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SubjectRe: [PATCH RFC 01/16] prcu: Add PRCU implementation
On Tue, Jan 23, 2018 at 3:59 PM,  <lianglihao@huawei.com> wrote:
> From: Heng Zhang <heng.z@huawei.com>
>
> This RCU implementation (PRCU) is based on a fast consensus protocol
> published in the following paper:
>
> Fast Consensus Using Bounded Staleness for Scalable Read-mostly Synchronization.
> Haibo Chen, Heng Zhang, Ran Liu, Binyu Zang, and Haibing Guan.
> IEEE Transactions on Parallel and Distributed Systems (TPDS), 2016.
> https://dl.acm.org/citation.cfm?id=3024114.3024143
>
> Signed-off-by: Heng Zhang <heng.z@huawei.com>
> Signed-off-by: Lihao Liang <lianglihao@huawei.com>
> ---
> include/linux/prcu.h | 37 +++++++++++++++
> kernel/rcu/Makefile | 2 +-
> kernel/rcu/prcu.c | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++
> kernel/sched/core.c | 2 +
> 4 files changed, 165 insertions(+), 1 deletion(-)
> create mode 100644 include/linux/prcu.h
> create mode 100644 kernel/rcu/prcu.c
>
> diff --git a/include/linux/prcu.h b/include/linux/prcu.h
> new file mode 100644
> index 00000000..653b4633
> --- /dev/null
> +++ b/include/linux/prcu.h
> @@ -0,0 +1,37 @@
> +#ifndef __LINUX_PRCU_H
> +#define __LINUX_PRCU_H
> +
> +#include <linux/atomic.h>
> +#include <linux/mutex.h>
> +#include <linux/wait.h>
> +
> +#define CONFIG_PRCU
> +
> +struct prcu_local_struct {
> + unsigned int locked;
> + unsigned int online;
> + unsigned long long version;
> +};
> +
> +struct prcu_struct {
> + atomic64_t global_version;
> + atomic_t active_ctr;
> + struct mutex mtx;
> + wait_queue_head_t wait_q;
> +};
> +
> +#ifdef CONFIG_PRCU
> +void prcu_read_lock(void);
> +void prcu_read_unlock(void);
> +void synchronize_prcu(void);
> +void prcu_note_context_switch(void);
> +
> +#else /* #ifdef CONFIG_PRCU */
> +
> +#define prcu_read_lock() do {} while (0)
> +#define prcu_read_unlock() do {} while (0)
> +#define synchronize_prcu() do {} while (0)
> +#define prcu_note_context_switch() do {} while (0)
> +
> +#endif /* #ifdef CONFIG_PRCU */
> +#endif /* __LINUX_PRCU_H */
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 23803c7d..8791419c 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -2,7 +2,7 @@
> # and is generally not a function of system call inputs.
> KCOV_INSTRUMENT := n
>
> -obj-y += update.o sync.o
> +obj-y += update.o sync.o prcu.o
> obj-$(CONFIG_CLASSIC_SRCU) += srcu.o
> obj-$(CONFIG_TREE_SRCU) += srcutree.o
> obj-$(CONFIG_TINY_SRCU) += srcutiny.o
> diff --git a/kernel/rcu/prcu.c b/kernel/rcu/prcu.c
> new file mode 100644
> index 00000000..a00b9420
> --- /dev/null
> +++ b/kernel/rcu/prcu.c
> @@ -0,0 +1,125 @@
> +#include <linux/smp.h>
> +#include <linux/prcu.h>
> +#include <linux/percpu.h>
> +#include <linux/compiler.h>
> +#include <linux/sched.h>
> +
> +#include <asm/barrier.h>
> +
> +DEFINE_PER_CPU_SHARED_ALIGNED(struct prcu_local_struct, prcu_local);
> +
> +struct prcu_struct global_prcu = {
> + .global_version = ATOMIC64_INIT(0),
> + .active_ctr = ATOMIC_INIT(0),
> + .mtx = __MUTEX_INITIALIZER(global_prcu.mtx),
> + .wait_q = __WAIT_QUEUE_HEAD_INITIALIZER(global_prcu.wait_q)
> +};
> +struct prcu_struct *prcu = &global_prcu;
> +
> +static inline void prcu_report(struct prcu_local_struct *local)
> +{
> + unsigned long long global_version;
> + unsigned long long local_version;
> +
> + global_version = atomic64_read(&prcu->global_version);
> + local_version = local->version;
> + if (global_version > local_version)
> + cmpxchg(&local->version, local_version, global_version);

It is called with irq-disabled, and local->version can't be modified on
other cpu. why cmpxchg is needed?

