TINY_PREEMPT_RCU) += rcutiny.o
> +obj-$(CONFIG_JRCU) += jrcu.o
> obj-$(CONFIG_RELAY) += relay.o
> obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
> obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
> Index: b/include/linux/hardirq.h
> ===================================================================
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -146,7 +146,13 @@ static inline void account_system_vtime(
> extern void account_system_vtime(struct task_struct *tsk);
> #endif
>
> -#if defined(CONFIG_NO_HZ)
> +#if defined(CONFIG_JRCU)
> +extern int rcu_nmi_seen;
> +#define rcu_irq_enter() do { } while (0)
> +#define rcu_irq_exit() do { } while (0)
> +#define rcu_nmi_enter() do { rcu_nmi_seen = 1; } while (0)
> +#define rcu_nmi_exit() do { } while (0)
> +#elif defined(CONFIG_NO_HZ)
> #if defined(CONFIG_TINY_RCU) || defined(CONFIG_TINY_PREEMPT_RCU)
> extern void rcu_enter_nohz(void);
> extern void rcu_exit_nohz(void);
> @@ -168,7 +174,6 @@ static inline void rcu_nmi_enter(void)
> static inline void rcu_nmi_exit(void)
> {
> }
> -
> #else
> extern void rcu_irq_enter(void);
> extern void rcu_irq_exit(void);
> Index: b/kernel/sched.c
> ===================================================================
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2658,6 +2658,21 @@ void sched_fork(struct task_struct *p, i
> }
>
> /*
> + * Fetch the preempt count of some cpu's current task. Must be called
> + * with interrupts blocked. Stale return value.
> + *
> + * No locking needed as this always wins the race with context-switch-out
> + * + task destruction, since that is so heavyweight. The smp_rmb() is
> + * to protect the pointers in that race, not the data being pointed to
> + * (which, being guaranteed stale, can stand a bit of fuzziness).
> + */
> +int preempt_count_cpu(int cpu)
> +{
> + smp_rmb(); /* stop data prefetch until program ctr gets here */
> + return task_thread_info(cpu_curr(cpu))->preempt_count;
> +}
> +
> +/*
> * wake_up_new_task - wake up a newly created task for the first time.
> *
> * This function will do some initial scheduler statistics housekeeping
> @@ -3811,7 +3826,7 @@ void __kprobes add_preempt_count(int val
> if (DEBUG_LOCKS_WARN_ON((preempt_count() < 0)))
> return;
> #endif
> - preempt_count() += val;
> + __add_preempt_count(val);
> #ifdef CONFIG_DEBUG_PREEMPT
> /*
> * Spinlock count overflowing soon?
> @@ -3842,7 +3857,7 @@ void __kprobes sub_preempt_count(int val
>
> if (preempt_count() == val)
> trace_preempt_on(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
> - preempt_count() -= val;
> + __sub_preempt_count(val);
> }
> EXPORT_SYMBOL(sub_preempt_count);
>
> @@ -3994,6 +4009,7 @@ need_resched_nonpreemptible:
>
> rq->nr_switches++;
> rq->curr = next;
> + smp_wmb(); /* for preempt_count_cpu() */
> ++*switch_count;
>
> context_switch(rq, prev, next); /* unlocks the rq */
> @@ -8209,6 +8225,7 @@ struct task_struct *curr_task(int cpu)
> void set_curr_task(int cpu, struct task_struct *p)
> {
> cpu_curr(cpu) = p;
> + smp_wmb(); /* for preempt_count_cpu() */
> }
>
> #endif
> Index: b/include/linux/preempt.h
> ===================================================================
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -10,18 +10,33 @@
> #include <linux/linkage.h>
> #include <linux/list.h>
>
> +# define __add_preempt_count(val) do { preempt_count() += (val); } while (0)
> +
> +#ifndef CONFIG_JRCU
> +# define __sub_preempt_count(val) do { preempt_count() -= (val); } while (0)
> +#else
> + extern void __rcu_preempt_sub(void);
> +# define __sub_preempt_count(val) do { \
> + if (!(preempt_count() -= (val))) { \
> + /* preempt is enabled, RCU OK with consequent stale result */ \
> + __rcu_preempt_sub(); \
> + } \
> +} while (0)
> +#endif
> +
> #if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_PREEMPT_TRACER)
> extern void add_preempt_count(int val);
> extern void sub_preempt_count(int val);
> #else
> -# define add_preempt_count(val) do { preempt_count() += (val); } while (0)
> -# define sub_preempt_count(val) do { preempt_count() -= (val); } while (0)
> +# define add_preempt_count(val) __add_preempt_count(val)
> +# define sub_preempt_count(val) __sub_preempt_count(val)
> #endif
>
> #define inc_preempt_count() add_preempt_count(1)
> #define dec_preempt_count() sub_preempt_count(1)
>
> #define preempt_count() (current_thread_info()->preempt_count)
> +extern int preempt_count_cpu(int cpu);
>
> #ifdef CONFIG_PREEMPT
>


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