> +}
> +
> +void prcu_read_lock(void)
> +{
> + struct prcu_local_struct *local;
> +
> + local = get_cpu_ptr(&prcu_local);
> + if (!local->online) {
> + WRITE_ONCE(local->online, 1);
> + smp_mb();

What's is the paired code?

> + }
> +
> + local->locked++;
> + put_cpu_ptr(&prcu_local);
> +}
> +EXPORT_SYMBOL(prcu_read_lock);
> +
> +void prcu_read_unlock(void)
> +{
> + int locked;
> + struct prcu_local_struct *local;
> +
> + barrier();
> + local = get_cpu_ptr(&prcu_local);
> + locked = local->locked;
> + if (locked) {
> + local->locked--;
> + if (locked == 1)
> + prcu_report(local);
> + put_cpu_ptr(&prcu_local);
> + } else {
> + put_cpu_ptr(&prcu_local);
> + if (!atomic_dec_return(&prcu->active_ctr))
> + wake_up(&prcu->wait_q);
> + }
> +}
> +EXPORT_SYMBOL(prcu_read_unlock);
> +
> +static void prcu_handler(void *info)
> +{
> + struct prcu_local_struct *local;
> +
> + local = this_cpu_ptr(&prcu_local);
> + if (!local->locked)
> + WRITE_ONCE(local->version, atomic64_read(&prcu->global_version));
> +}
> +
> +void synchronize_prcu(void)
> +{
> + int cpu;
> + cpumask_t cpus;

It might overflow the stack if the cpumask is large, please move it to
struct prcu.

> + unsigned long long version;
> + struct prcu_local_struct *local;
> +
> + version = atomic64_add_return(1, &prcu->global_version);

I think this line of code at least causes the following problem.

> + mutex_lock(&prcu->mtx);
> +
> + local = get_cpu_ptr(&prcu_local);
> + local->version = version;

The successful orders of mutex_lock() might not be the same
the orders of atomic64_add_return(). In this case,
local->version will be decreased.

prcu_report() can also happen here now. It is unsure who will
change successfully the local->version.

> + put_cpu_ptr(&prcu_local);
> +
> + cpumask_clear(&cpus);
> + for_each_possible_cpu(cpu) {
> + local = per_cpu_ptr(&prcu_local, cpu);
> + if (!READ_ONCE(local->online))
> + continue;

It seems like reading on local->online is unreliable.

> + if (READ_ONCE(local->version) < version) {

please handle the cases when version wraps around the maximum.

> + smp_call_function_single(cpu, prcu_handler, NULL, 0);

it smells bad when it is in for_each_possible_cpu() loop.

> + cpumask_set_cpu(cpu, &cpus);
> + }
> + }
> +
> + for_each_cpu(cpu, &cpus) {
> + local = per_cpu_ptr(&prcu_local, cpu);
> + while (READ_ONCE(local->version) < version)
> + cpu_relax();
> + }
>

Ouch, the cpu_relax() loop would take a long time.
Since it will wait until all the relevant cpus scheduled.
relevant cpus: prcu reader active cpus.
So this block of code equals to synchronze_sched()
in many cases when prcu is massively used. isn't it?


smp_mb() /* A paired with B */

> + if (atomic_read(&prcu->active_ctr))
> + wait_event(prcu->wait_q, !atomic_read(&prcu->active_ctr));
> +
> + mutex_unlock(&prcu->mtx);
> +}
> +EXPORT_SYMBOL(synchronize_prcu);
> +
> +void prcu_note_context_switch(void)
> +{
> + struct prcu_local_struct *local;
> +
> + local = get_cpu_ptr(&prcu_local);
> + if (local->locked) {
> + atomic_add(local->locked, &prcu->active_ctr);

smp_mb() /* B paired with A */

> + local->locked = 0;
> + }
> + local->online = 0;
> + prcu_report(local);
> + put_cpu_ptr(&prcu_local);
> +}
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 326d4f88..a308581b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -15,6 +15,7 @@
> #include <linux/init_task.h>
> #include <linux/context_tracking.h>
> #include <linux/rcupdate_wait.h>
> +#include <linux/prcu.h>
>
> #include <linux/blkdev.h>
> #include <linux/kprobes.h>
> @@ -3383,6 +3384,7 @@ static void __sched notrace __schedule(bool preempt)
>
> local_irq_disable();
> rcu_note_context_switch(preempt);
> + prcu_note_context_switch();
>
> /*
> * Make sure that signal_pending_state()->signal_pending() below
> --
> 2.14.1.729.g59c0ea183
>

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